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AGH-University of Science and Technology

Faculty of Physics and Applied Computer Science

Charles University

Faculty of Mathematics and Physics

Doctoral dissertation

Damian Rybicki

„Nuclear magnetic resonance study of

selected Ruddlesden-Popper manganites”

completed at the Department of Solid State Physics

Faculty of Physics & Applied Computer Science AGH University of Science & Technology, Cracow, Poland

under supervision of prof. dr hab. Czesław Kapusta

and

at the Department of Low Temperature Physics Faculty of Mathematics and Physics

Charles University, Prague, Czech Republic under supervision of Doc. RNDr. Helena Štěpánková, CSc.

Kraków, 2007

I thank all persons who contributed and have their share in this work.

Above all I wish to thank my beloved wife

for her patience and support.

I warmly thank both supervisors, Prof. Czesław Kapusta and

Doc. Helena Stepankova for their attention and guidance.

I also express my gratitude to V. Prochazka ,Z. Jirak

A. Lemański, D. Zając, W. Tokarz, J. Przewoźnik and others.

Contents

Contents:

Polskie streszczenie i

1. Introduction 1 2. Properties of cubic and bilayered manganese perovskites 2

2.1 Mixed valence manganese perovskites 2 2.2 Structural properties 3 2.3 Magnetic and electronic properties 4 2.4 Phase segregation 7 2.5 Bilayered manganese perovskites, 10 722122 OMnSrLa xx +−

2.6 Cubic manganese perovskites 13 2.6.1 13 31 MnOSrLa xx−

2.6.2 Nanoparticles of 14 31 MnOSrLa xx−

2.6.3 15 31 MnOCaLa xx−

2.7 Sample preparation 17 2.7.1 “Cubic” perovskites 17 31 ),( MnOCaSrLa xx−

2.7.2 Bilayered perovskite manganites 17 3. Nuclear Magnetic Resonance method 19

3.1 Nuclear Magnetic Resonance 19 3.2 Physical basis of the NMR 19

3.2.1 Quantum mechanical description 20 3.2.2 Classical description 22

3.3 Spin echo technique and relaxation times 25 3.4 NMR in magnetically ordered materials 29

3.4.1 The effective field at nucleus 29 3.4.2 Enhancement of the high frequency field 31

3.5 NMR spectrometers and magnetometers 33 4. “Cubic” perovskites: La0.875Sr0.125MnO3, La0.85Sr0.15MnO3 and

La0.9Ca0.1MnO3 38 4.1 La0.875Sr0.125MnO3 and La0.85Sr0.15MnO3 55Mn and 139La NMR 38

4.1.1 La0.875Sr0.125MnO3 - 55Mn NMR 38 4.1.2 La0.85Sr0.15MnO3 - 55Mn NMR 46 4.1.3 La0.875Sr0.125MnO3 and La0.85Sr0.15MnO3 – 139La 50

4.2 La0.9Ca0.1MnO3 - 55Mn NMR results 55

5. Nanoparticles of La0.75Sr0.25MnO3 60 5.1 55Mn NMR results of La0.75Sr0.25MnO3 nanoparticles 60

5.1.1 55Mn NMR spin echo spectra at 4.2 K and 77 K 61

Contents

5.1.2 The spin-spin and spin-lattice relaxations at 4.2 K and 77 K 65 5.1.3 The enhancement factor of 55Mn at 4.2 K and 77 K 70

5.2 139La signal at 4.2 K and at no applied field 72

6. Bilayered manganese perovskites, La2-2xSr1+2xMn2O7 73 6.1 55Mn NMR of bilayered manganese perovskites, La2-2xSr1+2xMn2O7 73 6.2 55Mn NMR results at 4.2 K 75

6.2.1 Mn4+ signal analysis 78 6.2.2 Mn3+ signal analysis 79

6.3 55Mn NMR at 77 K 83 6.4 139La NMR at 4.2K 86

7. Heavily doped La2-2xSr1+2xMn2O7 88

7.1 Magnetic measurements of heavily Sr doped La2-2xSr1+2xMn2O7, x≥0.5 88 7.2 55Mn NMR results of heavily doped La2-2xSr1+2xMn2O7 92 7.3 Spin-spin relaxation times, T2 in heavily doped La2-2xSr1+2xMn2O7 99

8. Conclusions 102 9. References 103

10. List of author’s publications 110

10.1 Publications related to thesis 110 10.2 Other publications 110

Streszczenie i

STRESZCZENIE Niniejsza rozprawa doktorska przedstawia wyniki badań właściwości magnetycznych i elektronowych związków tlenkowych manganu należących do szerokiej grupy materiałów krystalizujących w strukturze perowskitu. W szczególności w pracy badane są tzw. perowskity pseudokubiczne i dwuwarstwowe, których ogólny wzór chemiczny dany jest jako:

gdzie A i B oznaczają odpowiednio jon dwuwartościowy i trójwartościowy ziem alkalicznych i lantanowców. Badane związki charakteryzują się wieloma ciekawymi właściwościami fizycznymi, w szczególności dotyczącymi ich właściwości magnetycznych i elektronowych, do których można zaliczyć zjawisko segregacji fazowej. Wykazują one również efekt magnetooporu, co jest bardzo obiecujące z uwagi na możliwe zastosowania.

( ) 1311 ++− nnnxx OMnBA

Właściwości magnetyczne i elektronowe badanych w niniejszej pacy związków są ze sobą ściśle powiązane na skutek tego, że są one determinowane przez te same elektrony z orbitalu d, które są odpowiedzialne zarówno za magnetyzm tych związków jak również za ich przewodnictwo elektryczne. Wzajemne powiązanie wielu stopni swobody: strukturalnego, ładunkowego i spinowego z typem obsadzanego orbitalu 3d (uporządkowanie orbitalne), skutkuje dużą różnorodnością wykazywanych właściwości. Związki z badanych w pracy „grup” są ferromagnetycznymi izolatorami (FMI) lub metalami (FMM), antyferromagnetycznymi lub paramagnetycznymi izolatorami (AFI lub PI). Perowskity manganowe oferują również niezwykłą możliwość sprawdzenia teorii tworzonych do opisu obserwowanych eksperymentalnie zjawisk. W niniejszej pracy poddany analizie jest problem wpływu „wymiarowości” na lokalne właściwości magnetyczne i elektronowe badanych związków. Pseudokubiczne perowskity manganowe opisywane ogólnym wzorem są uważane za związki „trójwymiarowe”, podczas gdy perowskity dwuwarstwowe,

( ) 31 MnOBA xx−

( ) 7231 OMnBA xx− są uważane za quasi- dwuwymiarowe. W pracy badane są również inne ważne aspekty, takie jak: wpływ domieszkowania Ca lub Sr oraz wpływ wielkości ziaren na właściwości magnetyczne i elektronowe. Zbadano również jak przyłożenie pola magnetycznego modyfikuje lokalne właściwości, a dla niektórych związków zbadano wpływ temperatury na te właściwości. W pracy podjęto również próbę odpowiedzi na pytanie: czy i na ile różnice we właściwościach makroskopowych wiążą się ze zjawiskiem segregacji fazowej występującym w badanych związkach. W celu zbadania lokalnych właściwości związków będących przedmiotem rozprawy zastosowano metodę Magnetycznego Rezonansu Jądrowego (MRJ, ang. - NMR) w powiązaniu z pomiarami namagnesowania. W metodzie MRJ jako „sondę” właściwości lokalnych wykorzystuje się dipolowe momenty

Streszczenie ii

magnetyczne oraz elektryczne momenty kwadrupolowe jąder atomowych. Dostarczają one informacji o właściwościach magnetycznych i elektronowych ich atomów (jonów). Jądrowe dipolowe momenty magnetyczne dostarczają informacji o polach nadsubtelnych wytwarzanych przez elektrony, a jądrowe kwadrupolowe momenty elektryczne dostarczają informacji o gradientach pola elektrycznego wytwarzanych przez niesferyczny rozkład gęstości ładunku elektronowego w miejscu jądra. Dzięki temu MRJ dostarcza informacji o stanie magnetycznym i elektronowym pierwiastka w danym położeniu krystalograficznym, czy magnetycznym. Fluktuacje pola nadsubtelnego i gradientu pola elektrycznego wpływają na procesy relaksacji jądrowej. Pomiary MRJ w zewnętrznym polu magnetycznym pozwalają na zidentyfikowanie rodzaju uporządkowania momentów magnetycznych i sprzężenia między nimi. Praca zawiera następujące rozdziały:

• wprowadzenie • opis własności badanych związków • opis metody Magnetycznego Rezonansu Jądrowego • omówienie wyników eksperymentalnych • podsumowanie. W oparciu o analizę wyników przeprowadzonych badanń wyciągnięto

następujące wnioski: • Segregacja fazowa jest obserwowana w większości badanych

związków i nie zależy od ich „wymiarowości” czy rodzaju domieszkowanego pierwiastka ziem alkalicznych (Sr lub Ca). Obserwuje się ją dla związków zarówno z małą jak i dużą ilością domieszki, dla związków z mniej niż 15% zawartością Sr (w perwowskitach pseudokubicznych) i powyżej 50% (w perowskitach dwuwarstwowych). W przypadku związków pseudokubicznych z zawartością Sr poniżej 15% segregacja fazowa zaobserwowana została w widmach zarówno 55Mn jak i 139La.

• Pomiary temperaturowe dla związku La0.875Sr0.125MnO3 wykazały istnienie obszarów FMM nawet powyżej "makroskopowej" temperatury Curie. Stwierdzono silniejsze sprzężenie momentów magnetycznych manganu w obszarach FMM niż dla jonów Mn4+ w obszarach FMI.

• Sygnały od kationu Mn4+ są obserwowane w zakresie częstotliwości od poniżej 300 MHz w przypadku jonów Mn4+ uporządkowanych antyferromagnetycznie, do 330 MHz dla jonów Mn4+ w obszarach FMI.

• Sygnał od jonów Mn3+ składa się z więcej niż jednej linii, wskazując na znaczną anizotropię pola nadsubtelnego. Linie te występują w

Streszczenie iii

zakresie częstotliwości 400-500 MHz. Wykorzystując zmierzoną anizotropię pola nadsubtelnego w perowskitach dwuwarstwowych (x=0,35 i x=0,5) stwierdzono, że obsadzenie orbitalu 3d typu

223 rz − jest większe niż orbitalu 22 yx − oraz wyznaczono tę różnicę obsadzeń.

• Widma rezonansowe oraz zależności czasu relaksacji spin-spin od częstotliwości dla związku La0.85Sr0.15MnO3 w temperaturach 4,2 K i 77 K pokazują, że w przypadku tego związku oddziaływanie Suhla-Nakamury wpływa na relaksację momentów jądrowych 55Mn, które znajdują się w obszarach ferromagnetycznych metalicznych. W przypadku w związku La0.85Sr0.15MnO3

stwierdzono, że obszary FMM mają rozmiar znacznie większy niż 10 nm, a w przypadku La0.875Sr0.125MnO3, że ich rozmiary są mniejsze niż 10 nm.

• Próbki nanokrystaliczne związku La0.75Sr0.25MnO3 wykazują degradację właściwości magnetycznych ze zmniejszaniem się średniej wielkości ziarna. Zidentyfikowano sygnały pochodzące z metalicznych ferromagnetycznych wnętrz ziaren i z ferromagnetycznych izolujących zewnętrznych warstw ziaren. Zaobserwowano niewielką anizotropię pola nadsubtelnego Mn w próbkach nanokrystalicznych.

• W silnie domieszkowanych perowskitach dwuwarstwowych z 68,05,0 ≤≤ x zaobserwowano sygnały pochodzące zarówno z

obszarów uporządkowanych ferromagnetycznie jak i antyferromagnetycznie. Ma to miejsce również w przypadku związku, który nie posiada uporządkowania magnetycznego dalekiego zasięgu (x=0,68). W przypadku związków z x=0,75 i x=0,8 zaobserwowano wyłącznie sygnał z obszarów uporządkowanych antyferromagnetycznie, co zgadza się z pomiarami namagnesowania, które nie wykazują obecności fazy ferromagnetycznej.

Chapter 1: Introduction 1

1. Introduction

“Cubic” and bilayered manganese perovskites belong to a very broad family of oxide compounds of transition metals crystallizing in the perovskite structure. They are a part of the Ruddlesden-Popper phases system which is described by the formula: , where A and B are trivalent and divalent cations, respectively. The compounds studied exhibit intriguing magnetic and electronic properties and phenomena, including phase segregation. They also show magnetoresistance effect, which is very promising in terms of possible applications. Magnetic and electronic properties of these compounds are closely interrelated, due to the same (d) character of the “magnetic” and “conduction” electrons. The interplay of lattice, charge, spin and orbital degrees of freedom results in a variety of electronic and magnetic properties ranging from a metallic or insulating ferromagnetic, to antiferromagnetic insulating,

( ) 1311 ++− nnnxx OMnBA

or paramagnetic insulating behaviour. Manganese perovskites also offer a unique opportunity to study and verify concepts introduced by theoretical physicists and chemists, which can also be very useful in other fields of physics. In this work the effect of “dimensionality” of the compounds’ structure is studied. “Cubic” manganese perovskites with general formula are considered as three dimensional compounds, while bilayered perovskites,

are considered as (quasi) two dimensional. Other important aspects and factors which influence properties of these compounds are also studied, such as: the effect of Sr or Ca doping, the influence of the grain size on the magnetic and electronic properties and how the applied magnetic field affects the local properties. Additionally, for some compounds the temperature changes of their properties are investigated. Namely, an attempt is made to check if the phase segregation picture is a common feature for various manganese perovskites, which differ in terms of their macroscopic properties.

( ) 31 MnOBA xx−

( ) 7231 OMnBA xx−

In order to study local properties of the compounds of interest, the Nuclear Magnetic Resonance (NMR) method was employed and complemented with bulk magnetic measurements. NMR uses nuclear magnetic dipole moments and electric quadrupole moments as “local” probes of electronic and magnetic states of their parent atoms with the selectivity to the individual isotope of the element. Nuclear magnetic dipoles probe magnetic hyperfine fields produced by electrons and the nuclear electric quadrupoles probe electric field gradients produced by an electric charge density of aspherical distribution around the nucleus. It provides the information on the magnetic and electronic state of an element at the individual structural and magnetic sites. Fluctuations of the hyperfine fields and electric field gradients contribute to the nuclear relaxation. Performing the NMR experiment in an applied magnetic field enables us to study the local arrangement of magnetic moments and their coupling.

Chapter 2: Properties of “cubic” and bilayered manganese perovskites

2

2. Properties of “cubic” and bilayered manganese perovskites

2.1 Mixed valence manganese perovskites Mixed valence manganites with the perovskite structure (see subsection 2.2 Structural properties) with the general formula ( ) 31 MnOBA xx− became for the first time the subject of interest of physicists in the fifties of the last century. Many features of those compounds were described by Jonker and van Santen, namely ceramic sample preparation process, crystal structure and magnetic properties etc. [Jonker 1950, 1956, van Santen 1956]. When a trivalent cation, A3+ is substituted with a divalent cation B2+ with oxygen ion maintaining O2- state, the relative fraction of Mn3+ to Mn4+ decreases with increasing doping x.

In the fifties of the last century many different systems were studied and a rich variety of possible magnetic type orderings and first evidence of phase separation in those compounds were found, see for example in [Wollan 1955], where La1-xCaxMnO3 system was studied by neutron diffraction technique. The interest of the experimental physicists was followed by theoretical physicists and the theories established by C. Zener, J. Kanamori, J. Goodenough and the others are used by scientists today. The interest in manganites revived in the 1990s, when large magnetoresistance (MR) effects were found in Nd0.5Pb0.5MnO3 [Kusters 1989]. Magnetoresistance is an effect in which the electrical resistivity of the material changes when this material is placed in an external magnetic field. The effect was first predicted by E. Hall [Hall 1897] and the MR is usually defined as:

0

0

RRRMR B −

=

where RB is the resistivity of material at applied external magnetic field and R0 is the resistivity at zero field. Example of such effect for compound is presented in Fig. 2.1.

726.14.1 OMnSrLa

Fig. 2.1. Temperature dependence of the electrical resistivity measured on single crystal of at various applied magnetic fields, the resistivity was measured along two different directions [Perring 1998].

726.14.1 OMnSrLa


3

2.2 Structural properties Due to similar crystal structure to that of the mineral CaTiO3 (perovskite),

the compounds studied are called perovskites. Ideal cubic perovskite structure is shown on Fig. 2.2. It consists of the transition ion (small cation) at the centre of the cube, which is coordinated by six oxygen atoms in octahedron, the corners of the cube are occupied by large cations, which can be for example La, lanthanides, alkaline earths or elements from first group of the periodic table like K or Na. Only in ideal case the crystal structure is of cubic symmetry; if one has a mismatch of ionic radii between small and large cations, the symmetry is reduced to orthorhombic or tetragonal. Fig. 2.2 The unit cell of the ideal cubic manganese perovskite.

The studied compounds belong to the Ruddlesden-Popper phases with a general formula ( ) 1311 ++− nnnxx OMnBA , the n parameter can be equal to 1, 2, 3,… and ∞, x is the doping, A is a trivalent cation and B is a divalent cation. When n=∞ one obtains formula , which are so called three dimensional “cubic” perovskites. If n=2 the general formula is

( ) 31 MnOBA xx−

( ) 7231 OMnBA xx− , compounds from this series are sometimes referred as two dimensional bilayered perovskites, which is due to their quasi two dimensional crystallographic structure. Both crystallographic structures are presented in Fig. 2.3. The crystallographic structure of the "cubic" perovskites is no longer of cubic symmetry, but has orthorhombic symmetry due to rotations of the oxygen octahedra and due to the cooperative Jahn-Teller distortions (octahedra can be compressed or elongated along various axes), Fig 2.3a or a rhombohedral symmetry at higher doping (not presented). The crystallographic structure of bilayered perovskite presented in Fig 2.3c is tetragonal. Fig 2.3b presents the Mn-O bonds arrangement in “cubic” perovskites. The medium bond length is between Mn and apical oxygens, the short and long bonds are between Mn and planar oxygens and they alternate on going to neighbouring octahedron. Fig. 2.3d presents a scheme of Mn-O bonds arrangement in bilayered manganites. Depending on the doping parameter x, the short and medium bonds can be exchanged.


4

Fig. 2.3 a) Crystallographic structure of “cubic” manganese perovskite with Pbnm orthorhombic symmetry; b) bonds arrangements in “cubic” manganese perovskite with orthorhombic symmetry (L-long, M-medium, S-short, trivalent or divalent cations were omitted for clarity); c) crystallographic structure of bilayered manganese perovskite with I4/mmm tetragonal symmetry; d) bonds arrangements in bilayered manganese perovskite with tetragonal symmetry.

2.3 Magnetic and electronic properties

Before going into more detailed information on electronic and magnetic properties of perovskite manganites with mixed valence a brief and essential introduction to basic physics related to manganese compounds with perovskite structure is presented. As was mentioned earlier, when substituting a trivalent cation A with divalent cation B the valence of Mn increases from Mn3+ to Mn4+. Both these ions have their valence electrons in the 3d band, the splitting of the atomic 3d energy levels by the crystal field and by the Jahn-Teller effect in Mn3+ is presented in Fig 2.4.


5

Fig 2.4 Energy levels of 3d band of the Mn3+ in a free ion (five-fold degenerate levels) and splitting of levels by the crystal field (10Dq) in the octahedral symmetry into t2g and eg levels and further splitting by the Jahn-Teller distortion (ΔJT). One of possible modes of the Jahn-Teller distortion is also presented. The meaning of used symbols is described in text. In the free Mn3+ cation five 3d orbitals are degenerated, but the crystal field resulting from six oxygen anions in vertices of octahedron surrounding Mn3+ cation in the centre, splits energy levels into three lower energy levels, t2g ( , , ) and two higher energy levels, eg ( , ). The crystal field splitting (10Dq) amounts to 2-3 eV. The t2g and eg levels are further split by the Jahn-Teller distortion (ΔJT), which is typically one order of magnitude smaller than the crystal field splitting energy. One of possible modes of the Jahn-Teller distortion is presented in Fig 2.4. In the presence of the Jahn-Teller distortion the eg orbitals differentiate their energy, which leads to many interesting phenomena like orbital ordering in manganese perovskites [van den Brink 2002]. The large Hund’s coupling (JH) leads to the high spin state of the Mn ions (either three electrons on the t2g band and one electron on the eg band for Mn3+ or only three electrons on the t2g band for Mn4+).

xyd xzd yzd 22 yxd

− 223 rzd

−

For understanding the physics related to manganese perovskites it is also essential to introduce possible magnetic interactions, which can occur between Mn ions i.e. the super-exchange (SE) and double exchange (DE) interactions. Both these interactions are indirect exchanges with mediation of the oxygen, which is located between two Mn ions. The SE interaction occurs between two magnetic ions of the same valence through the occupied oxygen 2p orbital (Fig. 2.5). The SE interaction is a virtual exchange between electrons of magnetic ions with electrons from the same 2p orbital of oxygen, which are antiparallel according to Pauli exclusion principle. Therefore the orientation of spins of magnetic ions is also antiparallel and leads to antiferromagnetic ordering of the material. In some cases the SE interaction can lead to ferromagnetic ordering, it happens, when electrons from magnetic ions interact with electrons from


6

different oxygen orbitals [Goodenough 1955]. Since SE is only “virtual” exchange it does not affect the mobility of electrons.

The DE interaction occurs between two Mn with different valence also through the 2p oxygen orbital, but unoccupied (Fig. 2.5). The DE interaction occurs when the electron from the eg orbital (of Mn3+ ion) can hop to a neighbouring site (to Mn4+ ion), when there is a vacancy of the same spin [Zener 1951]. As a result of the first Hund’s rule and strong exchange interaction of the eg electron and three t2g electrons all electrons are aligned. Therefore the hopping of the eg electron from one site to other site with t2g electron spins antiparallel to eg electron spin is not energetically favourable. As a result, ferromagnetic alignment of neighbouring magnetic ions is required to maintain the high spin state of both ions. The DE and the electron hopping ability reduce kinetic energy and the system aligns ferromagnetically to minimise the total energy. Another result of electron hopping is the appearance of itinerant electrons in the system, which results in metallicity of the material. If the manganese spins are not parallel or if the Mn-O-Mn bond is bent the electron transfer is more difficult and electron mobility decreases.

Fig. 2.5 The schematic diagram of mechanism of the double exchange and super-exchange interactions in the case of two Mn cations.

It was proposed by Anderson and Hasegawa [Anderson 1955] that the

transfer integral, t follows following relation:

⎟⎠⎞

⎜⎝⎛=

2cos0

θtt (2.1)

where t0 is the normal transfer integral, which depends on the spatial wave functions and the term cos(θ/2) is due to spin wave function where θ is the angle between two spins directions (Fig. 2.5). An important factor, especially


7

for nuclear magnetic resonance, is also correlation time of the electron hoping, τ which is smaller than 10 ps, in La1-xNaxMnO3 (x=0.1-0.2) compounds τ was found to range from 10-13 s (at 60 K) to 10-11 s (at 300 K) [Savosta 1999].

2.4 Phase segregation

Due to the strong competition between the DE and SE interactions and electron-phonon interactions (i.e. Jahn-Teller effect) in manganites a rich variety of possible types of magnetic (paramagnetic, ferromagnetic, antiferromagnetic, canted antiferromagnetic) and electronic (metallic, insulating) phases can occur. Moreover, in manganites ferromagnetic metallic (FMM) phase can coexist with ferromagnetic insulating (FMI) or antiferromagnetic insulating (AFI) phases. If one also keeps in mind the possible charge order (Mn3+ and Mn4+ ions) and orbital order of the Mn3+ 3d eg orbitals ( or ) the number of physical parameters, which has to be taken into account, when explaining observed properties of manganites, is very high.

22 yxd

− 223 rzd

−

Fig. 2.6 An atomically sharp boundary separates an insulating charge-ordered phase (left, pink) from a weakly conducting charge-disordered phase (right, purple), as shown in this room-temperature STM image of Bi0.24 Ca0.76 MnO3, inserts present I-V curves for both regions [Renner 2002, Mathur 2003].

First observations of phase coexistence in manganese perovskites date to 1950s and were done with neutron diffraction, but nowadays imaging techniques provided much more detailed information on the nature and the length scales of this phases. The first experiment with imaging technique (transmission electron microscopy) revealed coexistence of the FMM phase and charge ordered insulating (COI) phase in compounds La5/8-yPryCa3/8MnO3 [Uehara 1999]. The authors also proposed a model, which explained the fact that observed saturation magnetisation, MS was smaller than it was supposed to be according to the compound’s stoichiometry and related theoretical spin magnetic moment. The model assumed that due to DE interaction between Mn3+ and Mn4+ ions, FM


8

clusters (domains) are formed, their size depends on the distortions and doping. However, there are also regions characterized as charge ordered and insulating, which are responsible for smaller MS of the sample.

The picture of phase separation obtained by scanning tunnelling microscope (STM) spectroscopy combined with the atomic resolution STM imaging for Bi0.24 Ca0.76 MnO3 is presented in Fig. 2.6. In the charge-ordered phase, the I-V curve displays insulating behaviour, whereas the charge-disordered phase shows an ohmic, metallic regime near zero voltage. The origin of this difference appears to be related to the structure. In the charge-ordered phase, the Mn3+ and Mn4+ ions arrange themselves in a regular, repeating pattern that doubles the unit cell [Renner 2002, Mathur 2003].

The phase separation problem has recently been studied by other imaging techniques. E.g. Loudon et al. [Loudon 2002] reported the electron holography and transmission electron microscopy results on La0.5Ca0.5MnO3 at 90 K. The authors found the phase exhibiting simultaneously charge ordering and ferromagnetic properties, which were prior believed to be mutually exclusive. J.H. Yoo et al in [Yoo 2004] studied La0.81Sr0.19MnO3 compound by means of the electron holography and the effect of the applied magnetic field on the coexisting paramagnetic and ferromagnetic phases was presented. Their studies revealed that well separated ferromagnetic domains located in paramagnetic matrix are connected by the formation of the channels of magnetic flux, while the previous works considered the interaction between the substantially close FM domains. When phase separation was found by experimentalists it also attracted interest of theoretical physicists. First computational results presented in [Yunoki 1998] with Kondo Hamiltonian have taken into account the DE interaction and Jahn-Teller distortions and the results showed strong tendency for phase separation in hole doped manganese perovskites. The models were further extended [Moreo 1999, 2000, Dagotto 2001, 2002] but the common feature was assuming a cluster state with FM islands (droplets etc.), which were aligning their magnetic moments on applying external magnetic field. However, such models had problems with explaining the experimental values of the resistivity. This problem was solved in [Burgy 2004] by adding into considerations the cooperative nature of Mn-O lattice distortions. In [Khomskii 2003] it was shown that in percolation picture, not only the total fraction of one or another phase, but also distribution of these phases in size and shape can be crucial. Also NMR measurements revealed electronic phase separation in manganese perovskites. First such results were presented in [Allodi 1997, Papavassiliou 1999, 2000, Renard 1999, Kapusta 2000a, 2000b]. Phase separation observed with the NMR method manifests itself with distinct resonant lines in the frequency swept NMR spectra. Resonances around 330 MHz or higher than 400 MHz are due to Mn4+ and Mn3+ ions in the


9

ferromagnetic insulating phase. Signals between 250-290 MHz are due to the resonance of Mn4+ ions in the antiferromagnetic insulating phase. The resonance around 370-380 MHz is due to manganese ions with averaged valence (between 3+ and 4+) as a result of the ferromagnetic coupling resulting from the DE interaction [Matsumoto 1970]. Observation of this resonance indicates that the characteristic time of electron hopping due to the DE interaction is shorter than the precession period of the nuclear magnetic moment, i.e. it is much shorter than the difference in precession periods in the two states (3+ and 4+) when the direction of hyperfine field does not change, i.e. <<5.10-9s. A schematic plot of the frequency swept spectrum with possible resonant lines is presented in Fig. 2.7. In [Allodi 1997] results of 55Mn and 139La NMR measurements, showed coexistence of antiferromagnetic and ferromagnetic subsystems in Ca doped LaMnO3 compounds. Coexistence of FMM and FMI regions is also possible as was concluded in [Papavassiliou 2000] for La1-xCaxMnO3 (with x ranging from 0.2 to 0.5). In low hole doped La1-xSrxMnO3 system coexistence of insulating and metallic phases was found as well [Renard 1999].

Fig. 2. 7. Schematic plot of the frequency swept NMR spectrum with possible resonant lines and their attribution to various phases.


10

OMnSr x+

2.5 Bilayered manganese perovskites, 722122 OMnSrLa xx +−

The general formula of bilayered manganites can be written as

. The magnetic and structural phase diagram for doping parameter x ranging from 0.3 to 1, determined by neutron powder diffraction [Mitchell 2001] is presented in Fig 2.8.

722122 OMnSrLa xx +−

Fig. 2.8 Magnetic and structural phase diagram of determined by neutron powder diffraction [Mitchell 2001]. Solid markers represent magnetic transition temperature (TC or TN), open markers the tetragonal to orthorhombic transition. The abbreviations mean: FMM (ferromagnetic metallic), AFI (antiferromagnetic insulating), LRO (long range order), CAF (canted antiferromagnet), CO (charge ordered), types of magnetic order are shown in Fig. 2.9.

722122La x−

As was stated earlier the compounds studied have tetragonal I4/mmm

symmetry at low temperatures, which does not change with doping up to x=0.8. Their body centred tetragonal structure consists of bilayers of MnO6 octahedra separated by (La,Sr)2O2 layers. MnO6 octahedra, similarly to “cubic” perovskites (with n=∞) suffer from Jahn-Teller (J-T) distortions, although they are much smaller and typical Mn-O-Mn angles in plane are close to 179° [Mitchell 2001]. However magnetic and electronic properties are considerably altered by the change of doping parameter. The (x=0.3) compound at low temperatures is an antiferromagnet (AFM) with magnetic moments along the c axis and ferromagnetic (FM) order within the bilayer while bilayers are AFM coupled (Fig. 2.9). This type of magnetic order was also confirmed by imaging with spin-polarised scanning electron microscope [Konoto 2004]. The compounds with doping from 0.33≤x<0.5 are FM with moments within the ab plane. In phase diagram constructed in [Mitchell 2001]

726.14.1 OMnSrLa


11

the x=0.4 compound has the same magnetic structure as the compound with x=0.35, while in [Kubota 1999, Hirota 2002] the structure is proposed to be a canted AFM. Canted AFM ordering occurs in [Mitchell 2001] in doping range 0.42≤x<0.5. Starting from x=0.5 A type AFM order was proposed. In this type of magnetic order, magnetic moments are within ab plane FM coupled in a single layer, but coupling between two layers is AFM. Very interesting region was found to exist in doping range 0.66≤x<0.74, where neutron powder diffraction showed no magnetic long range order (LRO) [Ling 2000]. Within the doping range 0.75≤x<0.92 the crystal structures changes to orthorhombic Immm space group. The neutron powder diffraction revealed two AF superstructures, referred as type C and type C*, in which the c is doubled [Ling 2000]. Above x=0.92 type G magnetic order was found [Mitchell 1998]. The ideal G type magnetic order, found for x=1, involves all Mn nearest neighbours coupled AF, with spins parallel to the c axis. Below x=1 spins tilt from the c axis towards ab plane [Ling 2000]. Therefore in accordance to Goodenough’s theoretical predictions, when increasing Mn4+ content (increasing number of holes in the eg orbital) one successively obtains less ferromagnetism (sheets to rods to points). Fig. 2.9 Magnetic structures of series of compounds at 5 K, determined by the neutron powder diffraction [Mitchell 2001].


As a result of reduced dimensionality of the compounds a highly

anisotropic behaviour of the resistivity is observed. The resistivity measured within the ab plane and along the c axis differs by few orders of magnitude [Perring 1998, Matsukawa 2000] (Fig. 2.1). Reduced dimensionality of the compounds gives also rise to an enhanced magnetoresistance (MR) [Moritomo 1996]. Spin wave measurements carried out for x=0.3 to 0.4 [Perring 1998] showed that the in-plane exchange depends weakly on doping, x. The exchange between planes of the bilayer changes by factor of four with increasing x from 0.3 to 0.4 and it was concluded that with doping there is a change of the orbital character from mixed 3d and orbital for compound with x=0.3 to 223 rz

d− 22 yx

d−


12

mostly for x=0.4. A stabilization of the 3d orbital with increasing doping was also proposed in another spin waves study [Hirota 2002]. Similar conclusions were derived by [Welp 2001] on the basis of magnetisation measurements on single crystals and magnetic anisotropy analysis. However there are also studies that suggest that in doping range of 0.3≤x≤0.5, occupancy of the 3d orbital is higher than of the orbital. In [Argyriou 2002,] from polarized neutron diffraction studies of the x=0.4 doped compound at 100 K, authors concluded that the 3d orbital has higher ocupation. Similar conclusion for the compound with the same doping was made in [Akimoto 1999] basing on the Madelung potential calculation derived from the structural data.

22 yxd

− 22 yxd

−

223 rzd

− 22 yxd

−

223 rzd

−


13

2.6 “Cubic” manganese perovskites

2.6.1 31 MnOSrLa xx−

Phase diagrams of all manganese perovskites are rich especially at low

temperatures and the material magnetic and electronic properties change strongly with dopant concentration (e.g. Sr or Ca). Fig . 2.10 presents one of the first constructed phase diagrams of series of compounds in the region of interest. The parent compound i.e is an antiferromagnet with ferromagnetic order within the plane and antiferromagnetic coupling between the planes [Wollan 1955] with Néel temperature TN≅140 K and magnetic moment within the plane of μ=3.87μB [Moussa 1996]. The spin-echo NMR 55Mn study by Sidorenko et al. [Sidorenko 2004] revealed signal from Mn3+ ions around 350 MHz at 10 K. The signal was vanishing fast with temperature due to fast transverse relaxation. With increasing doping the TN decreases slightly and when going out from parent compound canted antiferromagnetism (CAF) occurs. Starting from x≅0.1 we have ferromagnetic spin arrangement with TC increasing with doping parameter. The compounds of interest i.e. x=0.125 and x=0.15 have TC values 205 K and 240 K respectively. Dabrowski et al. [Dabrowski 1999, Xiong 1999], have studied extensively the transport, magnetic and structural properties of La1-xSrxMnO3 compounds with 0.1≤x≤0.2 and in temperature range 10≤T≤350K , the study has shown that the arrangement of spins varies from FM ordered, mainly along the b-axis (x=0.11) to FM almost

31 MnOSrLa xx−

3LaMnO

Fig. 2.10 Phase diagram of . CI, FI, FM, PI, PM, AFM denote spin canted insulating, ferromagnetic insulating, ferromagnetic metallic, paramagnetic insulating, paramagnetic metallic, antiferromagnetic [Dagotto 2001].

31 MnOSrLa xx−


14

along the c-axis (x=0.185). Up to x=0.16 compounds are insulating, both below and above the TC. Above x=0.16, below the TC compounds are still ferromagnetic, but metallic up to almost x=0.5. Unexpectedly compounds in the range 0.16<x<0.3, which are metallic at low temperatures, above the TC are insulating. In the region 0.5≤x≤0.6 the compounds are A-type AFM (see Fig. 2.9). At low temperatures starting from close to x=0.22 there is a change from orthorhombic symmetry at lower doping to rhombohedral at higher x [Paraskevopoulos 2000, Dabrowski 1999]. This structural transformation is due to an absence of coherent Jahn-Teller distortions, which are present in the orthorhombic phase. In perovskites in paramagnetic state there exist magnetic moments correlations called magnetic polarons [DeGennes 1960]. They were studied by several techniques up to now. One of first results presenting such polaronic behaviour in paramagnetic state was NMR study by Kapusta et al., [Kapusta 1999a], where authors found that such correlations existed due to the DE interaction. In system (for x=0.3 and 0.4) such polarons were experimentally found above the TC by Mannella et al., [Mannella 2004]. Formation of ferromagnetic polarons above the TC and below some certain temperature T* agrees also with theoretical calculations [Burgy 2001].

31 MnOSrLa xx−

Besides magnetic polarons also orbital polarons were found in these systems. The Hartree-Fock calculations indicated that in lightly doped manganites orbital polarons in which the orbitals of Mn3+ sites are directed towards the Mn4+-like site are created. Orbital polarons are stabilized by the breathing-type and Jahn-Teller-type lattice distortions and the magnetic coupling between these polarons is ferromagnetic [Mizokawa 2000]. Experimentally in lightly doped , the orbital polarons were confirmed to be present below the TC down to 5 K by the inelastic light scattering [Choi 2005].

31 MnOSrLa xx−

2.6.2 Nanoparticles of 31 MnOSrLa xx−

In the past few years, besides single crystals, powder samples with grains

of micrometric size, thin films and nanoparticle materials have also been studied [Zhu 2001, Li 2001, Bibes 2003]. They are prospective materials for biomedical applications, both for diagnostics and therapy [Mornet 2004, Gupta 2005]. For example for magnetic hyperthermia owing to their lower TC, they seem to be advantageous over commonly used magnetite or maghemite nanoparticles. Lower TC guarantees that the treated tissue is not overheated, the radiation which penetrates the tissue is only emitted if the nanoparticle is in magnetically ordered state [Pollert 2006]. Magnetic nanoparticles have already been used as a magnetic resonance imaging contrast enhancement, for tissue repair, detoxification of biological fluids, drug delivery and cell separation. The biomedical applications require that the nanoparticles have high magnetization


15

values, dimensions smaller than 100 nm and narrow range of the size distribution. For biomedical applications the surface of nanoparticles has to be coated, this coating has to be non-toxic, biocompatible and allow target delivery of the particle localization in the specific area.

The size of the grain and in consequence different volume ratios of the atoms on the surface to the total volume, are additional parameters that highly influence electrical and magnetic properties of the perovskite manganites. It is now well established in literature that the nanoparticles consist of inner core part, which has properties similar to the bulk material and surface part, which differs in properties from the bulk material. The surface regions are spin disordered and may have some structural faults like vacancies etc. As can be expected the size of the grains has very big influence on the magnetotrasport properties (i.e. magnetoresistance) due to higher electron scattering on disordered grain surfaces at zero field and smaller scattering on ordered spins after applying the external magnetic field. From this point of view nanoparticles of manganese perovskites can in the future be used by the modern spin electronics.

2.6.3 31 MnOCaLa xx−

The phase diagram of this system is presented in Fig. 2. 11. All

manganese cubic perovskites doped with divalent alkaline earth reveal similar structural, electronic and magnetic properties at similar doping values. Therefore always similar phases occur on their phase diagrams, although at slightly different doping, which is mostly determined by the band width. The low Mn4+ concentration region at low temperatures of the , similarly to the

system, transforms with doping from the AF insulator to canted AF insulator, FM insulator and to FM metal [Dagotto 2001, Biotteau 2001].

There is confusion in the literature about properties of the low doped system. In [Pissas 2004] it was found that it was due to the

different sample preparation processes used, namely different atmospheres during sample preparation process. The differences in the case of compound of interest, i.e. result in the fact that when prepared in helium atmosphere it has an A-type antiferromagnetic structure with an antiferromagnetic spin component along the b-axis and a ferromagnetic component along the c-axis (as was reported earlier [Biotteau 2001]) while the sample prepared in air atmosphere is ferromagnetic [Pissas 2004]. However, all our samples were prepared in air atmosphere, see for further details in sample preparation section.

31 MnOCaLa xx−

31 MnOSrLa xx−

31 MnOCaLa xx−

31.090.0 MnOCaLa


16

Fig. 2.11 The phase diagram the [Dagotto 2001], the denominations used are the same as in Fig 2.10.

31 MnOCaLa xx−


17

2.7 Sample preparation

2.7.1 “Cubic” perovskites 31 ),( MnOCaSrLa xx−

The samples desribed in chapter 4 were prepared by

standard ceramic procedure. First, heating in air of stoichimetric amounts of compounds containing desired elements at temperatures lower than 1000 °C, grinding and pressing powder into bars and another heating at temperatures higher than 1000 °C, see for example in [Algarabel 2003]. The preparation process of La0.9Ca0.1MnO3 samples is described in more details in the chapter 4.

31 ),( MnOCaSrLa xx−

All samples described in the chapter 5 (two with grains of nanometric and one of micrometric size) were fabricated by firing the sol-gel prepared precursors at different temperatures in order to obtain different average grain sizes. The starting compounds La2O3, SrCO3, and MnCO3 of the actual contents of the cationic components determined by chemical analysis, were separately dissolved in nitric acid, mixed together with citric acid and ethylene glycol in the ratio of (0.75[La3+]+0.25[Sr2+]+[Mn2+]) / 1.5[citric acid] / 2.25[ethylene glycol] and pH was adjusted to 9 by addition of NH4OH. Further steps included evaporation of water at 80–90o C, drying at 160oC and calcination at 400oC (4 hours) in air. Finally the powders were annealed at given temperatures in the range of 570–900 oC in air for 3 and 15 hours and then cooled down by switching off the furnace. The size of nanoparticles was determined from broadening of X-ray diffraction lines and checked independently by the electron microscopy [Pollert 2006].

2.7.2 Bilayered perovskite manganites

All studied samples of bilayered perovskites were powder samples, however samples in range 5.03.0 ≤≤ x were prepared as single crystals with the floating zone method and were later powdered. The crystal growth was performed on sintered polycrystalline rod with use of a halogen-lamp image furnace at a rate of 12 or 14 mm/h under an atmosphere of 1 atm O2. The preparation method of single crystals is thoroughly described in [Kimura 1998]. Higher Mn4+ containing region (beyond x=0.5) required more complex preparation process in order to obtain stoichiometric samples, however growth of single crystals beyond x=0.5 is still unsuccessful.

Polycrystalline samples of were prepared by high temperature, solid state reaction of La2O3 (Johnson-Matthey REacton 99.999%, pre-fired at 1000°C in air for 12h), SrCO3 (Johnson-Matthey Puratronic 99.994%, dried at 150°C in air for 12h) and MnO2 (Johnson-Matthey Puratronic 99.999%, pre-fired at 425 °C in flowing oxygen for 6h, then slow cooled at 1 °C/min to room temperature). Stoichiometric quantities of the starting materials



18

were mixed and fired in air as powders, at 900 °C for 24 h and then 1050 °C for a further 24 h. Samples were then pressed into 13 mm diameter pellets at 6000 lbs and ramped at 5°C/min to 1650 °C. After 18 h each compound was quenched directly from the synthesis temperature into dry ice. As is the case for Sr3Mn2O7+δ, [Urushibara 1995] the materials are metastable below 1650 °C and must be rapidly cooled to below 1000 °C to prevent decomposition. The black products were subsequently annealed for 12 h at 400 °C in the flowing oxygen. Materials were reground between each firing [Millburn 1999]. The Mn4+ content of the as-made and annealed materials was determined by iodometric titration against a standardized potassium thiosulfate solution [Millburn 1999].

Chapter 3: Nuclear Magnetic Resonance method

19

3. Nuclear magnetic resonance method

3.1. Nuclear Magnetic Resonance In this chapter the physical basis of the Nuclear Magnetic Resonance (NMR) and its application to solid state physics, with emphasis put on the magnetic properties, is described. The nuclear magnetic resonance in condensed matter was first observed independently in 1946 by two scientists, Felix Bloch and Edward M. Purcell [Bloch 1946, Purcell 1946]. They described it as a physicochemical phenomenon which was due to the magnetic properties of certain nuclei in the periodic system. They were both awarded with the Nobel Prize in Physics in 1952. Since then the NMR experiments are broadly used in many different fields of science. Nowadays various NMR techniques are applied in solid state physics (to study magnetic, electrical and structural properties) , medicine (i.e. Magnetic Resonance Imaging – MRI), biology, chemistry (for example structures of compounds such as polymers), biochemistry (studies of conformation and dynamics of biomolecules). Some of main advantages of the NMR are that this method is non destructive (not harmful to the sample or patient) and selective (element and even isotope selective). The NMR phenomenon relies basically on the interaction of the magnetic dipolar moment of the nucleus with the magnetic field and of the electric quadrupole moment with the electric field gradient. The information obtained from the experiment helps us to learn about the local magnetic, electrical and structural properties of the system under investigation. NMR also allows to study structure of particles, chemical reactions and bonding, crystallographic structure and, due to non invasiveness, to study living organisms and processes occurring in their bodies. For this thesis the most important is the possibility to study magnetic materials and the information resulting from such experiments, e.g. strength of magnetic interaction, magnetic moments and the magnetocrystalline anisotropy.

3.2. Physical basis of the NMR In 1924 Wolfgang Pauli suggested the possibility of an intrinsic nuclear spin which triggered the development of methods that would allow to prove and measure this new physical quantity. This was first done in 1933, when Otto Stern and Walther Gerlach were able to measure the effect of the nuclear spin by the deflection of a beam of hydrogen molecules. The basic description of the magnetic resonance of nuclear magnetic moments (NMR) and the electronic magnetic moments (ESR) were developed simultaneously because the basic


20

principles are the same with the difference in results coming from different

masses of the proton (mp) (resp. neutron) and electron (me): 1840≅e

p

mm

.

3.2.1 Quantum mechanical description

The quantum mechanical description of atomic nucleus formulated by Dirac in 1930, predicted the property of spin angular momentum. This property is characterised by the spin quantum number, I . Its value is an intrinsic property of every nucleus and it is quantized – it may take the half-integer or integer values only. The total spin angular momentum is . The simplest example is proton (hydrogen nucleus), with its nuclear spin I=1/2. In order to observe the nuclear magnetic resonance the nuclei have to possess non zero nuclear spin.

2/1))1(( +IIh

The nuclear magnetic moment nμr is related to the spin I of the nucleus by the equation:

In

rr γμ = , (3.1) where γ is the gyromagnetic ratio, which is isotope-specific and varies widely for different nuclei. In absence of the external magnetic field, the energy levels of nucleus are degenerated. This degeneracy is lifted by applying magnetic field and the energy, E of the nuclear magnetic moment placed in the external field,

is given by the scalar product: 0Br

0BE n

ro

rμ−= . (3.2) The static magnetic field, is conventionally taken to be in the direction, so 0B

rz

that the energy is expressed as: zIBE hγ0−= . (3.3)

However, the quantization of the angular momentum restricts Iz to integer and half integer values, thus:

mBmE hγ0)( −= , (3.4) where m is the eigenvalue of the operator Iz and goes in integer steps from -I to +I.

The applied static magnetic field causes the split of the energy into 2I+1 levels and is called the nuclear Zeeman effect. Fig 3.1 presents the diagram of energy levels and the energy difference between two spin states as a function of the external magnetic field, 0B

rfor the case of proton with I=1/2.

The energy difference between two energy levels, ΔE can be obtained as: 0BE hγ=Δ . (3.5)

If the transition occurs between two energy levels, e.g. due to absorption or emission of a photon with the energy 0ωh one obtains Larmor equation:

00 Bγω = (3.6)


21

which is the most important equation in NMR spectroscopy [Kittel 1966].

Fig. 3.1 The diagram of energy levels and the energy difference between two spin states as a function of the external magnetic field, 0B

rfor the case of proton

(I=1/2, γ>0).

If the P(m) represents the population of nuclei at the mth level, then, using the Boltzman distribution function, it can be written as:

⎟⎟⎠

⎞⎜⎜⎝

⎛−≈=

−

TkmEAAemP Tk

mE

β

β )(1)()(

for E(m)/kβ T<<1, (3.7)

where A is a normalisation factor, T is the absolute temperature, is the Boltzman constant and N is the number of NMR nuclei in the sample,

βk

∑ =m

NmP )( . (3.8)

Thus, the equilibrium nuclear magnetisation of the sample at temperature T can be expressed with the formula:

0)()(

==

== ∑∑YX

zzZ

MMmPmM μμ

(3.9)

where hmmz γμ =)( . Since 0=∑m

m and 3

)12)(1(2 ++=∑

IIImm

, the

magnetization along the direction can be written as: z

)12)(1(3

)22

++= IIITkB

AM Zβ

γ h, (3.10)

where 12 +

=INA , so that

)1(3

)22

+= IITk

NBM Z

β

γ h (3.11)

[Abragam 1961]. This formula describes the nuclear magnetisation of a paramagnetic system of nuclear spins related to an uneven occupation of their energy levels in the external magnetic field.


22

When one suddenly applies the external magnetic field, the nuclear magnetisation of the sample does not reach its equilibrium value )(∞M instantly, but the equilibrium state is gradually approached according to the formula:

⎟⎟⎠

⎞⎜⎜⎝

⎛−∞=

−11)()( Tt

eMtM , (3.12)

T1 is the longitudinal, or spin-lattice relaxation time. The value of T1 depends on the material and it varies with temperature. It can range between microseconds and hours [Abragam 1961].

3.2.2 Classical description If the nuclear magnetisation vector M

ris placed in a magnetic field B

r, it

experiences a torque. The equation of motion for Mr

can be expressed as:

BMdtMd rrr

×= γ . (3.13)

If the magnetic field Br

is static and acts along the direction, ,one can write the following equations:

z zBB ˆ0=r

yx MB

dtdM

0γ= , xy MB

dtdM

0γ= , 0=dt

dM z , (3.14)

and derive the solutions: tMM x 0cosω⊥= , tMM y 0sin ω⊥−= , ||MM z = , (3.15)

where M⊥ and M|| are the magnetization components perpendicular and parallel to the static field and0B

r0ω is the angular frequency of the precession, 00 Bγω = ,

which is the Larmor frequency [Kittel 1966].

Fig. 3.2 Precession of the magnetisation vector, Mr

in a static magnetic field applied along the axis. z


23

In the above situation the nuclear magnetisation vector Mr

precesses along the magnetic field direction as shown in Fig. 3.2. Let us now introduce a new reference frame. From the static laboratory frame (x, y, z) we can transform our system to a rotating reference frame (x’, y’, z’), where z’ is coincident with z. The field, rB

r seen by the magnetic moment in

the frame rotating with an angular frequency ω can be written as:

'ˆ0 zBBr ⎟⎟⎠

⎞⎜⎜⎝

⎛−=

γωr

. (3.16)

When the angular frequency fulfils the condition 0Bγω = the magnetic moment does not see the static effective magnetic field 0B

r and remains at rest in the

rotating frame, 0=rBr

. Now, with the static magnetic field 0B

rapplied along axis, let us

consider the time varying magnetic fieldz

1Br

, perpendicular to and oscillating at the angular frequency ω0:

0Br

)sinˆcosˆ()( 0011 tytxBtB ωω −=r

(3.17)

One can write equations fordt

dM x , dt

dM y and dt

dM z similarly to case with only 0Br

.

Their solutions show that by applying oscillating magnetic field at angular frequency ω0, the magnetisation simultaneously precesses around at ω0 and around at

0Br

1Br

11 Bγω = Making the transformation to the same rotating frame as previously, one

obtains: 'ˆ'ˆ 10

0 xBzBBr +⎟⎟⎠

⎞⎜⎜⎝

⎛−=

γωr

, (3.18)

At the resonance this equation simplifies to: 'ˆ1xBBr =r

, which means that the nuclear magnetisation in the rotating frame precesses around x axis. In NMR experiments radio frequency field is realised by placing the sample into a coil with radio frequency (rf) sinusoidal current. The “linearly polarised” rf field can be considered as two rotating fields with opposite helicities and two times smaller amplitudes:

+− +=++−== tytxBtxBtB B BBtytxrr

sinˆcosˆ(cosˆ)( 01

011 ωωr

)sinˆcosˆ(2

)2 00

10 ωωω

(3.19)


24

In the rotating reference frame, the angular frequency of the magnetic field B - and B+ is zero and 2ω0, respectively. Thus, the contribution of B+ can be neglected.


25

3.3. Spin echo technique and relaxation times Nowadays, in the NMR spectroscopy, pulsed rf experiments are being used mostly. One of such techniques is the spin echo, which is described in this section. The spin echo technique uses in its simplest case two rf pulses. The nuclear magnetisation M

r, experiencing the external magnetic field acting

along axis and the rf field 0Br

z 1Br

acting along x axis, precesses with the Larmor frequency 00 Bγω = . The resonance can be achieved when the angular frequency ω of the rf field is close to the ω0. Therefore, in the rotating reference frame, the magnetisation precesses around the total magnetic field experienced by the nucleus , which is called the effective field, and the turning angle is given by: effB

r

tB1γϕ = . If the duration of one of the two rf field pulses, t1 or t2, corresponds to

2πϕ = or π, the pulses are called the “

2π pulse” or the “π pulse”, respectively, as

presented schematically in Fig. 3.3. First pulse, the “2π pulse” turns the nuclear

magnetisation Mz, which was aligned along axis by the field , from the z axis to the xy plane, see Figs 3.4a and 3.4.b. The magnetisation vector precesses around the xy plane and induces a signal in the receiving coil. This signal is called the free induction decay (FID), Fig. 3.3.

z 0Br

zB ˆ0

Fig. 3.3 A scheme of the spin-echo pulse sequence used in NMR experiments. The free induction decay (FID) signal appears after the π/2 pulse and the spin-echo signal is formed after time 2τ from the first pulse. Due to the inhomogeneity of every material, there is the distribution of the effective magnetic field at nuclei, which gives rise to different angular frequencies and a “fan” of nuclear moments is created (Fig. 3.4c), which is the main reason of the fast decay of the FID signal. The “π” pulse applied a time τ

after the “2π ” pulse produces a spin-echo signal appearing after a time 2τ (Fig

3.4e). The spin-echo signal can be observed if the transverse (spin-spin)


26

ig. 3.4 Scheme of the spin-echo formation in a rotating reference frame. The

The transverse component of the nuclear magnetisation, and,

relaxation time, T2 is sufficiently long compared with the pulse separation time, τ.

Fpulse sequence in above picture is: (π/2)x -τ -(π)y, where indices x and y denote the axes of the pulses. See the text for details. ⊥Mconsequently, the signal induced in the coil, decrease exponentially according to the equation:

2TMMd ⊥−=

dt ⊥ (3.20)

∫∫ −=⊥

∞ ⊥

⊥τ

02

1 dtTM

dMM

M

(3.21)

and therefore: 2TeMM

τ−

= . ∞⊥ (3.22) This equation

[Suhl 1958, Nakamura 1958].

describes (beside the recovery of the nuclear magnetization to the direction of static magnetic field) the interaction between the nuclear spins and their irreversible “dephasing” due to transverse relaxation which is mainly due to spin-spin relaxation processes. The possible mechanisms of the transverse relaxation are dipolar spin-spin interaction, indirect coupling between nuclear spins by means of the hyperfine interaction with conduction electrons (Ruderman-Kittel interaction) [Ruderman 1954] and Suhl-Nakamura coupling of nuclear magnetic moments with emission/absorption of electronic spin waves


27

the nuclear magnetization, Mz to the equilibrium The longitudinal relaxation time (or spin-lattice relaxation time) denoted as T1, describes the recovery ofstate. Spin-lattice relaxation processes in magnetic materials are usually strongly related to the electron spin system. In conventional ferromagnetic metals the spin-lattice relaxation is dominated by a Korringa process in which the relaxing nuclear spin flips an electronic spin down [Weisman 1973]. Another possibility for the spin-lattice relaxation is the fluctuation of the hyperfine fields caused by migrating electron holes. One can write the following equation describing the rate of reaching the equilibrium state, M0:

1

0

TMMM

dtd z

z

−−= (3.23)

∫∫ =−

tM

z

z dtTMM

dMz

010 0

1 (3.24)

10

0lnTt

MMM

z

=−

and therefore:

(3.25)

⎟⎟⎠

⎞⎜⎜⎝

−= 1)( 0z MtM⎛ −

1Te . (3.26)

A measuremeexploiting the spin-echo pulse sequence π/2-τ-π, with varying pulse spacing, τ

t

nt of the spin-echo signal with the two-pulse method

allows us to determine the T2 relaxation time by using the formula: 2

0)( TeAAτ

τ−

= , (3.27) where A(τ) and A are the spin-echo amplitudes at the time τ and τ=0, respectively.

of the two-pulse spin-echo at a fixed, small, pulse separation, τ by varyin

0

The spin-lattice relaxation time, T1 can be determined by carrying out the measurement

g the repetition time of the sequence. The T1 can also be measured by the saturation-recovery method. A “comb” of pulses (three or more) saturates the longitudinal nuclear magnetisation, Mz to a value close to zero. The subsequent recovery of Mz is measured by the spin-echo produced by a two-pulse sequence as a function of its separation, t from the saturation comb. The echo intensity changes according to the formula:

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

−11)( 0

Tt

eAtA . (3.28)

Relaxation randomly varying magnetic field or a fluctuating electric field gradient at the nucleu

processes of the nuclear magnetic moments are due to a

s. These fluctuating magnetic fields or electric field gradient are usually caused by molecular and electronic motions and, in the case of magnetic


28

d,

materials, by excitations and fluctuations of electronic moments. Measurements of the relaxation times can provide valuable information on the molecular dynamics and the dynamics of the electronic and magnetic systems [Abragam 1961, Turov 1969]. In the magnetically ordered materials the spin-lattice relaxation time, T1 is usually a few orders of magnitude bigger than the spin-spin relaxation time, T2.

General expressions relating the relaxation rates (1/T1 and 1/T2) to the fluctuating local fiel H

rare following: δ

( )dtttfT ∫

+∞

∞−

= )cos()(12 ωγ (3.29) 1

1

( )dttfTT ∫

+∞

∞−

+= )(211

02

12

γ

where f0 and f1 are the correlation functions for the longitudinal and transverse field fluctuations, respectively.[Moriya 1956, Turov 1969].

(3.30)


29

3.4. NMR in magnetically ordered materials

3.4.1 The effective field at nucleus

The total effective field, effBr

experienced by the ion in a magnetically rdere : r

=

t p

o d solid can be expressed as0BBB locHFeff

rrr++ (3.33)

where r

is he ap lB

0B ied external magnetic field, locBr

is the local magnetic field due to other magnetic moments in the solid, HFB

r is the hyperfine field resulting

mainly from the spin and orbital moments of the electrons within the ion radius. The hyperfine field HFB

r usually dominates over other components in

magnetically ordered solids. The classical local field, locB

r is given by:

BBBBrrrr

++= demLorD r

loc (3.34)

where is the demdemB agnetising field (related to the particle shape for single domain particle or to macroscopic shape of the sample when magnetized), LorB

r

is the Lorenz field created by the magnetic pseudo charges at the surface of the Lorenz spherical cavity and DB

r is the field arising from the moments within the

Lorentz sphere except the central one. demBr

+ LorBr

=0 for a spherical sample and

DBr

=0 for atomic positions with cubic symmesite symmetry) [Bruno 1993, Panissod 2002].

The HFBr

may be written as a following sum: BBBBBrrrrr

++=

try of their neighbourhood (cubic

+ (3.35) 3d

TorbdSFermi − In the case of elem

HF

ents the biggest is the Fermi contact term, FermiBr

, ct interaction of the nuclear magnetic moment with which results from the conta

the m likagnetic moments of s-like electrons, polarized mostly by the d- e electronic magnetic moments [Watson 1961]. Two next contributions come from the spin dipolar interaction between the nuclear magnetic moment and the electronic spin ( dSB −

r) and from the interaction of unquenched orbital moment of

the electrons ( Br

) [Watson 1961]. The last term is the transferred hyperfine field, i.e. field caused by electron spin transfer from magnetic neighbours ( TB

orb r).

The Fermi contact field, FermiBr

is a result of the exchange polarization of s electrons by the unpaired electrons from not completely filled orbitals (i.e. 3d for transition metals or 4f for rare earths elements). This polarization of the s electrons produces a net spin density at nucleus, which produces the Fermi contact term of the effective magnetic field [Fermi 1933, Sternheimer 1952].


30

The Fermi contact field can be written as the difference of the electron densities with spins up and down:

( ) ( )( )0032

0 ↓↑−= ρρμμ BFermiB (3.36)

where: ( ) ( )0,0

↓↑ρ are the densities of electrons w

00

↓−

ith their spins up and down at the ρnucleus,

( ) ( )↑

ρρ is the electronic spin polarization at the nucleus. In the case of the 3d transition metal elements the Fermi contact field is

negative (i.e. t, μs) and, since antiparallel to electronic spin magnetic momenFermi is the dominant contribution to effBBr

r

, the effective field is also negative. For the rare earth elements, the Fermi contact field is caused mainly by the electrons

the magnetic 4f shells. For manganese one can estimate the value of the core polarisation contribution ( FermiBfrom r

) to the hyperfine field, knowing that FermiBr

is proportional to the spin moment of the parent ion with the ratio –10 T/μB [Asano 1987]. This theoretical prediction is confirmed by the electron resonance studies of Mn3+ ion in TiO2 [Geritsen 1963] and by the nuclear magnetic resonance studies [Kubo 2001]. The estimated values of the FermiB

spinr

amount to -41.48 T and -41.24 T respectively.

The orbital hyperfine field, orbBr

can be expressed as:

Lrμμ

302 orbB = (3.37)

where: μ0 – vacuum magnetic permeability

rr 1

3r - average reciprocal cube of the1 radius of 3d orbital

Lμr –

orbital magnetic moment in Bohr magnetons. The spin dipolar field ( dSB −

r) at nucleus is due to the electronic spins at

ividual orbitals and is given y the formula: ind b

( )∑

⎭⎬

⎩⎨

i r50

⎫⎧ − ii rrsrs 002 3

=− BsdS gBrr

orrr

μμ (3.38)

where gs=2.0023, is the spin of ith eisr lectron and 0rr is a unit vector along the

leading vector, r. This field can be evaluated by multiplying the above equation by the electron density, ee ΨΨ= *ρ and integrating over the electron coordinates of the orbital [Abragam 1970]. The results of calculations of the spin dipolar hyperfine field, dSB −

r at nucleus produced by a single electron occupying one of

the 3d eg orbitals are presented in the discussion of the results section.


31

The anisotropic terms of the total magnetic field experienced by the nuclei are: dSB −

r, orbBr

, part of the local field ( locBr

), namely the dipolar field, DBr

and om contributions to the transferred field, s TBe

r.

3.4.2 Enhancement of the hig reh f quency field

or ferrimagnetic aterials are not excited directly by the rf field

In NMR experiments, nuclear spins in ferromagnetic

1Br

m , but they experience oscillations of the hyperfine fields resulting from the electronic magnetic moment response to the 1B

r. Even small oscillations with the magnitude ϕ can

produce an oscillating transverse component of the effective field at nucleus, ⊥effB (Fig. 3.5). As a result, the rf field with small amplitude can produce the

field ⊥effB with much larger amplitude given by the equation:

1)1( B+=⊥Beff η (3.39) where (for spherical single domain particle):

Anizext BB +≈η (effB

BAniz is the mpresent) and η

3.40)

agnetocrystalline anisotropy field, Bext is the external field (if is called the NMR enhancement factor. It describes the induced

reinforcement of Beff by adding an oscillating transverse component that is directly responsible for the nuclear transitions. The enhancement factor is proportional to the magnetic AC susceptibility of the electronic system at the NMR frequency. Relative values of enhancement factors corresponding to magnetically different regions can be derived from measurements carried out at the optimal rf field amplitude, opt

rfB , corresponding to the first maximum of the spin echo, using the formula:

optrfBt

πγη

221 =− (3.41)

where t is the pulse length [Savosta 2004]depend on the

m

. The enhancement factor may also macroscopic demagnetisation field and, consequently, on the

shape of the particles in the sample. In ferromagnetic or ferrimagnetic materials the enhancement factor is usually bigger for nuclei in domain walls than in domain interiors. For nuclei in do ain interiors typical value of η is the range 10-103, while for nuclei in domain walls it can amount to 102-105 and its value varies throughout the domain wall. In antiferromagnetic materials the enhancement factor is close to unity. For the elements (ions) which exhibit anisotropy of the hyperfine field, the signals from domains and domain walls (domain wall edges and domain wall centres) can be resolved, as they appear at different frequencies. They can also


32

ig. cement effect in magnetically ordered aterials, Mel stands for electronic magnetisation, the rest of symbols is

xplained in the text

tically ordered materials is η2 times smaller than in dia- or param gnetic materials and the echo amplitude is η times larger.

be distinguished due to their different values of η, or by applying external magnetic field (domain walls disappear at high enough field) and also due to different relaxation times in domains and in domain walls. Signals from domain walls exhibit a faster nuclear spin relaxation than signals from domains [Davis 1976, Leung 1977, Weisman 1973].

F 3.5 A scheme of the NMR enhanme

Owing to the enhancement effect, the pulse power required in the NMR experiments on magne

a


33

3.5. NMR spectrometers and magnetometers

In this section a description of the equipment used for experiments is o magnetometers i.e. the

ibrating Sample Magnetometer (VSM) and the SQUID (superconducting

NMR spectrometer is described in more

ceiver section, with the signal amplifiers, the protection diodes and

• ith the averaging oscilloscope and the computer.

sinusoidal output is divided into a reference signal and a “to sample” excitation sig l onal chann

itch, [Riedi

a commercial BRUKER AVANCE high-resolution console. The original apparatus was adapted in order to cover broad freque

ery low intensity. e m

presented, including two NMR spectrometers and twVquantum interference device) magnetometer.

Since a detailed description of one of the spectrometers can be found in [Riedi 1994, Lord 1995], in view of similarities of the design of both spectrometers used, only the second (Bruker)

details in this thesis. The spectrometer used for the most experiments carried out in this thesis is a home made spectrometer designed to work over the frequency range 10 – 1000 MHz, with particular application to NMR measurements of magnetically ordered materials. However, both spectrometers consist of:

• the pulse generator, with the splitters and the power amplifier in the transmitter section,

• the remixers, the digital section, wA stable frequency is provided by a synthesized oscillator whose

na . The spin-echo signal, phase sensitive detected in two orthogels, is averaged in the digital oscilloscope and stored in the computer. The spin-echo signal can be separated from the FID and spurious signals

(including receiver recovery effects) following the “π/2-τ- π - τ” sequence by phase switching and, additionally, by an electronic transmit/receive sw

1994, Lord 1995]. The recovery time of the spectrometer is of 4-6 µs, depending on the frequency range.

The spectrometer used in the NMR laboratory in Prague for NMR/NQR

spectroscopy is a system based on

ncy range needed e.g. for experiments carried out on magnetic materials. The present setup covers the frequency range of 6-600 MHz. The spectrometer performs coherent summation (averaging) of NMR signals in the time domain for required number of scans, which guarantees a high sensitivity and possibility to detect signals which have vTh inimal time delay between a radiofrequency pulse end and the start of data acquisition (detection) is about 5 μs, which allows us to measure samples with correspondingly short spin-spin relaxation time T2. A wobbler unit allowing carrying out tuned measurements is also integrated in the spectrometer system.


34

Main parts of the spectrometer are shown in the block diagram in Fig. 3.6. The transmitter line consists of two synthesizers - Signal Generation Units (SGU units),. The phase, amplitude and frequency are generated by the Direct Digital Synthesis (DDS). SGU provide phase resolution better than 0.05° and the frequency resolution better than 0.05 Hz. The low level high frequency pulses from the SGU unit are amplified in the power amplifier. Two linear power amplifiers (transmitters) are implemented

transmit/receive (T/R) switch

in the spectrometer: BLAX 500 (500 W) works in frequency range of 6-365 MHz, and BLAH 300 (300 W) in frequency range of 200-600 MHz. Blanking pulses are used to block the amplifier in time intervals when no rf pulses are transmitted. The rf pulses of appropriate power are transmitted from the power amplifier via (integrated in multilink HPPR/2 unit) to the probe coil. The switch protects the receiver input during excitation pulses and disconnects the transmitter line during data acquisition in order to eliminate possible noises and leakages from the transmitter line. In the usual experimental setup the same coil acts as the excitation and the pick-up coil. The SGU generates also the Local Oscillator (LO) frequency for mixing procedure in the receiver line. The NMR signal induced in the coil (can be of

w μfe V only) is processed in the receiver line. The signal proceeds to the multilink HPPR/2 unit and to a low noise HP-preamplifier. Next, the signal is mixed with Local Oscillator (LO) frequency (LO is by the value of the intermediate frequency IF of 720 MHz higher from the excitation frequency). The output signal converted to the Intermediate Frequency (IF) 720 MHz, is filtered by the IF filter and amplified with adjustable gain. The amplified signal (of order of Volts) is then registered with quadrature detection. The digital quadrature detection (DQD) using Slow A/D Converter SADC and oversampling option is limited to 25 kHz band. Therefore, the analogue quadrature detection mode is commonly used for broad spectral lines, by means of the fast digitizer (FADC, 12 bit digitizer, 10 MHz sampling rate). Before digitizing the signals from two quadrature detection channels I, Q, the analogue bandwidth is reduced with a low pass filter system ('aliasing' filter, amendable bandwidth 4 MHz - 0.125 MHz). The digitized data (after coherent summation) are transferred to the NMR PC workstation and stored there. The acquisition computer of the spectrometer includes a complete acquisition computer system (Communication Control Unit - CCU) and the spectrometer specific function boards, TCU (Timing Control Unit), FCU (Frequency Control Unit) and Receiver Control Unit (RCU). The Timing Control Unit (TCU) is a pulse programmer which provides exact timing and flexibility in the data acquisition with 12.5 ns timing resolution, controlled by a fast RISC processor. The TCU generates the most of the realtime clock pulses for the spectrometer. Basing on this clock, the synchronization of the transmitter system (TCU, FCU) and the receiver system


35

(RCU) is maintained. The Communication Control Unit (CCU) with a high speed RISC processor, 16 MByte memory, dedicated Fast Ethernet and RS-232 connection ports creates the communication link to the NMR PC workstation. Pulse sequence programs, as well as the acquisition and processing parameters are set by the PC operator using either the original XWINNMR Bruker software display, or by means of the special software, developed for frequency swept spectra only.

Fig.3.6: The block diagram of Bruker AVANCE spectrometer used for NMR measurements. For details and explanation of abbreviations – see the text.

ample magnetometer (VSM) and the Superconducting Quantum Interference Device

Magnetization measurements were carried out with the vibrating s

(SQUID) magnetometer. The measurements of bilayered perovskites with doping range 15.0 ≤≤ x

presented in this thesis were carried out with the Lakeshore VSM magnetometer. In this law: magnetometer magnetisation is measured owing to the Faraday

dtd B

SEM

Φ−=ξ (3.42)

SEMξ where is the electromotive force (EMF) and BΦ is the magnetic flux. Variable magnetic flux in the VSM is created due to the harmonic motion of the magnetic sample inside the pick-up coils in the m netometer (Fig. 3.7). Changes of the magnetic flux induce the electromotive force, SEM

agξ in the pick-up

coils, which is proportional to the sample magnetisation. This signal is measured by a selective nanovoltometer assembly, which is a part of the magnetometer


36

principle of the vibrating sample magnetometer (VSM) operation.

ID magnetometer was used for magnetisation easurements of nanoparticles. The superconducting quantum

interfe form two pa

rconducting detect

set-up, which also contains a controller of sample vibrations. Sample probe is placed inside the cryostat in order to carry out measurements at variable temperatures. More detailed discussion about the VSM magnetometer used can be found in [Tokarz 2001].

Fig. 3.7 A Scheme illustrating the physical

The Quantum Design SQU

m 31

rence device (SQUID) consists of two superconductors separated by thin insulating layers to rallel Josephson junctions. The device may be configured as a magnetometer to detect very small magnetic fields.

Similarly to the VSM magnetometer, the measurement in the SQUID magnetometer is performed by moving a sample through the supe

MnOSrLa xx−

ion coils, which, as shown in Fig. 3.8, are located at the centre of the magnet. The sample moves along the symmetry axis of the detection coil and magnet. As the sample moves through the coils the magnetic dipole moment of the sample induces an EMF and the corresponding electric current in the detection coils. This is due to the fact that detection coils, connecting wires and the SQUID input coil form a superconducting loop and any change of the magnetic flux in detection coils produces a change in the persistent current in the detection circuit, which is proportional to the change of the magnetic flux. The thin film SQUID device located below the magnet and inside the superconducting shield essentially functions as a very sensitive current-to-


37

e magnet, detection coils and the sample

voltage converter, so that variations in the current in the detection coil circuit produce corresponding variations in the SQUID output voltage, which is proportional to the magnetic moment of the sample. In a fully calibrated system, measurements of the voltage variations from the SQUID detector as a sample is moved through detection coils provide highly accurate measurement of the magnetic moment of sample. Under ideal conditions, the magnetic moment of the sample does not change during movement through the detection coil [McElfresh].

Fig. 3.8 Geometrical configuration of thchamber in the SQUID magnetometer.

Chapter 4: “Cubic” perovskites: La0.875Sr0.125MnO3, La0.85Sr0.15MnO3 and La0.9Ca0.1MnO3

38

4. Cubic perovskites La0.875Sr0.125MnO3, La0.85Sr0.15MnO3 and La0.9Sr0.1MnO3

4.1 La0.875Sr0.125MnO3 and La0.85Sr0.15MnO3 55Mn and 139La NMR results and discussion

4.1.1. La0.875Sr0.125MnO3 - 55Mn NMR

In this section results and discussion of the 55Mn NMR spin echo

measurements of the La0.875Sr0.125MnO3 compound are presented. In Fig. 4.1 the 55Mn frequency swept spectra measured at various temperatures ranging from 4.2 K to 225 K are shown. In all the presented NMR results at no applied magnetic field the measured signal comes mostly from domain walls owing to their larger enhancement factor. The dependences of the spin echo signal on the pulse length were measured at the maximum pulse amplitude for most of the resonance lines observed in the frequency swept spectra, starting from short pulses (mostly 0.1 μs) to 1-2 μs in order to find the first maximum of the echo signal, i.e. to determine the optimal excitation conditions. Unless stated differently, the presented frequency swept spectra are measured for excitation conditions which are optimal for the double exchange line. Usually, the enhancement factor of the Mn4+ line was found to be slightly larger and for the Mn3+ resonances - slightly smaller than for the DE line. A correction of the frequency response of the spectrometer was provided by inserting a 6 dB attenuator at the top of the probehead, which was found to reduce substantially the standing waves arising from a lack of impedance matching of the untuned coil. As the exact frequency response of the spectrometer and the frequency dependence of the enhancement factor were not known, the spectra are presented with no frequency correction.

At 4.2 K we observe several lines, namely Mn4+ line (at 327 MHz), the DE line (around 400 MHz) and two Mn3+ lines (ranging from 400-550 MHz). The appearance of several lines in the 55Mn spectra suggests the occurrence of phase separation in the compound studied, in agreement with results of [Allodi 1997, Papavassiliou 1999, 2000, Renard 1999, Kapusta 2000a, 2000b, Novak (2004)] as was described in the Introduction.

The origin of the Mn4+ and Mn3+ lines are the Mn cations, which are in ferromagnetic insulating (FMI) regions, while the DE line is associated with Mn cations of intermediate valence, which is related to a fast, DE driven, 3d eg electron hopping between the Mn3+ and Mn4+ ions leading to ferromagnetism and metallicity (FMM) of these regions [Matsumoto 1970]. The ground state of the La0.875Sr0.125MnO3 compound in the literature [Dabrowski 1999] is reported to be FMI, however, at low temperatures we observe signals not only from FMI, but also from FMM regions. This indicates that there exists a phase separation into FMM and FMI regions in the compound studied. As was mentioned in the Introduction, the phase separation problem in lightly doped perovskite manganites has recently been studied by many researchers with means of many different techniques.


39

250 300 350 400 450 500 550

TC=180K

225K

212K

4.2K

182K

170K

160K

136K

121K

97K

Frequency [MHz]

77K

TCO=160K

Nor

mal

ized

spi

n ec

ho in

tens

ity [a

rb. u

nits

]La0.875Sr0.125MnO3

Fig. 4.1. The 55Mn frequency swept spectra of La0.875Sr0.125MnO3 at various temperatures. TCO and TC are the charge ordering temperature and Curie temperature respectively, reported in the literature [Dagotto 2001].

In [Mostovshchikova 2004] the amount of the FMM phase was obtained by

analysing the temperature dependence of the light absorption and dc conductivity in lightly Sr and Ca doped La1-x(Sr,Ca)xMnO3 compounds. In the case of La0.9Sr0.1MnO3 compound they evaluated the volume of the FMM phase at 100 K at 0.1% only. The size of the metallic-like regions in lightly doped manganites reported in the literature varies from 10 Å to 100 Å [Hennion 1998].

As can be seen in Fig. 4.1, the DE line exists well above the bulk magnetic ordering temperature, TC of 180 K [Dagotto 2001, Hennion 2006] and the signal was measured even at 225 K. This fact is attributed to long lived FMM regions (FM polarons [Salafranca 2006]). The fast diminishing of the Mn3+ signal with increasing temperature is due to a fast decrease of the spin-spin relaxation time, T2 and at 77 K very weak Mn3+ signals are observed at 420 MHz and at 500 MHz. A fast decrease of the Mn3+ signal from FMI regions with increasing temperature was also observed in lightly Ca doped cubic manganese perovskite, La0.9Ca0.1MnO3 [Algarabel 2003].

The FMI Mn4+ line is observed up to 160 K, which is reported to be the temperature of transition to the charge ordered state [Dagotto 2001]. One can also


40

notice that the resonant frequencies of the Mn4+, DE and Mn3+ lines decrease as the temperature is increased, which is due to a decrease of the manganese magnetic moment, <Sz>, with temperature.

0 50 100 150 200 25020

25

30

35

40 DE line Mn4+ line

Effe

ctiv

e fie

ld [T

]

Temperature [K]

Fig. 4.2. Temperature dependence of the effective field, Beff of the FMI Mn4+ and DE lines of the La0.875Sr0.125MnO3 compound. Solid lines are guides for eyes only.

0 25 50 75 100 125 150 175 200 2250,75

0,80

0,85

0,90

0,95

1,00

0,75

0,80

0,85

0,90

0,95

1,00

Temperature [K]

DE Beff

Mn4+ Beff

Nor

mal

ized

effe

ctiv

e fie

ld -

Bef

f

Fig.4.3. Normalised effective field, Beff versus temperature (for the FMI Mn4+ line and Mn ions in the DE regions of the La0.875Sr0.125MnO3 compound). Lines are guides for eyes only.


41

The temperature dependences of the effective field at nucleus, Beff of the Mn4+ and Mn cations in the DE regions are shown in Fig. 4.2. Points on the plot are central frequencies of Gaussian curves fits to the experimental data. At 4.2 K the Beff at the Mn4+ nuclei in the FMI regions amounts to 31.26 T and at the Mn nuclei in FMM regions (averaged Mn4+ and Mn3+

valence state) it is higher and amounts to 37.9 T (400 MHz). The value of the Beff for the DE line decreases merely by 17% between 4.2 K and 212 K. Since the Beff is approximately proportional to the average spin magnetic moment, this indicates almost fully saturated Mn magnetisation in the FMM regions in this temperature range. The existence of the FMM regions above the TC was observed by NMR also in other manganese perovskites [Kapusta 1999]. The respective change of the Mn4+ Beff is larger and amounts to 18%, between 4.2 K and 154 K, compared to 10.7% for the DE line at this temperature range. This indicates a weaker magnetic coupling in the FMI than in the FMM regions. A similar effect was found in the lightly Ca doped manganite system [Kapusta 2000].

300 350 400 450 500 550Frequency (MHz)

τ = 10μs

τ = 75μs

τ = 150μs

τ = 200μs

τ = 300μs

Nor

mal

ised

spi

n ec

ho in

tens

ity [a

rb.u

nits

]

Fig. 4.4 55Mn NMR spin echo spectra of the La0.875Sr0.125MnO3 compound at 4.2 K and different values of pulse spacing, τ. Lines are guides for eyes only.

Signals from the Mn3+ ions at 4.2 K in the FMI regions range from 425 MHz

(40.28 T) to 550 MHz (52.12 T). Such a broad distribution is due to the large anisotropy of the Mn3+ hyperfine field. Mn3+ is a Jahn-Teller ion and thus a


42

different occupation of the eg orbitals can be expected. Additionally, in the case of Mn3+ ion the orbital moment is not quenched and can be of 0.05-0.1 μB (4-6% of the spin magnetic moment) in the lightly Sr doped cubic manganese perovskites [Koide 2001].

In order to identify the origin of the Mn3+ lines at 440 MHz and at 530 MHz at 4.2 K, measurements with various pulse spacing, τ were carried out. Their results for τ ranging from 10 μs to 300 μs are presented in Fig. 4.4. As τ increases, the intensity of the DE line decreases and the Mn4+ and Mn3+ lines at 325 MHz and 530 MHz, respectively, remain dominant. The Mn3+ line at 530 MHz persists for higher pulse spacing, indicating that nuclei contributing to this signal have a longer spin-spin relaxation time T2 compared with nuclei contributing to the Mn3+ line at 440 MHz. Nuclear moments corresponding to shorter T2 relax out before the appearance of the second pulse and they do not contribute to the spin echo signal. The line with shorter T2 (at lower frequency) is attributed to the domain wall centre and the line which has a longer T2 (at higher frequency) is attributed to the domain wall edge, where the ionic magnetic moments are within or close to the easy magnetisation direction (EMD) [Weisman 1973, Davis 1976, Leung 1977]. The signal from domain wall centre corresponds to the ions with magnetic moments along (or close to) the hard magnetisation direction(s) (HMD). However, according to this line attribution, the line at 440 MHz attributed to domain wall centre should

275 300 325 350 375 400 425 450 475 500 525 550 575

275 300 325 350 375 400 425 450 475 500 525 550 575

Nor

mal

ized

spi

n ec

ho in

tens

ity [a

rb. u

nits

]

Frequency [MHz]

0T 3T 0T magn.

at 3T

Figure 4.5 55Mn NMR spin echo spectra of the La0.875Sr0.125MnO3 compound at 3.2 K at no applied field (black line), at 3 T (blue line) and at 0 T after switching the field off (red line).


43

have different enhancement factor, which is not the case. The enhancement factor of the line at 440 MHz is almost the same as for the line at 530 MHz, the optimal pulse length (first maximum on the dependence of the spin echo signal on the pulse length) was 1.2 μs and 1 μs for the line at 440 MHz and 530 MHz respectively.

Additionally, measurement at 3 T and at 3.2 K was carried out to verify the attribution of Mn3+ lines described above (see Fig. 4.5). As it can be seen at 3 T the FMI Mn4+ and FMM DE lines shift towards lower frequencies, as the Mn3+ line at 530 MHz also does. However, the Mn3+ line at 440 MHz does not shift, but only decreases in intensity compared with other lines.

The shift towards lower frequencies confirms that the observed signals, from the Mn4+ ions and from regions where the DE interaction is effective, come from ferromagnetically ordered regions. The hyperfine field, BHF which is the dominant component of the effective field, is negative (antiparallel to the electronic spin moment, μs and to the applied field, B0). Therefore with increasing applied field the effective field and the corresponding resonant frequency decrease (see Fig. 4.6). Resonant frequencies decrease from 327 to 299 MHz (28 MHz) and from 393 to 375 MHz (18 MHz) for the Mn4+ line and the DE line, respectively. A smaller change in the case of the DE line is probably due to a higher demagnetising field and/or magnetocrystalline anisotropy in the DE regions.

Fig. 4.6 Scheme illustrating decrease of the effective field, Beff as the external magnetic field, B0 is applied (bottom).

In the case of Mn3+ lines, the line at 530 MHz, which has longer spin-spin relaxation time, shifts towards lower frequencies at 3 T while the line at 440 MHz remains at the same frequency and only decreases in intensity. Assuming that observed signals come from domain walls, which is usual in NMR experiments due to higher enhancement factor of nuclei in domain walls than within domains, the shift of the line at 530 MHz means that magnetic moments of ions contributing to this line are parallel to the applied field. This means that they may be located in domain wall edges.

In the domain wall centres magnetic moments are perpendicular to the applied field. Since the effective field is much larger than the applied field (42 T comparing to 3 T) no shift of the line at 440 MHz is observed at 3 T. The strong decrease of the intensity of this line can be due to domain walls removal at the applied field.


44

Fig. 4.7 Schematic view of the structure with two manganese positions (Mn1 and Mn2), blue arrows indicate magnetic moments. Mn1 and Mn2 sites have different angles between magnetic moment direction and directions of Mn-O bonds.

The situation is in fact more complicated, because in the compound studied

there are two non equivalent Mn sites. Taking into account only crystallographic structure there is only one Mn lattice site, but introducing magnetic order related to distortions and tilting of manganese-oxygen octahedra results in two non equivalent Mn sites. For these two sites angles between direction of the magnetic moment and main directions in the octahedron are different (see Fig. 4.7). Presence of two non equivalent Mn sites can be the reason of the asymmetric line shape of the Mn4+ FMI line at 330 MHz (see Fig. 4.4). However, the Mn3+ lines observed (at 440 and 530 MHz) cannot be due to two non equivalent Mn sites, because they both should shift towards lower frequencies at the applied field, which is not observed for the line at 440 MHz.

An alternative explanation for the lack of the shift in the applied field could be its assignment to antiferromagnetically coupled moments. However, as was mentioned the enhancement factor, η is similar for both lines while it should be much smaller in the case of the signal from antiferromagnetically coupled moments than for ferromagnetically coupled [Turov 1970].

In Fig. 4.5 also the measurement at no applied field, but after magnetising sample at 3 T is presented. The frequency swept spectrum looks almost the same as in the measurement with the non magnetized sample. This indicates that no metamagnetic transition was induced by the field of 3 T in contrast to e.g. another manganite Pr0.67Ca0.33MnO3 in which a field of 7 T resulted in a transition from antiferromagnetic state to the ferromagnetic metallic state and the dominant DE line was observed in the remanent state [Oates 2005].


45

We have also carried out measurements of the spin-spin relaxation time, T2 for nuclei contributing to each line, for the FMI Mn4+ at 337 MHz, for the DE line at 382 MHz and for the two Mn3+ lines at 442 and 532 MHz (Fig. 4.8a) at various temperatures. In the fitting procedure a single exponential decay of the spin echo was assumed and it provided a good agreement of the fit with the experimental data. The following formula was used:

⎟⎟⎠

⎞⎜⎜⎝

⎛−+= =

20

2exp)(T

AAA n

ττ τ (4.1)

where )(τA and are the spin-echo amplitudes at the time τ, and τ=0 (see also chapter: Introduction to NMR). The magnitude of the An parameter corresponds to the noise level, so a possible contribution from large T2 processes can be discarded within the error margin.

0=τA

0 50 100 150 200

10

20

30

40

50

60

0 50 100 150 200

10

20

30

40

50

60

T 2 [μ

s]

Temperature [K]

DE line Mn4+ line 432MHz 532MHz

b)

0 20 40 60 80 100

Spi

n ec

ho in

tens

ity [a

rb. u

nits

]

Pulse spacing [μs]

Mn4+ line DE line 442 MHz 532 MHz

a)

Fig. 4.8 a) Spin echo decay curves (in linear scale) of the La0.875Sr0.125MnO3 compound for resonant frequencies of the Mn4+ line, the DE line and Mn3+ lines at 442 and 532 MHz at 4.2 K. Red lines are fits to the experimental data (points); b) temperature dependence of the T2.

At 4.2 K the shortest T2 is observed for the DE line indicating that manganese nuclei in FMM regions relax faster than the Mn3+ and Mn4+ nuclei in FMI regions, which have longer spin-spin relaxation time. At higher temperatures T2 values of Mn4+ and DE lines are obtained only. The signals from Mn3+ nuclei are not observed at higher temperatures due to fast decrease of the T2 with increasing temperature (at 77 K T2 of Mn3+ nuclei is the shortest measured, Fig.


46

4.8b). Above 100 K the T2 values of nuclei contributing to the DE line and Mn4+ line do not change much and remain in the range 5-10 μs (Fig. 4.8b).

4.1.2 La0.85Sr0.15MnO3 - 55Mn NMR

With increasing content of Sr (hole doping) the average ionic radius of the perovskite A-site cations increases (the ionic radius of Sr2+ is larger than La3+). This is called the chemical pressure effect [Xu 2003], which results in the increase of the Mn-O-Mn bond angle and a decrease of the distortions [Xiong 1999]. Therefore, one might expect a higher effectiveness of the DE interaction (effectiveness of the DE interaction increases with increase of the Mn-O-Mn bond angle, see chapter 2). The FMM phase content should also increase with increasing Sr doping, thus it should be higher in the x=0.15 Sr doped compound than in the compound with x=0.125. As expected, we observe this kind of behaviour in the NMR spectra.

250 300 350 400 450 500 550

250 300 350 400 450 500 550

Nor

mal

ised

spi

n ec

ho in

tens

ity [a

rb.u

.]

Frequency (MHz)

τ = 9μs

τ = 150μs

τ = 300μs

Fig. 4.9. 55Mn NMR spin echo spectra of the La0.85Sr0.15MnO3 compound at 4.2 K with three different values of the pulse spacing, τ.

The 55Mn NMR frequency swept spectra at different pulse spacing, τ at 4.2

and 77 K, of the La0.85Sr0.15MnO3 compound are presented in Figures 4.9 and 4.10. Similarly to La0.875Sr0.125MnO3, the spectra of La0.85Sr0.15MnO3 at 4.2 K reveal the co-existence of ferromagnetic insulating regions and ferromagnetic metallic regions. The line at 334 MHz is due to Mn4+ ions and signals ranging from 410 to


47

550 MHz are due to Mn3+ ions in the FMI regions while the line at 396 MHz is due to FMM regions.

250 300 350 400 450 500 550

250 300 350 400 450 500 550

Nor

mal

ised

spi

n ec

ho in

tens

ity [a

rb.u

.]

Frequency (MHz)

τ = 8μs

τ = 100μs

Fig. 4.10. 55Mn NMR spin echo spectra of the La0.85Sr0.15MnO3 compound at 77 K obtained with two different values of the pulse spacing, τ.

One can also notice that the DE line at 4.2 K is not symmetrical (Fig. 4.9).

Similar behaviour was also observed in the case of compound with x=0.16 and it was suggested that there are two DE lines present at low temperatures [Savosta 2003]. According to [Savosta 2003] these two lines originate from hole-rich and hole-poor Mn sites due to charge density wave, possibly accompanied by a partial orbital ordering. The difference between hyperfine fields of manganese ions on those two sites was found to be of 2.3 T (24.3 MHz) at 22 K. However, we propose that the asymmetric line shape is due to the anisotropy of the hyperfine field. A similar observation was made for La0.75Sr0.25MnO3 (see chapter 5) and bilayered manganites La1.2Sr1.8Mn2O7 and LaSr2Mn2O7 (see chapter 6).

At 77 K resonant frequencies of the FMI Mn4+ and the DE lines are lower than at 4.2 K due to a decrease of the magnetic moment with temperature. The DE line shifts towards lower frequencies by 17 MHz (1.6 T) to 382 MHz. The shift of the FMI Mn4+ is harder to estimate because the intensity of this line decreases strongly with increasing temperature. The Mn3+ line is not observed at 77 K due to a fast increase of the nuclear relaxation rate of Mn3+ in the charge localised FMI regions with increasing temperature.

In Figures 4.9 and 4.10 it is clearly visible that a dip in the centre of the DE line appears with increasing pulse spacing. The effect can be explained by the Suhl-Nakamura interaction [Suhl 1958, Nakamura 1958, Davis 1974] between nuclear spins of neighbouring Mn ions. The nuclear spin, which sees the electronic


48

spin of its own ion through the hyperfine coupling, excites a spin wave through this coupling, and another nuclear spin absorbs it through its hyperfine coupling [Suhl 1958]. The observed dependence of the spectra on the pulse spacing means that nuclear spins, which precess at or near the central frequency, reveal a faster spin-spin relaxation than spins contributing to the wings of the resonance line. As the centre of the DE resonance line originates mainly from the spins inside the DE regions, a minimum at the line centre denotes that the neighbouring manganese ions are magnetically equivalent, which makes the Suhl-Nakamura (SN) interaction effective. This is much less effective at the boundaries of DE regions where manganese neighbours differ in terms of magnetic moments and/or their directions, which prevents the exchange of virtual spin waves and results in a slower spin-spin relaxation. An indication of the Suhl-Nakamura interaction is the frequency dependence of the spin-spin relaxation time, T2 which should have a minimum at the centre of the DE line. Such minimum is observed in the case of La0.85Sr0.15MnO3, but not for La0.875Sr0.125MnO3 (see Fig. 4.11). Values of the spin-sin relaxation time presented in Fig. 4.11 were calculated using frequency swept spectra measured with different pulse spacing, τ (Figures. 4.4 and 4.9).

340 360 380 400 420 440

50

60

70

80

90

100

T 2 [μs]

Frequency [MHz]

x=0.15

x=0.125

DE

Fig. 4.11 Frequency dependence of the spin-spin relaxation time, T2 for x=0.125 and x=0.15 compounds. The DE mark denotes the resonant frequency. Lines are guides for eyes only.

For the La0.875Sr0.125MnO3 compound the DE line vanishes with increasing pulse spacing and no dip at the line centre appears (Fig. 4.4) in contrast to the La0.85Sr0.15MnO3 compound (see figures 4.9 and 4.10). The Suhl-Nakamura interaction was also observed in other ferromagnetic metallic manganites [Savosta 2001, Rybicki 2004]. On this basis we conclude that the Suhl-Nakamura interaction is effective only in the x=0.15 doped compound and that the regions, where DE interaction is dominant are much larger in size in the x=0.15 than in the


49

x=0.125 doped compound. For the sample with x=0.15 the dip at the centre of the DE line is also observed at 77 K. This is a clear evidence that DE regions are large enough to ensure the Suhl-Nakamura interaction between neighbouring nuclear spins to be effective at this temperature. The earlier NMR results by Savosta et al. [Savosta 2001], which revealed that the Suhl-Nakamura interaction is effective in manganites at higher temperatures were obtained for ferromagnetic metallic La0.7Sr0.3MnO3, while our results concern lower Sr doped ferromagnetic insulating compound, where the amount of DE regions is expected to be smaller. The ferromagnetic metallic clusters of the size of 10 nm were also found in the x=0.15 compound by the small angle neutron scattering (SANS) technique [Ibarra, unpublished]. Spin-spin relaxation times obtained from pulse spacing measurements carried out for La0.85Sr0.15MnO3 at 4.2 K and 77 K are presented in the table 4.1. Similarly to x=0.125 compound the shortest value of T2 is obtained for manganese nuclei in FMM regions and the longest - for Mn3+ nuclei in FMI insulating regions (signals above 400 MHz). All values of the T2 obtained for La0.85Sr0.15MnO3 are smaller than those obtained for La0.875Sr0.125MnO3 compound and the biggest difference is found for the T2 of nuclei contributing to the DE line. This difference amounts to 9 μs (24%) and can be attributed to the Suhl-Nakamura interaction, which is less effective in the La0.875Sr0.125MnO3 compound. One can also notice that for La0.85Sr0.15MnO3 the T2 of nuclei contributing to the DE line does not decrease so dramatically with increasing temperature as in La0.875Sr0.125MnO3. At 77 K it amounts to 15 μs versus 6 μs for the higher and the lower Sr doped compounds, respectively (see table 1 and Fig. 4.8b).

At 4.2 K At 77 K Freq.

[MHz] T2 [μs] Δ T2 [μs]

Freq. [MHz] T2 [μs] Δ T2

[μs] 336 52.8 1.2 351 16.9 1.9 390 36.7 0.5 381 15.1 0.4 447 45.2 0.7 411 16.5 1.9 512 58.1 2.1

Table 4.1 Spin-spin relaxation times, T2 and their uncertainties, for nuclear magnetic moments of manganese ions obtained using equation (4.1) for La0.85Sr0.15MnO3 at 4.2 K and 77 K.

4.1.3. La0.875Sr0.125MnO3 and La0.85Sr0.15MnO3 – 139La

In manganese perovskites, besides 55Mn NMR also signals from 139La nuclei

have been measured in many compounds doped with calcium [Allodi 1997, Papavassiliou 1997, 2001] or with sodium [Savosta 1999]. Lanthanum nucleus (nuclear spin I=7/2) is subjected to the transferred hyperfine field (from manganese neighbours) and dipolar interactions with electronic moments as well


50

as to the quadrupolar coupling with the electric field gradient. However, satellite lines which would indicate the presence of quadrupolar interactions have never been observed experimentally in manganese perovskites. The NMR signal of 139La originates indirectly from the overlap of the Mn |3d> with oxygen |2p> wave functions, in conjunction with σ bonding of the oxygen with |sp3> hybrid states of the La3+ cation [Papavassiliou 1997].

The frequency swept spectra of 139La NMR are presented in Fig. 4.12. For both compounds with different Sr doping two lines are observed, however with different relative intensities. Resonant frequencies are obtained by fitting Gaussian curves to the data and they amount to 17,64±0.09 MHz and 32,80±0.11 MHz for x=0.125 Sr doped compound. For x=0.15 Sr doped one resonant frequencies amount to 18.57±0.04 MHz and 32.92±0.14 MHz. The corresponding values of the effective field, Beff at the 139La nuclei amount to 2.94 T and 5.46 T for the x=0.125 compound and 3.09 T and 5.48 T for the x=0.15 one. The difference between effective fields of those two lines for both compounds is close to 2.5 T and it cannot be explained by the anisotropy of the dipolar field, BD which is of order of a few tenths of Tesla. The transferred hyperfine field on gallium ion at the site of manganese in Pr0.5Ca0.5Mn0.97Ga0.03O3 compound, amounting to 5.3 T was obtained previously by the NMR [Oates 2005]. This value is similar to the value obtained by us for 139La (the upper line).

The 139La signal at frequency 20 MHz and a tail at higher frequencies (up to 35 MHz) were observed also for La0.875Ca0.125MnO3 compound [Papavassiliou 2001]. The authors suggested that the higher frequency signal is due to the formation of Mn octant cells with enhanced Mn-O wave functions overlap resulting in a higher effective field on 139La nuclei. The signal at higher frequency is not observed for the FMM manganese perovskites doped both with Sr (see the spectrum for x=0.3 Sr doped compound, Fig 4.12a, and for x=0.25, chapter 5) and with Ca [Papavassiliou 1998], where a single line at the lower frequency is observed only. Therefore, one can assume that the line at lower frequency is due to 139La ions in FMM regions and the higher frequency line is due to 139La in charge localised (CL) FMI regions. This assumption can be supported by the fact that the relative intensity of the CL line is higher for x=0.125 doped compound, thus meaning that the amount of the FMI phase is higher for this compound than for the x=0.15 one.


51

10 15 20 25 30 35 40 45

10 15 20 25 30 35 40 45

x=0.3

Nor

mal

ised

spi

n ec

ho in

tens

ity [a

rb. u

nits

]

Frequency (MHz)

x=0.15

x=0.125

a)

10 20 30 40 50

10 20 30 40 50

1

0

1.5/3 μs

0.2/0.4 μs

x=0.125

Frequency [MHz]

1.5/3 μs

0.2/0.4 μs

x=0.15

Nor

mal

ized

ech

o in

tens

ity [a

rb. u

nits

]

b)

Fig. 4.12. a) 139La NMR frequency swept spectra at 4.2 K, for x=0.125, x=0.15 Sr doped compounds and for x=0.3 for a comparison; b) the spectra measured with shorter exciting pulses for both compounds (red lines). The solid curves are Gaussian fits to the experimental data.

As can be seen in Fig. 4.12b the line at lower frequency requires a smaller rf pulse amplitude or pulse length, whereas the line at higher frequency is not observed for a small pulse length (0.2 μs). This indicates that the enhancement factor, η of nuclei contributing to the signal at lower frequency is bigger.

A possibility that one of observed lines could be due to some remaining antiferromagnetic phase (the AF – FM phase boundary occurs for 10% of Sr doping) can be excluded, since the 139La signal from AF phase can only be observed at high applied fields. Such a signal was observed at 7 T in the antiferromagnetic La0.5Ca0.5MnO3 [Allodi 1998] and at similar fields in La0.95Sr0.05MnO3 [Kumagai 1999] and (La0.25Pr0.75)0.7Ca0.3Mn18O3 [Yakubowskii 2000]. Even if 139La signal from the AF phase could be observed at 0 Tesla, it should correspond to much lower frequencies than the signal from the FM phase. Since the hyperfine transferred field from AF ordered manganese neighbours of lanthanum would be smaller than from FM ordered manganese ions. So this hypothetical La signal from AF phase at lower frequencies should, according to previous studies [Allodi 1998, Kumagai 1999, Yakubowskii 2000], have a smaller enhancement factor and shorter relaxation times, which is not the case (see Fig. 4.12b and table 4.2).


52

Spin-spin relaxation times, T2 for 139La nuclei are longer than for nuclei of 55Mn (see table 4.2), both for the CL FMI regions and for the FMM regions. In the case of La0.875Sr0.125MnO3 and La0.85Sr0.15MnO3 the T2 of 139La nuclei in FMM regions are considerably longer (more than two times) than those in FMI regions.

Freq.

[MHz] T2 [μs] Δ T2 [μs]

17 406

Table 4.2 Spin-spin relaxation times, T2 and their uncertainties, for 139La obtained using equation (4.1) for La0.875Sr0.125MnO3 and La0.85Sr0.15MnO3 compounds at 4.2 K. As was previously discussed for the frequency swept spectra, there is no indication of the electric quadrupolar interaction, however, such an indication was reported earlier in measurements of the spin-lattice relaxation time with the recovery of the spin echo method [Allodi 1998, Savosta 2003]. Therefore, for 139La also the spin-lattice relaxation time, T1 has been measured. In principle, as the separation time between the saturating comb of pulses and the probing two-pulse sequence (π/2 and π pulses) increases, the spin echo recovers according to the equation:

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

−

∞=11)( Tt

t eMtM (4.2),

where is the nuclear magnetisation for time t=∞. If the nuclear magnetisation is not saturated to zero at time t=0 (Mt=0≠0), above equation changes to:

∞=tM

⎟⎟⎠

⎞⎜⎜⎝

⎛−+=

−

∞==11)( 0

Tt

tt eMMtM (4.3)

It is assumed that the signal is a sum of two contributions with different enhancement factors and saturation is reached for one contribution only. If the not saturated component has much longer T1, the Mt=0 means the long-relaxing component. As was found by Allodi et al., in manganese perovskites the recovery of the spin echo is not single exponential. They attributed it to the quadrupole interaction [Allodi 1998]. The spin lattice relaxation of a I=7/2 nucleus with quadrupole splitting of the nuclear Zeeman levels is governed by the master equations which predict a multiexponential behaviour [Gordon 1978]. Assuming that the spin-lattice relaxation mechanism is predominantly of magnetic origin and only the central -1/2→1/2 transition is saturated, the nuclear magnetisation in the spin-lattice relaxation time measurement recovers according to the equation:

11x=0.125 32.5 131 818.5 281 4x=0.15 33.5 134 40


53

⎟⎠⎞

⎜⎝⎛ −−−−= −−−−

∞=WtWtWtWt

t eeeeMtM 5630122

429175

9120

112

2141)( (4.4)

where the transition probability 1

12T

W = .

As can be seen in Fig. 4.13a, the recovery of the spin echo can be relatively well fitted with the single exponential function (equation 4.3). In contrast, fitting with the equation 4.4 does not give satisfactory results unless one adds the parameter Mt=0, corresponding to unsaturated nuclear magnetisation, similarly to the recovery described by the single exponential function (equation 4.3). However, in this case the obtained spin-lattice relaxation times are more than one order of magnitude bigger than those derived from single exponential fits (see table 4.3). For a comparison also the literature results for the La0.8Na0.2MnO3 compound at 77 K are presented in Fig. 13b, after [Savosta 2003]. As can be seen in Fig. 13b, discrepancies between single exponential fit and experimental data occur at larger t (the time between the saturating comb of pulses and probing pulse sequence of π/2 and π pulses) values, while our measurements were carried out up to t value of 5 ms (Fig. 12a). A "misfit" of the equation 4.4 (with Mt=0 parameter included) and 4.3 at small values of t reveals an additional contribution with short T1, which denotes the presence of two different spin-lattice relaxation processes.

Fig. 4.13 a) Recovery of the longitudinal component of the 139La nuclear magnetization, M for La0.875Sr0.125MnO3 measured at 17 MHz (FMM line) using single exponential function, equation 4.3 (black line), multiexponential function equation 4.4 without (blue line) and with (red lines) parameter Mt=0. Black and red lines coincide. b) Literature results [Savosta 2003], see text for details.

0 1000 2000 3000 4000 5000

0,25

0,30

0,35

0,40

single exponential multiexponential multiexponential with Mt=0

M (t

) [ar

b. u

nits

]

t [μs]

Pulses: 0.2 µs 0.4 µsPulse-Spacing: 12 µsFrequency: +17 MHz

a)


54

Freq.

[MHz] T1 [ms] Δ T1 [ms] T1me [ms]

Δ T1 me [ms]

17 2.78 0.15

38.7 1.9x=0.125 32.5 1.67 0.40

32.6 10.1

x=0.15 17 2.64 0.14

48.1 3.4 Table 4.3 Spin-lattice relaxation times, T1 and their uncertainties, for 139La obtained using equation (4.3) for La0.875Sr0.125MnO3 and La0.85Sr0.15MnO3 compounds at 4.2 K. T1me are spin lattice relaxation times with obtained by fitting multiexponential function.


55

4.2 La0.9Ca0.1MnO3 - 55Mn NMR results In this section NMR results obtained on four samples (three powder samples and a single crystal) of La0.9Ca0.1MnO3 are presented. As was mentioned, all powder samples were calcined in air atmosphere, later milled and then sintered either air (Zaragoza and SB#2 samples) [Algarabel 2003, Ghivelder 1998] or in oxygen atmosphere (FCB11 sample) [Ghivelder 1998]. However, the Zaragoza sample was sintered in air at 1250 °C and at 1400 °C for 12 hours with intermediate grinding while the SB#2 sample was sintered at 1400 °C only for 5 hours. The single crystal was also prepared in air atmosphere.

Fig. 4.13 The phase diagram of La1-xCaxMnO3 for a) the air atmosphere (AP) prepared samples, solid circles denote the paramagnetic–ferromagnetic transition, open circles correspond to the onset temperature TB of the jump in the ZFC dc magnetization curves; and b) for R (He atmosphere annealed) samples, solid circles denote the transition from the paramagnetic to the canted antiferromagnetic or ferromagnetic state. Taken from [Pissas 2004].

As was found recently, the atmosphere in which sintering process is carried out is crucial not only for stoichiometry, but also for crystallographic and magnetic structure of the compound [Horyń 2004, Pissas 2004]. In the case of low doped


56

La1-xCaxMnO3 series of compounds new phase diagrams were established, depending on the preparation process, see Fig. 4.13 [Pissas 2004].

Properties of the compound of interest, i.e. La0.9Ca00.1MnO3, depend on the oxygen partial pressure during preparation, which influences the Mn4+ content and in order to prepare stoichiometric samples a low oxygen partial pressure is needed [Pissas 2004]. Samples prepared in atmospheric conditions for x<0.16 were found to be cation deficient in such a way that the Mn4+ concentration remains constant regardless of x. Moreover, as can be seen from Fig. 4.13 samples prepared in atmospheric conditions at low temperatures are not canted antiferromagnets, as reported for samples prepared in helium, [Pissas 2004] or argon atmosphere [Terashita 2005], but they exhibit ferromagnetism [Pissas 2004]. For the Zaragoza sample neutron diffraction showed no evidence of antiferromagnetic domains [Algarabel 2003]. The values of TC and temperature denoted as TB agree well with those derived from AC susceptibility measurements for our samples. For single crystal and SB#2 samples TB was found to be of 110 K and TC amounts to 158 K and 165 K respectively [Yates 2003]. Basing on this as well as on the magnetic and other measurements presented in [Ghivelder 1998, Yates 2003, Algarabel 2003] we conclude that the ground state of the samples studied by us is not a canted antiferromagnet but ferromagnet.

Fig. 4.14 55Mn NMR spin echo spectra of the SB#2 sample at 4.2 K and different pulse spacing, τ. Lines are guides for eyes only.

Fig. 4.15 55Mn NMR spin echo spectra of the single crystal sample at 4.2 K and different pulse spacing, τ. Lines are guides for eyes only.

Chapter 4: La0.875Sr0.125MnO3 and La0.85Sr0.15MnO3 - NMR results

57

Fig. 4.16 55Mn NMR spin echo spectra of the FCB11 sample at 4.2 K with different values of pulse spacing, τ. Lines are guides for eyes only.

Fig. 4.17 55Mn NMR spin echo spectra of the Zaragoza sample at 4.2 K with different values of pulse spacing, τ. Lines are guides for eyes only.

After this explanation we present the 55Mn NMR results carried out at 4.2

K. Figures 4.14-4.17 present the frequency swept spectra carried out at various pulse spacing, τ.

The interpretation of the frequency swept spectra for all the samples of La0.9Ca00.1MnO3 is analogous to La0.875Sr0.125MnO3. Starting from the low frequency side there is a sharp peak at around 320 MHz, due to Mn4+ in ferromagnetic insulating regions. The line around 380 MHz is attributed to the ferromagnetic metallic regions where the DE driven fast hopping of carriers between the adjacent Mn4+ and Mn3+ sites takes place [Matsumoto 1970, Kapusta 2000]. The higher frequency lines appearing in the range from 400 to 550 MHz are attributed to Mn3+ ions in the ferromagnetic insulating regions. One can notice that there are three distinct lines in this region, which are denoted in Fig. 4.14 as line I, II, III, in contrast to the frequency swept spectra for La0.875Sr0.125MnO3 compound where two Mn3+ lines were observed. Similar NMR frequency swept spectra of all samples of La0.9Ca00.1MnO3 compound to La0.875Sr0.125MnO3 also suggest that their ground state is similar, i.e. the ground state of La0.9Ca00.1MnO3 is a ferromagnet rather than a canted antiferromagnet. For one of the samples (i.e. Zaragoza sample) also measurements at the applied field (up to 6 T) were carried out [Algarabel 2003]. Their results are


58

similar to those obtained for La0.875Sr0.125MnO3 presented earlier in this chapter. Again the lines: FMI Mn4+, the DE and the Mn3+ line at 530 MHz shift towards lower frequencies in the applied field and the Mn3+ line at 450 MHz does not shift even at 6 T and only decreases in intensity [Algarabel 2003]. This suggests that, as in the case of La0.875Sr0.125MnO3, the Mn3+ line at 450 MHz is due to magnetic moments directed perpendicular to the applied field direction. It is worth noting that in the samples FCB11 and Zaragoza at the longest pulse spacing the Mn3+ line at 530 MHz remains, similarly to the La0.875Sr0.125MnO3 compound (see Fig. 4.4). The signal which stays at the longest pulse spacing (longest relaxation times) can be attributed to ions located in domain wall edge, where the magnetic moments are within or close to the easy magnetisation direction (EMD) [Weisman 1973, Davis 1976, Leung 1977]. The spin-spin relaxation time, T2 of the Mn3+ lines is the longest for the line above 500 MHz (see table 4.4). However, for the SB#2 and the single crystal samples the Mn3+ line at 400 MHz remains at longest pulse spacing. This may be due to a smaller anisotropy field in the SB#2 and the single crystal samples and, correspondingly, to a different easy magnetisation direction.

It is worth noting that the 55Mn NMR spectra of the Zarogaza sample (sintered in air atmosphere) are similar to those of the FCB11 sample, which was sintered in oxygen atmosphere, rather than to the NMR spectra of other samples prepared in air atmosphere (SB#2 and single crystal samples). This can be explained by a much longer time of sintering process of the Zaragoza sample and, possibly, to an excess of oxygen in the FCB11 and Zaragoza samples. The problem of oxygen nonstoichiometry and its effects on the properties of manganese perovskites have been studied by many researchers, see for example [Wołcyrz 2003, Horyń 2003]. The effect of oxygen nonstoichiometry on 55Mn NMR spectra can be very significant, see for example in [Kapusta 1999].

SB#2 Single crystal Zaragoza FCB11 Freq.

[MHz] T2

[μs] ΔT2 [μs]

Freq. [MHz]

T2 [μs]

ΔT2 [μs]

Freq. [MHz]

T2 [μs]

ΔT2 [μs]

Freq. [MHz]

T2 [μs]

ΔT2 [μs]

314 325 5 314 473 7 317 220 4 314 136 4381 105 1 351 168 5 363 93 1 321 107 3404 83 1 381 169 2 381 77 0 364 69 2437 125 2 505 258 6 411 73 1 381 65 1514 203 7 443 93 1 391 64 1

537 282 19 434 62 1 533 166 20

Table 4.4 Spin-spin relaxation times, T2 and their uncertainties, for 55Mn resonance lines, fitted with the equation (4.1) for all the studied samples of La0.9Ca0.1MnO3 at 4.2 K. Similarly to both compounds doped with Sr, described previously, the spin-spin relaxation time, T2 is the shortest for the DE line. However, for the


59

La0.9Ca0.1MnO3 samples the values of T2 are much larger than for the Sr doped compounds (see table 4.4, table 4.1 and Fig. 8b). Publication related to problems raised in this chapter: Rybicki D., et al., Acta Physica Polonica 105, 183 (2004)

Chapter 5: Nanoparticles of La0.75Sr0125MnO3, 60

5. Nanoparticles of La0.75Sr0.25MnO3 5.1 55Mn NMR results of La0.75Sr0.25MnO3 nanoparticles This chapter presents results and discussion of magnetisation measurements

carried out on the SQUID magnetometer and 55Mn NMR spin echo measurements at 4.2 K and at 77 K of three samples of La0.75Sr0.25MnO3 compound carried out on the spectrometer in Kraków. Sample with grains of micrometric size (SM) and two samples with average grains size 114 nm (S114) and 33 nm (S33) were measured. The results of magnetization measurements are presented in Fig. 5.1. As it was found earlier by other groups, for manganese perovskite compounds with grains of nanoparticle size, the saturation magnetization, MS decreases with decreasing average grain size [Balcells 1998, Bibes 2003, Savosta 2004]. The effect is related to the presence of non-ferromagnetic (non-collinear, antiferromagnetic or nonmagnetic) outer layers of the grains. A higher relative volume contribution of this layer to the total volume in smaller grains corresponds to a smaller value of MS. The thickness (l) of this layer can be estimated using following expression [Balcells 1998]:

⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

⎥⎦

⎤⎢⎣

⎡−≈

31

)()(

12 bulkM

Ml

S

S φφ (5.1)

where φ is the average grain diameter. The derived values of l amount to 1.7 nm and 1.54 nm for the sample S33 and S114 respectively.

-0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5

-80

-60

-40

-20

0

20

40

60

80

-80

-60

-40

-20

0

20

40

60

80

0 50 100 150 200 250 300 3500

15

30

45

60

75

M

[em

u/g]

T [K]

M [e

mu/

g]

μ0H [T]

S33 S114 SM

Fig. 5.1 Magnetisation plots versus applied field (M vs H) for samples S33 (red), S114 (black) and SM (blue) of La0.75Sr0.25MnO3 at 4.2 K. Inset presents the temperature dependence of the magnetisation, M.


This is in a good agreement with results obtained for nanoparticles of La2/3Sr1/3MnO3, approx. 1.2 nm for the sample with 20nm grains [Balcells 1998].

The Curie temperature increases with the average particle size, from 335 K to 352 K in the case of average grain size of 33 nm and 120 nm respectively [Pollert 2006]. The coercive field, HC increases with decreasing particle size, which can be attributed to a higher anisotropy field, BA in smaller grains and a possible influence of domain walls in bigger particles. Also the character of domain walls is likely to change with decreasing grain size and approaching the single domain limit. This result is consistent with a change of the NMR enhancement factor (η) for samples with different grain sizes and it is discussed later on. Values of the TC derived from Arrot plots decrease with decreasing average grain size, from 360 K for sample with grains of micrometric size (SM sample) to 353 K and 344 K for S114 and S33 samples [Pollert 2006].

5.1.1 55Mn NMR spin echo spectra at 4.2 K and 77 K

Figure 5.2 presents 55Mn NMR spin echo spectra at zero field, at 0.2 T and at

0.5 T for samples SM, S114 and S33. The dominant main line centred close to 380 MHz corresponds to a fast hopping of the electron (hole) among the Mn sites in ferromagnetic metallic (FMM) regions, at a rate faster than the NMR (Larmor) frequency due to the double-exchange (DE) interaction. A weak signal at lower frequencies (315-340 MHz), is ascribed to Mn4+ ions in ferromagnetic insulating (FMI) regions located in the outer layers of the grains [Matsumoto 1970, Kapusta 2000]. Our observation of two resonant lines is similar to that in the NMR 55Mn study of La0.66Ca0.33MnO3 nanoparticle materials [Bibes 2003, Savosta 2004] and in epitaxial thin films of La0.67Sr0.33MnO3 [Sidorenko 2006]. Similarly to the results presented in [Bibes 2003, Savosta 2004] the intensity of this signal per unit mass decreases with increase of the average grain size (Fig. 5.2). The relative amount of the FMI phase estimated from the line intensities of the spectra at zero field is of 3% and 1% for the samples S33 and S114, respectively (see inset in Fig 5.3). The values of the resonant frequencies of the DE line obtained from Gaussian curve fits to the spectra for samples S114, S33 and SM at zero field are 384.5 MHz, 381.9 MHz and 383.4 MHz respectively (Fig. 5.4).

Since the magnitude of the hyperfine field, BHF is approximately proportional to the electronic spin moment (<S>), SABHF

ˆ=r

, where A is the hyperfine coupling tensor and S is a average electronic spin moment, therefore different values of the BHF for all samples could be attributed to a slightly different Mn3+/Mn4+ ratio in all studied samples. This may be due to a possibly different oxygen stoichiometry in different samples resulting in non stoichiometric


Fig. 5.2 Normalized NMR 55Mn spin echo spectra of La0.75Sr0.25MnO3 with grains of micrometric size (SM), with 114nm (S114) and 33nm (S33) grains at 4.2 K and 77 K, at 0 T (black lines), 0.2 T (blue lines) and 0.5 T (red lines).

Fig. 5.3 Normalized 55Mn spin echo spectra of bulk (blue line) La0.75Sr0.25MnO3 and samples with 114 nm (red line) and 33 nm grains (black line) at 4.2 K and at no applied field. Zoomed view of low frequency region is presented in the inset.

300 325 350 375 400 425 450

Nor

mal

ised

ech

o ite

nsity

S33 S114 SM

320 340 360

Frequency [MHz]

300 320 340 360 380 400 420 440

300 320 340 360 380 400 420 440

SM

S114

Ech

o in

tens

ity [a

rb. u

nits

]

Frequency [MHz]

at 0T at 0.2T at 0.5T

S33

at 77K

300 320 340 360 380 400 420 440

300 320 340 360 380 400 420 440

SM

S33

Ech

o in

tens

ity [a

rb. u

nits

]


S114

Frequency [MHz]

at 4.2K


Mn3+/Mn4+ ratio and/or an influence of the related negative pressure in small grains, which would decrease the value of BHF [Kapusta 2001].

The DE line shifts towards lower frequencies with increasing applied field as expected for the hyperfine field antiparallel to the Mn magnetic moment. For the sample S114 at 0.5T, the Beff is reduced to 36.02 T from 36.45 T at no applied field, for the sample S33 it decreases from 36.20 T to 35.72 T and for the microcrystalline sample from 36.34 T to 35.98 T (see also Fig. 5.4). One can conclude that the demagnetizing field, Bdem is smaller than 0.2 T and that even at the smallest applied magnetic field our samples are the most likely in a single domain state.

0,0 0,1 0,2 0,3 0,4 0,5

0,0 0,1 0,2 0,3 0,4 0,5

368

370

372

374

376

378

380

382

384

Res

onan

t fre

quen

cy [M

Hz]

External field [T]

S33 at 4.2K S33 at 77K S114 at 4.2K S114 at 77K SM at 4.2K SM at 77K

Fig. 5.4 Resonant frequencies obtained from Gaussian curve fits to the DE lines for the NMR 55Mn spin echo spectra, at fields 0 T, 0.2 T, 0.5 T and at temperatures 4.2 K (solid lines) and 77 K (dashed lines), for samples SM (black lines), S33 (blue lines) and S114 (red lines).

However, the spin-spin relaxation time (T2) for all studied samples reveals a

non-exponential behaviour at 0 T, whereas T2 decreases with a single exponential character at 0.5 T (Fig. 5.5). This indicates the presence of some domain wall-like magnetic inhomogeneities in all studied samples at 0 T, which disappear after applying magnetic field of 0.5 T, similar behaviour was also observed by Savosta et al. [Savosta 2004]. On this basis we assume that the DE line originates from Mn ions located both in domains and domain-wall like inhomogeneities (i.e. close to surface regions, where magnetic moments change their orientation similarly to the behaviour of magnetic moments in domain walls). The existence of typical domain


walls is put in question due to very small size of grains in the samples S33 and S114. A similar assumption was also made in [Savosta 2004].

10 20 30 40 50 60 70 80 90 10010-7

10-6

10-5at 0T

Pulse spacing [μs]

Echo

inte

nsity

[arb

. uni

ts]

10 20 30 40 50 60 70 80 90 100

10-7

10-6

10-5

b)

SMS114S33

at 0.5Ta)

Fig. 5.5 Spin echo decay curves for resonant frequencies of the DE lines at 4.2 K for all studied samples of the La0.75Sr0.25MnO3; a) at 0.5 T and b) at 0 T. Note the logarithmic scale of the echo intensity.

0,0 0,1 0,2 0,3 0,4 0,5

0,0 0,1 0,2 0,3 0,4 0,5

14

16

18

20

22

14

16

18

20

22

Full

wid

th a

t hal

f max

imum

[MH

z]

External field [T]

S33 S114 SM

Fig. 5.6 The DE line widths (given as full widths at half maximum) obtained from Gaussian curve fits to the NMR 55Mn spin echo spectra, at fields 0 T, 0.2 T, 0.5 T and at the temperature 4.2 K, for samples SM (black line), S33 (blue line) and S114 (red line) of the La0.75Sr0.25MnO3.


The width of the DE line at 4.2 K decreases with the increasing average grain size. This effect can be attributed to a narrowing of the distribution of the hyperfine field values due to less amount of defects for samples with higher average particle size and, possibly due to the more pronounced anisotropy of the hyperfine field in samples with smaller particles and also due to the differences in distributions of demagnetizing fields. The line width also decreases with application of the field for all the samples studied (see Fig. 5.6) and this decrease is the most dramatic (by 25%) for the sample with grains of micrometric size, while the relative decrease for S114 and S33 amounts to 18% and 8% respectively. This can be explained by the effect of the domain wall (or domain wall like regions) rearrangement and their partial removal induced by applying magnetic field. In the sample SM the amount of domain walls is the biggest so the effect of the field is the largest. Accounting for the shift of the DE line with the applied field, the line narrowing on application of 0.2 T and 0.5 T occurs at the expense of the high frequency part of the line, which suggests that the signal from domain walls or domain wall like regions corresponds to that part of the spectrum (see Fig. 5.2).

5.1.2 The spin-spin and spin-lattice relaxations at 4.2 K and 77 K

There are two possible methods of obtaining values of the spin-spin

relaxation times, T2. The first one is to measure the decay of the spin-echo signal as a function of the pulse spacing, τ (as in Fig. 5.5). The second is to make several frequency swept spectra with different pulse spacing (Figures 5.6 and 5.7) and using values of the spin echo signal for given frequency from all spectra, calculate the spin-spin relaxation time, T2.

The measurements of the T2 at 0 T (at 4.2 K and 77 K) for all samples were carried out by measuring the decay of the spin-echo signal as a function of the pulse spacing, τ. As was mentioned above in this chapter, the decay of the spin-echo signal at 0 T in not a single exponential function, but two exponents have to be used in order to fit the data. This is due to presence of signals both from domains ( ) and from domain walls (or domain wall like regions) ( ), since in a ferromagnetic materials signals from nuclei in domains and nuclei in domain walls can be detected. The nuclei in domain walls have shorter spin-spin relaxation time than nuclei in domain interiors [Weisman 1973, Davies 1976, Leung 1977]. The experimental data were fitted with the following curve:

DT2DWT2

⎟⎟⎠

⎞⎜⎜⎝

⎛−+⎟⎟

⎠

⎞⎜⎜⎝

⎛−= == DW

DWD

D

TA

TAA

20

20

2exp2exp)( τττ ττ (5.2)

where and are the spin-echo amplitudes at the time τ, and τ=0 (for DWDA ,0=τ


domains and domain walls (or domain wall like regions), respectively. The obtained values of the spin-spin relaxation times for nuclear magnetic moments,

and for all the samples at 0 T, at 4.2 K and 77 K are presented in table 5.1. DT2DWT2

4.2K 77K

Sample DWT2 [μs] DT2 [μs] DWT2 [μs] DT2 [μs]

SM 4.3 ± 0.2 31.8 ± 1.2 11.1 ± 0.2 35.6 ± 0.6 S114 12.1 ± 0.1 38.7 ± 0.6 9.2 ± 1.2 24.4 ± 4.7 S33 10.7 ± 0.1 33.3 ± 0.5 7.6 ± 1.5 22.5 ± 6.6

Table 5.1. Spin-spin relaxation times for nuclear magnetic moments in domains ( ) and in domain walls (or in domain wall like regions) ( ), obtained using equation 5.2 for all studied samples of La0.75Sr0.25MnO3 at 0 T, 4.2 K and 77 K measured at the resonant frequency of line maximum for each sample.

DT2DWT2

Fig. 5.7a Normalized 55Mn spin echo spectra of La0.75Sr0.25MnO3 S114 sample at 4.2 K, at 0 T with various pulse spacing, τ.

Fig. 5.7b Normalized 55Mn spin echo spectra of La0.75Sr0.25MnO3 S114 sample at 4.2 K, at 0.5 T with various pulse spacing, τ.

300 320 340 360 380 400 420 440

τ=20μs

τ=100μs

τ=50μs

τ=10μs

S114 sample4.2K, 0T

Frequency [MHz]

Nor

mal

ised

ech

o in

tens

ity

300 320 340 360 380 400 420 440

Nor

mal

ised

ech

o in

tens

ity

τ=10μs

τ=20μsτ=50μs

τ=100μs

τ=150μs

Frequency [MHz]

τ=300μs

S114 sample 4.2K, 0.5T


In order to study the spin-spin relaxation more systematically, the spectra of all samples at 0 T and at 0.5 T for various pulse spacing, τ were measured at 4.2 K. Figures 5.7a and 5.7.b present measurements for sample S114 at 0 T and 0.5 T. Similar measurements for other two samples were also carried out, but the results look almost the same and, therefore, their spectra in dependence on τ are not presented here in details. It is clearly visible that with increasing pulse spacing a dip in the centre of the DE line appears similarly to La0.85Sr0.15MnO3 [see chapter 4]. The effect can be explained by the Suhl-Nakamura interaction [Suhl 1958, Davis 1974] between nuclear spins of neighbouring Mn ions.

350 360 370 380 390 40050

75

100

125

150

175

200

225

250

275

300

at 4.2K at 0.5T S33 S114 SM

Frequency [MHz]

T 2 [μs]

Fig. 5.8 Frequency dependence of the spin-spin relaxation time, T2 for samples S33, S114 and SM of La0.75Sr0.25MnO3 at 4.2 K and at 0.5 T with error bars marked.

The presence of the Suhl-Nakamura interaction can be shown in the other way

in the frequency dependence of the spin-spin relaxation time, T2 which should have a minimum at the resonant frequency. Fig. 5.8 presents such a dependence for all the studied samples at 4.2 K and at 0.5 T. The values of the T2 were calculated from all measured frequency swept spectra with different pulse spacing used. They were obtained assuming a single exponential decay of the spin-echo signal (see Fig 5.5a) i.e. a single domain state of the sample at 0.5 T. Similar behaviour of the frequency dependence of T2 is observed in the case of measurements at 0 T. However, a weakening of the spin-echo signal with increasing pulse spacing and a two exponential behaviour of T2 (signals from nuclei in domains and in domain walls)


result in larger uncertainty of the fitting. The Suhl-Nakamura interaction for 55Mn ion in bulk metallic La0.7Ca0.3MnO3 compound has the effective range of 35 Ǻ at 140 K [Savosta 2001].

As can be seen in Fig. 5.8, samples S114 and SM have similar values of the T2, for a given frequency, while T2 of the S33 sample is significantly smaller. This means that the relaxation of nuclear spins is faster, which can be attributed to less effective coupling mechanisms between nuclear spins in the S33 sample due to a smaller size of grains. In metallic-like perovskite manganites there are two main mechanisms responsible for the spin-spin relaxation of nuclear magnetic moments [Savosta 2004]. The first one are fluctuations of the hyperfine fields caused by hopping of electron holes, . The second mechanism is the spin-spin relaxation due to the Suhl-Nakamura interaction, , which is not effective at the sides of the resonant line, where only mechanism due to electron (hole) hopping has to be considered. Therefore, both relaxation processes can be separated using the formula [Savosta 2004]:

)(2 hopT)(2 SNT

)()( 12

12

12 SNThopTT −−− += (5.3)

where is the value for given frequency, taken from Fig. 5.8. From this formula the contribution of the Suhl-Nakamura interaction to the relaxation rate is derived. The values of the relaxation rates, amount to 6.97 ms-1, 6.61 ms-1 and 7.63 ms-1 for S33, S114, SM samples respectively for the DE line and one can conclude that in all the samples studied the effective range of the Suhl-Nakamura interaction is comparable or smaller than the size of DE regions.

12−T

)(12 SNT −

Nanosized samples exhibit smaller Suhl-Nakamura contribution to the relaxation rate than sample with grains of micrometric size. This can be explained knowing that the effectiveness of the Suhl-Nakamura interaction depends on the number of nuclear spins, which precess at or near the resonance frequency and this number can be smaller in samples with nanometric grains. The obtained results of the DE line widths (given as full widths at half maximum) from the Gaussian curve fits support this finding (see Fig. 6). The DE line of the S33 sample is the broadest (the broader the resonance line - the less nuclear spins, which precess at or near the resonance frequency). Similar observation was made for the nanoparticles of La0.7Sr0.3MnO3 [Savosta 2004]. The is larger for S33, i.e. for sample with smallest grains ( ≅ 1/100 μs-1) than for samples S114 and SM ( ≅ 1/260 μs-1). This implies, that in the sample with the smallest grains electrons and holes are moving slower, since

)(12 hopT −

)(12 hopT − )(1

2 hopT −

hopT τ~12− [Savosta 2004], where τhop is the correlation

time of electron (hole) hopping [Savosta 1999]. Values of the spin-spin relaxation times, T2 at 0 T could not be derived in a


similar way to those at 0.5 T, because the signal to noise ratio in the measurements at 0 T with increasing pulse spacing was decreasing much faster than in the case of measurements carried out at 0.5 T (compare Fig. 5.6 and 5.7 for the same value of pulse spacing, τ). Due to this fact frequency swept spectra within shorter range of pulse spacing values could be measured only before signal was lost in noise, and additionally at 0 T the spin-spin relaxation is two exponential; therefore a reasonable fit to these data is not possible.

One can also notice that spin-spin relaxation times, T2 of nuclear magnetic moments measured at 0.5 T are considerably longer that those obtained at 0 T. The T2 at 0 T lies in the range 25-30 μs for all samples (table 2) while at 0.5 T it amounts to 62 μs, 93 μs and 86 μs for S33, S114 and SM samples respectively. This fact is a result of the two factors. First, the contribution to the spin-spin relaxation time from nuclei within domain walls (or domain wall like regions) decreases and is later eliminated as domain walls (domain wall like regions) are removed by the applied field [Leung 1977]. As was shown the spin-spin relaxation time is shorter for nuclei within domain walls than for nuclei in domains. The second factor is the fact that the Suhl-Nakamura interaction depends on the external field, so that the spin-spin relaxation, T2 increases as the external field increases [Hone 1969, Davis 1974]. This effect was observed in the manganese ferrite [Davis 1976] and in La0.69Pb0.31MnO3 [Leung 1977].

The spin-lattice relaxation times, T1 for all the samples measured at 4.2 K and at 77 K at 0 T are presented in Fig. 5.9. The recovery of the nuclear magnetisation is not a single exponential function and it fits very well to two exponents:

(5.4)

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−−+⎟⎟

⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−−=

bTaT TtA

TtAtM ba

11

exp1exp1)(11

The results of the fits with above formula are presented in table 5.2. Necessity of using two exponents in the fitting curve in order to obtain good fit means that spin-lattice relaxation contains the contribution from domains and domain walls (or domain wall like regions), similarly to the spin-spin relaxation. Also the spin-lattice relaxation times are shorter for samples with grains of nanometric size and decrease with the particle size, which can be attributed to additional T1 relaxation present in nanometric particles, but absent in a bulk material. This additional relaxation is due non-ferromagnetic atoms on the particle surface layer, such observation was also made for nanometric particles of ferromagnetic cobalt in [Kaplan 1968], where the authors concluded that additional T1 relaxation observed in nanometric particles is due to paramagnetic like atoms in the surface layer of particles and nuclear spin diffusion


to paramagnetic like regions [Kaplan 1968].

0 500 1000 1500 2000

0 1000 2000 3000 4000 5000 6000 7000 8000

b)

Arb

. uni

ts

Recovery time [μs]

A

rb. u

nits

a)

0 500 1000 1500 2000

Arb

. uni

tsRecovery time [μs]

c)

Fig. 5.9 Measurements of the spin-lattice relaxation times, T1 at 4.2 K and 77 K at 0 T for all the samples of La0.75Sr0.25MnO3 studied; a) for sample SM (blue squares); b) for samples S33 (red squares) and S114 (green squares); c) at 77 K for all three samples with the same colours as in a) and b). Lines are fits to the experimental data using equation 5.3. The plots are arbitrary off-set for clarity.

4.2K 77K

Sample aT1 [μs] bT1 [μs] aT1 [μs] bT1 [μs]

SM 2923 ± 80 34 ± 2 395 ± 8 83 ± 3 S114 410 ± 5 36 ± 2 206 ± 11 48 ± 3 S33 225 ± 7 22 ± 1 139 ± 14 22 ± 4

Table 5.2. Spin-lattice relaxation times, and obtained using equation 5.4 for all of La0.75Sr0.25MnO3 samples studied, at 4.2 K and 77 K and at 0 T.

aT1bT1

5.1.3 The enhancement factor of 55Mn at 4.2 K and 77 K In order to analyse the NMR enhancement factor η, which is related to the

susceptibility of the electronic system at the NMR frequency, χRF the optimal pulse lengths corresponding to the maximum of the spin echo for the resonant frequency of the DE lines have been compared for all the samples studied. For measurements at 4.2 K they are presented in table 3 and for measurements at 77 K only the relative


changes of the optimal pulse lengths are given (see table 5.4). The enhancement factor, η is inversely proportional to the optimal pulse length,

t [Turov 1972] (see equation 3.41 in chapter 3):

RFtχη ∝∝

1 (5.4)

therefore at 0 T η is the largest for sample SM with grains of micrometric size and the smallest for S33 sample with the smallest grains.


S33 0.8μs 1.1μs 1.6μs S114 0.5μs 1μs 1.5μs SM 0.2μs (+4dB) 1.9μs 3.7μs

Table 5.3 Optimal lengths of the first exciting pulse (second pulse is always kept two times longer) used for all the samples studied at 4.2 K for resonant frequency of the DE lines. For SM sample at 0 T additional pulse attenuation was used in order to observe maximum in the dependence of the spin echo signal intensity on the pulse length.

Table 5.4 presents ratios of the enhancement factors, e.g. η at 0.5 T to η at 0 T for all studied samples at 4.2 K and at 77 K, calculated as inverse ratios of the optimal pulse lengths (equation 5.4). For all the samples at both temperatures enhancement factors decrease after applying magnetic field, which is due to rearrangement and/or removal of the domain walls (domain wall like regions), nuclei of atoms in domain walls or in domain wall like regions have larger enhancement factor than those in domain interiors. However, the biggest change of the enhancement factor is observed for sample SM with the largest grains: at 4.2 K after applying 0.5 T η decreases nearly 17 times. Such a big decrease compared to that of η114 and η33 is attributed to the presence of domain walls in the microcrystalline sample, which are removed by the applied field, whereas in the samples S33 and S114 there are much less domain walls or domain wall like regions.

)0()5.0(

33

33

TT

ηη

)0()5.0(

114

114

TT

ηη

)0()5.0(

TT

bulk

bulk

ηη

4.2K 0.4 0.33 0.06 77K 0.33 0.15 0.02

Table 5.4. Ratios of enhancement factors at 0.5 T and at 0 T for all studied samples at 4.2 K and at 77 K.


5.2 139La signal at 4.2 K and at no applied field The 139La signal was measured at 4.2 K and the results are presented in Fig. 5.10. Unlike in the case of lower Sr doped compounds (see chapter 4), the frequency swept spectra of La0.75Sr0.25MnO3, both with micrometric and nanometric grains, consist of a single resonance line. The resonance line of the S114 sample is centred at 26 MHz (6 MHz higher than for the SM sample) and is much broader. It is worth noting that the manganese resonant line of the S114 sample is also larger than that of the SM sample (see Fig. 5.6). The reason for a much larger line width is the bigger distribution of the transferred hyperfine fields at the La site due to smaller grain sizes and, correspondingly, to a higher influence of inhomogeneities in smaller grains. The signal from the sample with the smallest grains (S33 sample) was too weak to enable its observation in a reasonable time.

10 15 20 25 30 35 40

10 15 20 25 30 35 40

S114 sample

Nor

mal

ized

ech

o in

tens

ity [a

rb. u

nits

]

Frequency [MHz]

SM sample

Fig. 5.10. Normalized NMR 139La spin echo spectra of La0.75Sr0.25MnO3 with grains of micrometric size (SM sample) and with 114nm grains (s114 sample) at 4.2K and at no applied field. Some of the results shown in this chapter are presented in: Rybicki D., et al., Physica Status Solidi C 3, 155 (2006)

Chapter 6: Bilayered manganese perovskites, La2-2xSr1+2xMn2O7

73

6. Bilayered manganese perovskites, La2-2xSr1+2xMn2O7

6.1 55Mn NMR of bilayered manganese perovskites, La2-2xSr1+2xMn2O7 In this chapter 55Mn and 139La NMR results and discussion on bilayered manganese perovskites, La2-2xSr1+2xMn2O7 with 13.0 ≤≤ x are presented together with magnetisation measurements carried out with the VSM magnetometer (for compounds 15.0 ≤≤ x ). Before presenting the NMR results a closer look is given at the contribution to the hyperfine field, HFB

r called spin-dipolar field, . As was

mentioned in the subchapter dedicated to hyperfine fields in magnetically ordered materials, it is due to spins of electrons at individual orbitals and is given by the formula:

dSB −

r

( )

∑⎭⎬⎫

⎩⎨⎧ −

=−i

iiBsdS

rrrsrsgB

500

2

0

3 rro

rrrμμ (6.1)

where gs=2.0023, is the spin of ith electron and isr 0rr is a unit vector along the

leading vector, r. This field can be computed by multiplying the above equation by the electron density, ee ΨΨ= *ρ and integrating over the electron coordinates [Abragam 1970]. Calculation of the dSB −

r field for 3d orbitals is essential for the

explanation of observed 55Mn NMR frequency swept spectra (see paragraph 6.2.2). The wave function describing the electron consists of the radial part and the angular part. For 3d the radial part of the wave function, R3d is given by the equation:

223

23 309

1 ρ

ρ−

⋅⋅⋅= eZR d , where nZr2

=ρ (6.2)

r is the radius in atomic units, in our case n=3 and Z is the effective nuclear charge. The value of the effective nuclear charge, Zeff used for calculation amounted to 10.53 [Clementi 1963, Clementi 1967], which is the value for the 3d orbital for Mn ion including screening effect by the inner electrons. The angular parts of the electronic wave function are different for electrons on different 3d orbitals. These orbitals are usually denoted as xy, xz, yz, x2-y2, 3z2-r2 and the angular parts, Y3d of the respective orbitals are given by equations:

π41

460

2)(3 ⋅⋅=rxyY xyd (6.3)

π41

460

2)(3 ⋅⋅=rxzY xzd (6.4)

π41

460

2)(3 ⋅⋅=ryzY yzd (6.5)


74

π41)(

415

2

22

)(3 22 ⋅−

⋅=− r

yxYyxd

(6.6)

π41)2(

45

2

222

)3(3 22 ⋅−−

⋅=− r

yxzYrzd

(6.7)

where . The graphical representations of these angular wave functions are presented in Fig. 6.1.

2222 zyxr ++=

Fig. 6.1. Graphical representations of

compounds studied the Mn3+ ion has four electrons in the 3d band in a h

rbitals the

angular wave functions for five 3d orbitals, t2g orbitals are off axes and eg orbitals lie along coordination axes.

In the igh spin state, e.g. 3 electrons with spin up on t2g orbitals and one electron

with spin up on one of the two eg orbitals (see also chapter 2). Calculations of the spin-dipolar field, dSB −

r at nucleus produced by a single electron occupying

one of the 3d eg orbitals were carried out in the Mathematica program using numerical integration and method called Multidimensional, which is as an adaptive Genz-Malik algorithm [Genz 1991]. The results of calculations are presented in table 6.1. In order to test the results, other methods available in Mathematica for numerical integration were used The quasi Monte Carlo method (non-adaptive Halton-Hammersley-Wozniakowski algorithm) gave the result 0.2% bigger and the non adaptive Monte Carlo method gave result 7% smaller than the method Multidimensional. Due to the symmetry of the three t2g o dSB −

rfield produced by the

three electrons occupying t2g orbitals is zero and one obtains a nonzero value of the dSB −

r produced by the electron occupying one of the two eg orbitals. If

magnetic moments lie along one of the axes x, y or z, the corresponding spin dipolar field also has its component only along one of these axes.


75

dSB −

rfor eg orbital

sμr direction x − 2y 2232 rz −

[100] 10,34T -10,34T

[001] -20,68T 20,68T

Table 6.1. Calculated values of the spin dipolar hyperfine field at Mn dSB −

r

nucleus produced by a single electron occupying one of the 3d eg orbitals. The sign of the dSB −

r is with respect to the direction [xyz] of the spin moment, sμ

r .

6.2 55Mn NMR results at 4.2 K

The 55Mn NMR spectra at 4.2 K of all the compounds studied, obtained with v

ed in Fig. 6.2 sh

arious pulse spacing, are presented in Figures 6.2 and 6.3. In Fig 6.2 the spectra for the La1.4Sr1.6Mn2O7 (x=0.3) compound and the frequency dependence of the spin-spin relaxation time (T2) are presented. The spectrum measured with τ=8 μs shows a broad line centred at 380 MHz attributed to the DE controlled ferromagnetic metallic (FMM) regions and two much less intense lines, at the lower and at the higher frequency, corresponding respectively to the Mn4+ and Mn3+ valence states in the charge localised ferromagnetic insulating (FMI) regions. This situation is very similar to that observed in the metallic-like cubic manganites doped close to 30% with Sr or Ca for La, where the DE line was also observed [Matsumoto 1970, Kapusta 1999 and 2000, Savosta 2001]. With increasing doping the Mn4+ and Mn3+ lines have a higher intensity comparing to that of the DE line (Figures 6.3a and 6.3b). This reflects a decreasing amount of the metallic phase with increasing Sr content up to the x=0.5 compound, which is found to be an insulator. Simultaneous observation of the aforementioned NMR lines clearly indicates phase separated ground state in the compounds studied. The phase separation problem in manganese perovskites was described in more details in chapter 2. In this chapter it is shown how it can be studied by the NMR method. Recently, it was reported, using bulk magnetic measurements, that La1.1Sr1.9Mn2O7 (x=0.45) compound has an inhomogeneous magnetic ground state arising as a consequence of phase separation [Nair 2006].

The frequency swept spectra of La1.4Sr1.6Mn2O7 (x=0.3) presentow that the DE line reveals a minimum at its centre with increasing pulse

spacing. This is related to the frequency dependence of the spin-spin relaxation time T2 which reveals a minimum at the centre of the DE line. These two features result from the Suhl-Nakamura interaction and were already described in details in the chapter 4 presenting NMR results on the Sr doped cubic manganese perovskites.


76

300 325 350 375 400 425 450 475 500

5075

100125150

DEMn4+

τ=150μs

τ=60μs

Nor

mal

ized

inte

nsity

Frequency [MHz]

τ=8μs

La1.4Sr1.6Mn2O7 (x=0.3)

Mn3+

T 2 [μ

s]

a)

Fig. 6.2 55Mn NMR spin echo spectra at 4.2 K at no applied field of La1.4Sr1.6Mn2O7 (x=0.3) for different pulse spacing. The solid lines at the bottom spectrum are Gaussian fits and the other lines are guides for eyes only. The upper plot presents the frequency dependence of the spin-spin relaxation time with error bars marked.

The occurrence of a dip at the centre of the DE line is not observed for LaSr2Mn2O7, i.e. x=0.5 (Fig. 6.3b) and there is no minimum in the frequency dependence of the T2 at the central frequency. On this basis we can conclude that the S-N interaction is not effective in the x=0.5 doped compound and that the size of the DE regions is smaller than the effective radius of the S-N interaction in this compound, which can be due to antiferromagnetic ordering within the bilayer in this compound [Mitchell 2001]. On the contrary, the size of the DE regions is much larger than the effective radius of the S-N interaction in the x=0.3 compound. This radius was evaluated at 35 Ǻ for La0.7Ca0.3MnO3 at 140 K by Savosta et al. [Savosta 2001] and a similar value can be assumed for the bilayer system studied. This means that the DE regions in the x=0.5 compound are of a nanometer size, whereas they are at least tens of nanometers in the x=0.3 compound. The strongest signal from the DE regions observed in the x=0.3 compound can be due to its Mn3+/ Mn4+ ratio more preferable for DE interaction, but it can also be related to the fact that in this compound magnetic moments are perpendicular to the bilayer plane, while in the region

5.033.0 ≤≤ x magnetic moments lie in the bilayer plane (Fig. 2.9). This can


77

correspond to different magnetocrystalline anisotropy and/or to a difference of the DE integral.

For the compounds with x=0.4 and 0.5 one can notice that the DE line shifts towards higher frequencies at the larger pulse spacing, which can be attributed to the anisotropy of the effective field. Interestingly, for the x=0.3 compound such a behaviour is not observed, suggesting an isotropic effective field.

Fig. 6.3 55Mn NMR spin echo spectra at 4.2 K and no applied field a) of La1.2Sr1.8Mn2O7 (x=0.4) for different pulse spacing and b) of LaSr2Mn2O7 (x=0.5). Arrows on the spectra of both compounds for τ=30μs indicate positions of the Mn3+ lines. The upper plot presents frequency dependence of the spin-spin relaxation time with error bars marked. Solid lines are guides for eyes only.

325 350 375 400 425 450 475 500

40

50

60

70325 350 375 400 425 450 475 500

La1.2Sr1.8Mn2O7 (x=0.4)

τ=150μs

τ=90μs

τ=30μs

Nor

mal

ized

inte

nsity

Frequency [MHz]

τ=10μs

T 2 [μ

s] a)

325 350 375 400 425 450 475 500

30

40

50325 350 375 400 425 450 475 500

T 2 [μ

s]

LaSr2Mn2O7 (x=0.5)

τ=150μs

τ=60μs

τ=30μs

Nor

mal

ized

inte

nsity

Frequency [MHz]

τ=10μs

b)

As was mentioned in the chapter 3.4.1 devoted to the effective magnetic

field at nucleus, the total magnetic field experienced by the nuclei of a “magnetic” ion at zero external magnetic field is a sum of the hyperfine field,

resulting mainly from the spin and orbital moments of the electron within the ion radius and the “local” field,

HFBr

locBr

. The local field includes the classical dipolar field ( ) resulting from other magnetic moments in the sample, the demagnetising field, related to the macroscopic shape of the sample and the Lorenz field .The can be written as a following sum:

DBr

demBr

LorBr

HFBr

TorbdSFermiHF BBBBBrrrrr

+++= − (6.8)


78

where is the Fermi contact field, FermiBr

dSB −

r and orbB

r are contributions from the

spin dipolar interaction between the nucleus and the electronic spin and from the interaction between the nucleus and unquenched orbital moment of the electron respectively. The last element is the transferred hyperfine field, i.e. the field produced by magnetic neighbors ( TB

r). The anisotropic terms of the total

magnetic field experienced by the nuclei are mainly: dSB −

r, orbBr

, and . DBr

TBr

6.2.1 Mn4+ signal analysis

The values of Mn4+ resonant frequencies and corresponding effective

fields derived from Gaussian curve fits to the resonance lines are presented in table 6.2. No results are shown for the compound with x=0.3, because the signal is too weak compared with the DE line. The resonant frequencies at the maximums of spectral lines and the corresponding magnitudes of the effective fields are decreasing with increasing pulse spacing τ, which is attributed to the anisotropy of the effective field at nucleus. The signal from magnetic domain wall centre has a shorter T2 and it dominates for short pulse spacing. At large pulse spacing the signal from domain wall edges, which corresponds to the directions of magnetic moments close to those in domains, remains in the spectrum, as it has a longer T2. A possible influence of the S-N interaction can be excluded as Mn4+ ions are separated one from another and do not form homogeneous clusters large enough, which are necessary for the S-N interaction to be effective. Therefore the difference of the hyperfine field between short pulse spacing and long pulse spacing in this case corresponds to the anisotropy of the hyperfine field.

One of the possible anisotropic contributions to the effective field is the dipolar field, which is nonzero for symmetries lower than cubic. We have evaluated the

DBr

DBr

as a lattice sum of classical dipolar fields produced by neighbouring magnetic moments, which are situated within the sphere of the radius R. We increased the value of the radius R until the sum converged, which was obtained for R=30 Å (the average distance between Mn ions in a bilayered manganese perovskite is smaller than 4 Å at low temperatures). The magnitude of the is of 0.15 T and its anisotropy, i.e. the difference between the alignment of spins along c-axis and within the ab-plane, is smaller than 0.2 T.

DBr

As the observed shift of the resonance line due to the anisotropy of the effective field is of 0.6-07 T therefore the DB

r alone cannot explain the effect. A

similar decrease of the resonant frequency and the corresponding magnitude of the effective field with increasing τ has been observed in cubic perovskites, i.e. La0.9Ca0.1MnO3 (see chapter 4). As this could not be explained solely by the anisotropy of the dipolar field ( DB

r), an anisotropic spin-dipole contribution,


79

( ), arising from a slight distortion of the Mn4+ octahedra accompanying the J-T distortions of the Mn3+ octahedra, has to be taken into account.

dSB −

r

One can estimate the value of the core polarisation contribution to the hyperfine field ( FermiB

r), which is proportional to the spin moment of the parent

ion at –10 T/μB for manganese [Asano 1987]. With 3 electrons on the 3d orbital in a high spin state Mn4+ has the theoretical spin moment of 3 μB, so the core polarisation contribution to the hyperfine field of 30 T is expected, which corresponds to the resonant frequency of 316 MHz. However, the value observed is higher both in cubic and in bilayered manganese perovskites. The difference between the value expected for the core polarisation alone and the observed effective field in La0.9Ca0.1MnO3 reaches 6.6 MHz (≈0.62 T) while in LaSr2Mn2O7 it is 21 MHz (≈2 T). It can be attributed to the super-transferred hyperfine field from the neighbours, nB

r and to the dipolar contribution, DB

r. A

smaller difference observed for cubic perovskites can be explained by a lack of the contribution (due to the local symmetry closer to cubic one). The DB

rDBr

has a non-zero value in bilayered system, which has a tetragonal structure.

x=0.4 x=0.5 τ [μs] f

[MHz] Be [T] f

[MHz]Be [T]

30 336,7 31,91 337,0 31,93 60 333,2 31,57 333,1 31,56 90 331,0 31,37 331,1 31,37 120 330,2 31,29 329,9 31,26 150 329,3 31,20 329,0 31,18

Table 6.2. The values of the resonant frequencies obtained by the Gaussian curve fits to Mn4+ lines and the corresponding values of the effective fields effB

r

for spectra obtained with various pulse spacing, τ.

6.2.2 Mn3+ signal analysis

In the spectrum for x=0.3 taken at τ=8 μs (Fig. 6.2) and at the pulse power adjusted for optimal conditions for the DE line, a relatively weak Mn3+ line centred around 420 MHz is observed, which disappears quickly with increasing pulse spacing due to a fast spin-spin relaxation of the Mn3+ nuclei (i.e. short T2). Another set of measurements on x=0.3 doped compound with the pulse power adjusted at the Mn3+ and Mn4+ lines is presented in Fig. 6.4, which reveals the Mn3+ line persisting even at τ=60 μs.


80

300 325 350 375 400 425 450 475

Nor

mal

ised

inte

nsity

Frequency (MHz)

τ=12us

τ=30us

τ=60us

La1.4Sr1.6Mn2O7 (x=0.3)

Fig. 6.4 55Mn NMR spin echo spectra of La1.4Sr1.6Mn2O7 (x=0.3) at 4.2K at no applied field for different pulse spacing and the pulse power adjusted at the Mn3+ and Mn4+ lines.

The spectra of the x=0.4 and x=0.5 compounds at short pulse spacing

(Figs. 6.3a and 6.3b), show two Mn3+ lines. With increasing pulse spacing the Mn3+ line at the lower frequency disappears and the one at the higher frequency stays at the longest pulse spacing used (Fig. 6.3). Similarly to the situation observed for the Mn4+ resonance, the line which has a shorter T2 (lower frequency) is attributed to the domain wall centre and the line which has a longer T2 (higher frequency) is attributed to the domain wall edge, where the ionic magnetic moments are within or close to the easy magnetisation direction (EMD) [Weisman 1973, Davis 1976, Leung 1977]. The signal from domain wall centre corresponds to the ions with magnetic moments along (or close to) the hard magnetisation direction(s) (HMD).

As the x=0.3 compound has the EMD along the c axis and with increasing doping the easy axis changes to the easy plane (e.g. to the ab plane), the variation of resonant frequencies can be attributed to the change in the EMD. Therefore we can conclude that the Mn3+ line at the lower frequency comes from ions which have their magnetic moments along the c axis and the line at the higher frequency results from ions, which have their magnetic moments within the ab plane.

Let us denote and as the effective fields corresponding to the lower and the upper resonance line, respectively. Using the equation (6.8) for the hyperfine field,

1effBr

2effBr

HFBr

, one can write the following vector sums: ]001[1

dSBBBBBB orbDTFermieff −

++++=rrrrrr

(6.9) and

]100[2

dSBBBBBB orbDTFermieff −

++++=rrrrrr

, (6.10)


81

where the superscript in the dS

B−

r indicates the spin dipolar field produced by

electrons of Mn3+ ions with magnetic moments along the c axis ([001]) or within the ab plane ([100]).

One can group contributions to effective fields 1effBr

and into isotropic

and anisotropic parts. The isotropic part is

2effBr

FermiBr

, which can be estimated for Mn3+ in a similar way to that for the Mn4+ ion and it amounts to 40 T, that corresponds to the resonant frequency of 422 MHz. As was already mentioned in this chapter, anisotropic contributions are dSB −

r, orbBr

, DBr

and . The large anisotropy of the Mn3+ ion effective field observed cannot be explained solely by the anisotropy of the local contribution of dipolar fields ( ) from magnetic neighbours. This anisotropy was found to be less than 0.2 T in the compound studied, whereas the observed anisotropy of the Mn3+ effective field is of 2 T. Also the anisotropy of is the most likely smaller. For example, in the iron garnet the anisotropy of Fe3+ at the distorted octahedral position was found to be of 0.2 T [Stepankova 2000]. Therefore the observed Mn3+ effective field anisotropy has to originate from the anisotropy of the

TBr

DBr

TBr

orbBr

and/or . dSB −

r

The orbital field, results from an unquenched orbital moment, which was confirmed to be nonzero in cubic manganese perovskites both by theoretical calculations (for LaMnO3 compound) [Solovyev 1997, Radwanski 2004] and experimentally by means of X-ray circular dichroism (for the series of Sr doped cubic perovskites) [Koide 2001]. The experimentally obtained values of the orbital magnetic moment were up to 0.13 μB (for La0.67Sr0.33MnO3) and that theoretically calculated for LaMnO3 was of 0.24 μB [Radwanski 2004]. From polarized neutron diffraction results on the La1.2Sr1.8Mn2O7 compound the orbital moment was derived to be of 0.35 μB at 100 K [Argyriou 2002].

orbBr

Assuming that the hyperfine field anisotropy comes solely from the anisotropy of the orbital moment and following Streever’s calculations [Streever 1978, 1979] we obtained Eq. 6.11 that allowed us to derive the value of the orbital magnetic moment anisotropy:

[ ]( ) 33 ..2 −−

Δ=Δ

uarH

B

effBL μ

μμ (6.11)

where ΔHeff is the difference between values of the observed effective fields for Mn3+ (in cgs units), 3−r is the average reciprocal cube of the radius of the 3d orbital, which amounts to 4.7897 [Watson 1963] and [a.u.] is the atomic unit. The obtained value of the orbital magnetic moment anisotropy, ΔμL for x=0.4 and x=0.5 compounds is close to 0.04 μB. Taking the value of the spin-orbit coupling energy ESO=λLS, with λ=ζ/2S (ζ=46 meV [Van der Laan 1991] and S=2, for Mn3+ in the high spin state) one obtains ESO of order of 107 erg/cm3 while the observed magnetocrystalline anisotropy is of order of 105 erg/cm3


82

[Renard 2003]. Thus, the observed anisotropy of the Mn3+ hyperfine field can not be attributed to the anisotropy of the orbital moment, which would correspond to unphysical high magnetocrystalline anisotropy.

Therefore, the main contribution to the hyperfine field anisotropy has to arise from , which is given by the equation 6.1 and calculated at the beginning of this chapter for the electrons on the 3d orbital of the Mn ion. The results are presented in table 6.1.

dSB −

r

Assuming that the occupancy, α of the 22 yx − or 223 rz − orbitals in all the Mn3+ cations is the same and only the orientation of the magnetic moments varies from along the c axis to within the ab plane, this different orientation would result in an anisotropy of the spin dipolar field produced by the eg electron. If we have two partly occupied 3d eg orbitals one can write the following expression for produced by an electron on these orbitals: dSB −

r

( ) 2222 3]100[]100[

]100[ 1 rzyx BBBdS

−− −+=−

αα (6.12) Similar equation can be written for and α is the occupancy of the given

orbital. Assuming the sole anisotropy of the

]001[

dSB

−

dSB −

r the effBΔ can be obtained as:

( ) ( )[ ]22222222 3]001[]001[

3]100[]100[ 11 rzyxrzyx

eff BBBBB −−−− −+−−+=Δ αααα (6.13) Then one can easily derive α as:

22222222

2222

3]001[]001[

3]100[]100[

3]001[

3]100[

rzyxrzyx

rzrzeff

BBBBBBB

−−−−

−−

+−−

+−Δ=α (6.14)

For the x=0.4 compound one obtains α=0.48 and for x=0.5 α=0.46), which indicates a slightly higher occupancy of the 223 rz − orbital than the

one. This finding does not agree with the results derived from magnetic Compton profiles [Li 2004], where at low temperatures α for compound x=0.4 was found to be of 0.78. One has to note that our calculation uses a value of the

obtained for electrons in a free ion, not in the crystal. Moreover, as was mentioned earlier there are also suggestions in the literature that the

22 yx −

dSB −

r

223 rz − orbital can have its occupation larger than the 22 yx − one [Argyriou 2002].

It would be worth of comparing our way of calculation with ab initio approaches: the produced by one electron on the dSB −

r223 rz − orbital was

calculated by means of the LDA+U method for the Fe2+ ion (six electrons on a 3d band) in FeI2 compound at pressure 2.7 GPa and the calculated amounts to 15.3 T [Kunes 2003]. Following our calculations for the Mn ion one obtains the value of 24.75 T for Fe2+ (effective nuclear charge Z ff=11.18) [Clementi 1963 and 1967]. For the Fe3+ core in a high spin state, the of five electrons at the t2g and eg orbitals amounts to zero.

dSB −

r

e

dSB −

r


83

6.3 55Mn NMR at 77 K The 55Mn NMR spectra obtained at 77 K are presented in Figs. 6.5 and

6.6. The spectrum of x=0.3 doped compound consists of the DE line and a very weak line from Mn4+ ions (approx 330 MHz). For all the compounds studied the Mn3+ resonance is not observed due to a fast increase of the spin-spin relaxation rate of Mn3+ ions with temperature, similarly to the effect observed earlier in cubic perovskites [Algarabel 2003].

275 300 325 350 375 400 425 450

275 300 325 350 375 400 425 450

x=0.3

x=0.4

Frequency [MHz]

Nor

mal

ised

inte

nsity

x=0.5

Fig. 6.5. 55Mn NMR spin echo spectra of La2-2xSr1+2xMn2O7 compounds with Sr doping 5.03.0 ≤≤ x at 77 K and no applied field. The spectrum for x=0.3 was measured at τ=5 μs and for x=0.4 and x=0.5 at τ=7 μs.

The resonant frequency of the DE line of the x=0.3 Sr doped compound

decreases from 379 MHz (at 4.2 K) to 368 MHz (at 77 K), which corresponds to a change of the effective field by ≈1 T. The relative intensity of the Mn4+ line increases with Sr doping, indicating increasing Mn4+ content (Fig. 6.5). The spectra of the x=0.4 and x=0.5 Sr doped compounds at 77 K are more complex and more difficult to resolve due to the broader range of effective fields from the DE regions (possible coexistence of signals from domain walls and domains), similarly to the spectra measured at 4.2 K (Figs. 6.3a and 6.3b respectively). The x=0.5 compound is known from literature to reveal more complex properties than the other compounds in the doping range 5.03.0 ≤≤ x , and its spectrum at


84

77 K contains the signals from Mn4+ ions (300- 340 MHz) and from the DE regions (at about 380 MHz).

Fig. 6.6 55Mn NMR spin echo spectra of La1.4 Sr1.6Mn2O7 (x=0.3) at 77 K and no applied field for different pulse spacing, τ a) normalised and b) as measured.

275 300 325 350 375 400 425 450

275 300 325 350 375 400 425 450

τ = 5μs

τ = 10μs

τ = 30μs

Frequency [MHz]

Nor

mal

ised

ech

o in

tens

ity [a

rb. u

nits

]

τ = 60μs

La1.4Sr1.6Mn2O7 x=0.3

a)

275 300 325 350 375 400 425 450

2,0x10-7

4,0x10-7

6,0x10-7

8,0x10-7

τ=5 μs

τ=10 μs

τ=30 μs

Spi

n ec

ho in

tens

ity [μ

Vs]

Frequency [MHz]

τ=60 μs

The measurements of x=0.3 and x=0.4 Sr doped compounds at various

pulse spacing have also been carried out. The frequency swept spectra obtained for the x=0.3 doped compound are presented in Fig. 6.6. Fig. 6.6a presents normalised spectra and Fig. 6.6b - the spectra as measured, to show how the signal decreases with increasing pulse spacing. Fig. 6.7 presents measurements (normalised) for x=0.4 compound with various pulse spacing. For and the x=0.4 compound the Mn4+ line is not observed at the longest pulse spacing due to a fast relaxation rate, so that the nuclear spins relax out and the spin echo does not appear.


85

275 300 325 350 375 400 425 450

275 300 325 350 375 400 425 450

τ = 60μs

τ = 30μs

τ = 15μs

Nor

mal

ised

ech

o in

tens

ity [a

rb. u

nits

]

Frequency [MHz]

τ = 7μs

La1.2Sr1.8Mn2O7 x=0.4

Fig. 6.7 Normalised 55Mn NMR spin echo spectra at 77K and no applied field for different pulse spacing for La1.2 Sr1.8Mn2O7 (x=0.4).


86

6.4 139La NMR at 4.2K Fig. 6.8 presents the 139La NMR spectra at 4.2 K obtained for all the

compounds studied within the Sr doping range 5.03.0 ≤≤ x . As was already mentioned in the chapter presenting results on cubic perovskites, the NMR signal on 139La originates indirectly from the overlap of the Mn |3d> with oxygen |2p> wave functions, in conjunction with σ bonding of the oxygen with |sp3> hybrid states of the La3+ cation [Papavassiliou 1997]. The frequency swept spectra consist of a single line centred at 26.5 MHz for the, x=0.4 and x=0.5 compounds and at 24.5 MHz for the x=0.3 compound. The resonant frequencies are higher than in the case of cubic metallic perovskite La0.7Sr0.3MnO3, where 139La signal was observed at around 20 MHz (see Fig. 4.12a). However, the resonant frequency of the 139La signal in bilayered perovskites is similar to that observed in the La0.75Sr0.25MnO3 compound with 114 nm grains (see Fig. 6.8). The 139La resonant line in cubic perovskite with grains of nanometric size is broader (i.e. the distribution of the effective fields of La ions is bigger), which can be due to the smaller grain size.

15 20 25 30 35 40

15 20 25 30 35 40

x = 0.5

x = 0.4

x = 0.3

Nor

mal

ised

inte

nsity

Frequency [MHz]

La2-2xSr1+2xMn2O7

La0.75Sr0.25MnO3

Fig. 6.8 139La NMR spin echo spectra at 4.2 K and no applied field for La2-2xSr1+2xMn2O7 with 5.03.0 ≤≤ x and for La0.75Sr0.25MnO3 (sample with 114 nm grains). Solid curves are Gaussian fits to the experimental data, the straight line provides a comparison of line positions.


87

The difference between effective fields derived for x=0.3 and the other two compounds of bilayered system studied amounts to ≈0.22 T. This can be attributed to a different EMD (easy magnetization direction) for the compound with x=0.3 (c-axis) than in the other compounds studied (ab-plane) and to the corresponding anisotropy of the effective field at the lanthanum nuclei. Dipolar field contribution to the total effective field is different when Mn magnetic moments are along the c-axis, than when they are perpendicular to it. This difference was derived from a lattice sum over neighbouring manganese moments up to 3 nm distance and it amounts to 0.2 T and 0.13 T for the two different La sites in the structure, respectively (there are 2 and 4 La ions at different sites in the unit cell). The weighted average value amounts to 0.15 T, which is close to the observed difference of the La effective field, which amounts to ≈0.22 T. The line width is much larger than the difference of the dipolar fields at two La sites which makes the two fields unresolved. Publication related to problems raised in this chapter: Cz. Kapusta, D. Rybicki, P.C. Riedi, C.J. Oates, D. Zając, M. Sikora, C. Marquina, M.R. Ibarra, J. Mag. Mag. Mat. 272, 1759 (2004)

Chapter 7: Heavily doped La2-2xSr1+2xMn2O7

88

7. Heavily doped La2-2xSr1+2xMn2O7 7.1 Magnetic measurements of heavily Sr doped La2-2xSr1+2xMn2O7, x≥0.5

Before presenting the 55Mn MMR results magnetic measurements of

heavily Sr doped La2-2xSr1+2xMn2O7 (x≥0.5) carried out with the VSM magnetometer are discussed. The magnetisation, M versus applied field at several temperatures and M versus temperature, both zero field cooled (ZFC) and field cooled at 200 Oe (FC) curves, are shown in figures 7.1-7.6. M versus field curves of the compounds with x=0.5, x=0.62 and x=0.68 (Figs 7.1, 7.2 and 7.5 respectively) show a ferromagnetic-like behaviour. This indicates that, even if the compounds with x=0.5 and x=0.62 are reported in the literature to be antiferromagnets or with no long range magnetic order, for the x=0.68 compound a ferromagnetic component is observed. The magnitude of the magnetisation jump in the low fields ( the half of the distance between the two tangentals to the M versus field curve at high fields) amounts to 0.11 μB, 0.032 μB and 0.008 μB per Mn ion. for x=0.5, x=0.62 and x=0.68 Sr doped compounds respectively. Assuming that these values are proportional to the amount of the ferromagnetic phase, the latter could be estimated and it amounts to 36%, 10% and 3% for the x=0.5, x=0.62 and x=0.68 Sr doped compounds, respectively. From insets it can be seen that the coercive field also decreases with increasing Sr doping. Magnetisation versus field curves for x=0.75 and x=0.8 (Fig. 7.6) behave as for typical antiferromagnets.

-1,0 -0,5 0,0 0,5 1,0

-0,3

-0,2

-0,1

0,0

0,1

0,2

0,3

-0,1 0,0 0,1-0,05

0,00

0,05

M [μ

b/Mn

ion]

x=0.5 at 4K at 80K at 200K

μ0H [T]

Fig. 7.1 Magnetisation, M versus applied field at 4, 80 and 200 K for the LaSr2Mn2O7 (x=0.5) compound.


89

Both, the M versus applied field and M versus temperature curves show that the magnetisation decreases with increasing Sr doping. This is a result of a decrease of the average magnetic moment (it decreases from 3.5 μB for x=0.5 compound to 3.2 μB for x=0.8 compound), but the main reason is a “strengthening” of the antiferromagnetism. The magnetic structures reported for x=0.5 and x=0.62 compounds consist of ferromagnetic planes coupled antiferromagnetically, while x=0.75 and x=0.8 they have only ferromagnetically coupled “rods” [Mitchell 2001].

-1,0 -0,5 0,0 0,5 1,0

-0,06

-0,03

0,00

0,03

0,06

-0,05 0,00 0,05-0,02

0,00

0,02

M [μ

b/M

n io

n]

x=0.62 4K 120K

μ0H [T]

Fig. 7.2 Magnetisation M versus applied field at 4 K and 120 K for the La0.76Sr2.24Mn2O7 (x=0.62) compound.

Fig. 7.3 Magnetisation M versus temperature: zero field cooled (ZFC) and field cooled at 200 Oe (FC) for LaSr2Mn2O7 (x=0.5) and La0.76Sr2.24Mn2O7 (x=0.62) compounds. Arrows indicate Néel temperature, TN and charge ordering temperature, TCO reported in the literature [Mitchell 2001].

0 50 100 150 200 250 3000,000

0,005

0,010

0,015

0,020

0,025

0,030x=0.5 (200 Oe)

FC ZFC

M [μ

b/Mn

ion]

T [K]

TN

TCO0 50 100 150 200 250 300

0,002

0,003

0,004

0,005

0,006

0,007

0,008

0,009

M [μ

b/Mn

ion]

T [K]

x=0.62 ZFC FC

TNTCO

Magnetisation versus temperature measurements for x=0.5 and x=0.62

reveal Néel temperatures, TN similar to those reported in the literature [Mitchell


90

2001]. Additionally, a change of the M versus temperature behaviour is observed at 125 K, which can be attributed to the long range charge ordering temperature, TCO. Charge order was observed in the compounds with doping range x=0.5 to x=0.62, above TCO up to around 200-225 K by X-ray and/or electron diffraction [Li 1998, Argyriou 2000, Ling 2000, Li 2001, Mitchell 2002]. The behaviour of the M versus temperature dependence for the x=0.68 compound is similar to that of the x=0.62 doped one, but the magnetization measured is much smaller, which is due to the lack of long range magnetic order reported in the literature [Mitchell 2001].

Fig. 7.4 Magnetisation M versus temperature, zero field cooled (ZFC) and field cooled at 200 Oe (FC) for the La0.64Sr2.36Mn2O7 (x=0.68) compound.

Fig. 7.5 Magnetisation M versus applied field at 4 K, 40 K and 120 K for the La0.64Sr2.36Mn2O7 (x=0.68) compound.

-1,0 -0,5 0,0 0,5 1,0

-0,04

-0,02

0,00

0,02

0,04

-0,03 0,00 0,03-0,004

0,000

0,004

M [μ

b/M

n io

n]

x=0.68 4K 40K 120K

μ0H [T]

0 50 100 150 200 250 300

0,0016

0,0018

0,0020

0,0022

0,0024

M [μ

b/M

n io

n]

x=0.68 ZFC FC

T [K]


91

Fig. 7.6 Magnetisation M versus applied field: a) at 6 K and 200 K for the La0.5Sr2.5Mn2O7 (x=0.75) compound and b) at 5 K and 295 K for the La0.4Sr2.6Mn2O7 (x=0.8) compound.

-1,0 -0,5 0,0 0,5 1,0-0,03

-0,02

-0,01

0,00

0,01

0,02

0,03M

[μb/

Mn

ion]

x=0.75 6K 200K

μ0H [T]-1,0 -0,5 0,0 0,5 1,0

-0,03

-0,02

-0,01

0,00

0,01

0,02

0,03

M [μ

b/Mn

ion]

x=0.8 5K 295K

μ0H [T]


92

7.2 55Mn NMR results of heavily doped La2-2xSr1+2xMn2O7

In this section 55Mn NMR results of heavily doped bilayered manganese perovskites, La2-2xSr1+2xMn2O7 with 15.0 ≤≤ x are presented. Unlike low Sr doped samples studied in previous section, which were prepared as single crystals and were afterwards powdered, higher Sr doped samples studied in this section, were prepared as powders by the solid state reaction method (see chapter 2 in the section devoted to the sample preparation for more details). The NMR measurements were carried out on the Bruker Avance NMR spectrometer at 4.2 K. The lowest pulse spacing used on this spectrometer was about 12 μs, which was a limitation for measurements at 0 T.

Fig. 7.7 55Mn zero field NMR spin echo spectra at 4.2 K of La2-2xSr1+2xMn2O7 (for x=0.5, x=0.62 and x=0.68). Abbreviations used denote: AFI 4+ (Mn4+ ions in antiferromagnetic insulating regions), FMI 4+ (Mn4+ ions in ferromagnetic insulating regions), FMI 3+ (Mn3+ ions in ferromagnetic insulating regions), FM DE (Mn3+/4+ ions in ferromagnetic metallic regions where the double exchange interaction is effective).

250 300 350 400 450 500

250 300 350 400 450 500

FMI 3+

FM DE

FMI 4+x=0.68

x=0.62

Nor

mal

ised

ech

o in

tens

ity [a

rb. u

nits

]

Frequency [MHz]

x=0.5

AFI 4+

Fig. 7.7 presents normalised 55Mn zero field NMR spin echo spectra at 4.2

K of La2-2xSr1+2xMn2O7 (for x=0.5, x=0.62 and x=0.68) compounds. For samples


93

with x=0.5 and x=0.62 the spectra presented are an envelope of several frequency swept spectra with excitation conditions optimal for several frequencies in order to take into account the possible differences in excitation conditions and also to reduce the influence of standing waves in the transmission line. For the sample with x=0.68 two spectra are presented. The blue spectrum is an envelope of two measurements with optimal excitation conditions at 335 MHz (Mn4+ in FMI regions) and 380 MHz (the DE line). The green spectrum is obtained for optimal excitation conditions at 290 MHz (Mn4+ in AFI regions).

Starting from x=0.5 we see that spectrum presented in Fig. 7.7 is different from the spectrum for the same doping, but prepared as single crystal and powdered afterwards (see Fig. 6.3b, the bottom plot for pulse spacing 10 μs). Let us analyse possible reasons of observed differences. First of all, the preparation process of both samples is different (for more details see the chapter 2.7.2 dedicated to the sample preparation), also some differences in the frequency response between the two spectrometers used, i.e. the Bruker Avance spectrometer in Prague and the spectrometer in Cracow cannot be excluded.

Despite differences in relative intensities of observed resonance lines of both mentioned spectra, for both samples the same lines are observed i.e. the line due to Mn4+ ions in FMI regions, the DE line and the line due to Mn3+ ions in FMI regions due to the phase segregation in studied compounds. For x=0.5 Sr doped sample prepared as a powder (results presented in this chapter) these lines are observed at 329.47 MHz (31.23 T), 375.85 MHz (35.63 T) and above 400 MHz for the FMI Mn4+ line, the FM DE line and FMI Mn3+ signals, respectively.

In the x=0.62 compound, besides lines observed for x=0.5, i.e the Mn4+ line in FMI regions at 330 MHz, the DE line at 378 MHz and weak signals from Mn3+ ions in FMI regions above 400 MHz, an additional line centred at 284 MHz is observed. In the literature presenting the 55Mn NMR results on

manganese perovskites the lines below 300 MHz are ascribed to the Mn ions in antiferromagnetic insulating (AFI) regions [Allodi 1997, Kapusta 2000, Savosta 2000]. The resonant frequency and corresponding effective field, is lower in the case of the antiferromagnetic neighbourhood, because the sign of the transferred hyperfine field depends on the orientation of neighbouring spins, therefore the effective field for AF coupled neighbours is of opposite sign compared with that in the FM neighbourhood. It can be seen, if one writes the formula for the effective field in the following way:

effBr

0

2 BSBSAgBj

jjiiB

ieff +⎟

⎠⎞

⎜⎝⎛ += ∑μ

γπ (7.1)

where iS and jS are on-site and nearest neighbour electron spins, Ai and Bj are respective hyperfine couplings and B0 is the external field.


94

The distinction, which line is due to antiferromagnetic or ferromagnetic structure is not trivial, however it is possible with the measurements in the applied magnetic field. As was already mentioned in the chapter 5 devoted to La0.75Sr0.25MnO3 nanoparticles, if spins are coupled ferromagnetically, the resonant frequency shifts towards lower frequencies, which is due to the fact that the hyperfine field is dominant and antiparallel to the manganese moment. On the contrary, for an antiferromagnet with a low magnetocrystalline anisotropy, or easy magnetisation direction is of the easy-plane type, as is expected for the studied compounds in the doping range 15.0 <≤ x , the sublattice magnetizations will tend to align perpendicularly to the applied field component in the plane because the perpendicular susceptibility is larger than the parallel one. Hence, the resonant frequency corresponding to the effective field, which is a vector sum of the hyperfine field and a small applied field, changes only a little comparing to that at no applied field [Allodi 1997]. In such a situation the AF line only broadens without noticeable shifting in frequency [Allodi 1998].

Fig. 7.8 presents field measurements of La0.76Sr2.24Mn2O7 (x=0.62) compound at 0 T, 1 T and 1.5 Tesla at temperature 4.2 K. As expected for the signal from the antiferromagnetically ordered magnetic moments the line at 284 MHz does not shift in the applied field in contrast to the DE line at 378 MHz which shifts towards lower frequencies as expected for the ferromagnetically ordered moments. The resonant frequency of the DE line shifts to 367 MHz at 1 Tesla what agrees well with the value of the gyromagnetic ratio of the 55Mn, which is 10.553 MHz/T (11 MHz drop corresponds to decrease of the effective field by 1.05 T). The Mn4+ FMI line also shifts to lower frequencies in the applied field but due to its overlapping with other lines one can not determine its exact shift in applied field.

Second indication of the type of the magnetic ordering is the comparison of the enhancement factors, η. The enhancement factor for an antiferromagnet, ηAF (in the most effective case: external field in the easy plane, sublattice magnetizations perpendicular to the external field) is typically between 10 and 100, which is few orders of magnitude smaller than the enhancement factor for a ferromagnet, ηF. In the case of the x=0.62 compound optimal excitation conditions for the DE line were: pulse length 1μs while for the AF line at 284 MHz pulse length used was 5 μs and pulse amplitude, which corresponds to the rf field, was 10 times larger than for the DE line. Keeping in mind that the enhancement factor is inversely proportional to the product of the pulse length and radio frequency field (see chapter Introduction to NMR and equation 3.41) one obtains that ηAF is of 50 times smaller than ηDE (the enhancement factor in the ferromagnetically ordered DE regions).


95

225 250 275 300 325 350 375 400 425 450

225 250 275 300 325 350 375 400 425 450

Frequency [MHz]

at 1.5 Tesla at 1 Tesla at 0 Tesla

Nor

mal

ised

ech

o ite

nsity

[arb

. uni

ts]

Fig. 7.8. The 55Mn NMR spin echo spectra at 4.2 K for La0.76Sr2.24Mn2O7 (x=0.62) compound at 0 T, 1 T and 1.5 Tesla.

One has to keep in mind that the enhancement factor in the ferromagnetic material is much smaller for nuclei in domains than for nuclei in domain walls. However the possibility that the signal at 280-290 MHz comes from nuclei in ferromagnetically ordered domains (smaller enhancement factor) can be excluded since in the applied field such line should also shift towards lower frequencies, which is not the case.

In Fig. 7.7 also two frequency swept spectra for the x=0.68 compound are presented. As was already mentioned the blue one is an envelope of two measurements with optimal excitation conditions at 335 MHz (Mn4+ in FMI regions) and 380 MHz (the DE line). The green one is for optimal conditions at 290 MHz (Mn4+ in AFI regions). Separate spectra are presented due to much different optimal excitation conditions (much different enhancement factors) and the overlap of the AFI and FMI Mn4+

lines. For the x=0.68 compound, similarly to x=0.62 compound, three lines are

observed, the AFI Mn4+ line at 286 MHz, the FMI Mn4+ line at 317 MHz and the DE line at 380 MHz, but there are no signals above 400 MHz due to Mn3+ ions.. The neutron powder diffraction showed that in the doping region 0.66≤x<0.74 there is no magnetic long range order (LRO) [Ling 2000]. However the muon spin rotation study by Coldea et al. [Coldea 2002] revealed that in the x=0.68 compound at low temperatures short range magnetic order exists. Our NMR results support this finding and, furthermore, they indicate that the antiferromagnetic and the two different in electronic properties ferromagnetic regions (insulating and metallic) coexist.


96

300 325 350 375 400 425 450

300 325 350 375 400 425 450

τ=50 μs

τ=25 μs

Nor

mal

ized

ech

o in

tens

ity [a

rb. u

nits

]

Frequency [MHz]

τ=12 μs

x=0.68

Fig. 7.9 The 55Mn NMR spin echo spectra at 4.2 K of La0.64Sr2.36Mn2O7 (x=0.68) at no applied field and pulse spacing, τ=12 μs, 25 μs and 50 μs.

In order to be able to say more about the size of the ferromagnetic

metallic regions (due to the DE interaction) several frequency swept spectra with different value of pulse spacing were carried out and they are presented in Fig. 7.9. Unlike in the case of e.g. La0.75Sr0.25MnO3 compound, there is no clear minimum at the centre of the DE line in the spectra obtained at large pulse spacing. This indicates that the DE regions are smaller than the Suhl-Nakamura interaction range. Additionally, the FMI Mn4+ line disappears faster than the DE line with increasing pulse spacing, which is due to a faster relaxation rate of the Mn4+ nuclear moments. Comparing the width of the FMI Mn4+ line observed for the x=0.68 compound with that of the lower Sr doped compounds, one obtains that the line width of the x=0.68 compound is the biggest and amounts to 37 MHz. The line widths in the case of x=0.5 and x=0.62 compounds amount to 22 MHz and 26 MHz respectively. This difference is attributed to a larger distribution of the static local fields in the x=0.68 compound with no LRO. This result is also consistent with that of muon spin rotation measurements [Coldea 2002].


97

225 250 275 300 325 350 375

225 250 275 300 325 350 375

Nor

mal

ized

ech

o in

tens

ity [a

rb. u

nits

]

at 0T at 1T at 2.5T

x=0.75

x=0.8

Frequency [MHz]

Fig. 7.10 The 55Mn NMR spin echo spectra at 4.2 K of La0.5Sr2.5Mn2O7 (x=0.75) at 0 T (black), 1 T (blue) and 2.5 T (red) and of La0.4Sr2.6Mn2O7 (x=0.8) at 0 T (black) and 1 T (blue).

The 55Mn NMR spin echo spectra of La0.5Sr2.5Mn2O7 (x=0.75) and La0.5Sr2.5Mn2O7 (x=0.8) at 0 T and at the applied field 1 T and 2.5 T measured at 4.2 K are presented in Fig. 7.10. For both compounds at no applied field only one line centred at 290 MHz and 287 MHz for the x=0.75 and x=0.8 compound is observed, respectively. Note that magnetisation versus applied field curves presented earlier in this chapter did not show any ferromagnetic behaviour.

Also in the applied field one line is observed and it does not shift, which indicates that the line is due to Mn4+ in antiferromagnetically ordered regions. The enhancement factor for x=0.75 and x=0.8 compounds is smaller than for Mn4+ AFI line in x=0.62 and x=0.68 compounds. Even with first pulse of the spin-echo sequence 10 μs long and 300 Watt pulse amplifier, which was used for all measurements presented in this chapter, carried out on the Bruker spectrometer in Prague, we were unable to obtain maximum on the signal versus pulse attenuation scan. This also influenced the quality of the frequency swept spectra, due to a larger influence of the standing waves at the non-optimal excitation conditions. Usually, at the applied field the optimal excitation


98

conditions require more power (and/or longer pulses), which in our case means that we were more away from optimal conditions in measurements carried out at the applied field than at 0 T. The necessity for using very long pulses at the full power clearly indicates that the enhancement factor η for x=0.75 and x=0.8 compounds is smaller that in the case of the AFI line for x=0.62 and x=0.68 compounds. This indicates a more perfect AF order or/and a higher magnetocrystalline anisotropy in the former compounds. In fact, this is consisted with the magnetic structures determined by the neutron diffraction [Mitchell 2001]. The compound with x=0.62 has ferromagnetically ordered planes coupled antiferromagnetically (4 neighbours coupled FM and 2 AF coupled), while x=0.75 and x=0.8 compounds have ferromagnetically ordered "rods" (2 neighbours coupled FM and 4 AF coupled). Both, the small enhancement factor and a lack of shift of the resonance line at the applied field (see Table 7.1) indicates that the observed signal is due to antiferromagnetically ordered regions, similarly to the x=0.68 compound. Despite of several attempts of measurements of frequency swept spectra at the conditions which were optimal for the Mn4+ FMI line and the FM DE line in the other compounds, we did not observe lines due to ferromagnetic regions. This indicates that in x=0.75 and x=0.8 compounds such ferromagnetic regions do not exist or are too small to enable their observation by the NMR method.

0 T 1 T 2.5 T line width [MHz] 21.09 24.47 36.53

x=0.75 resonant freq. [MHz] 289.62 289.75 291.93 line width [MHz] 10.89 16.34

x=0.8 resonant freq. [MHz] 286.71 286.18 Table 7.1. The results of the Gaussian curve fits to the frequency swept spectra (see Fig. 7.10). Table presents the line width and central frequency for x=0.75 and x=0.8 compounds at 4.2 K and at 0 T, 1 T and at 2.5 T for x=0.75 compound.

We failed to observe any NMR spin-echo signal from the x=1 compound (all Mn next neighbours coupled antifferomagnetically). Attempts in the frequency range 200-350 MHz with excitation conditions similar to those used in the case of x=0.75 and x=0.8 compounds both at no applied field and at 1 T did not succeed and no spin echo signal could be observed.


99

7.3 Spin-spin relaxation times, T2 in heavily doped La2-2xSr1+2xMn2O7 For all the studied compounds of bilayered manganese perovskites within Sr doping range 15.0 ≤≤ x measurements of the spin-spin relaxation time, T2 were carried out. The representative results of the spin echo decay measurements are presented in Fig. 7.11. As can be seen from Fig. 7.11 unlike the spin-spin relaxation of La0.75Sr0.25MnO3 compound, where the spin-spin relaxation was two exponential, in heavily doped bilayered perovskites we have single exponential decay of the spin echo. This can mean a too short (much shorter than the minimum achievable pulse separation, 12 μs) spin-spin relaxation of the domain walls signals, to be observed. The results of the fits of a single exponential decay are presented in Table 7.2, where the following formula was used:

⎟⎟⎠

⎞⎜⎜⎝

⎛−+= =

20

2exp)(T

AAA n

ττ τ , (7.2)

similarly to the formula 4.1 in chapter 4, )(τA and are the spin-echo amplitudes at the pulse separation τ and τ=0, An parameter corresponds to the noise level.

0=τA

10 20 30 40 50 60 70 80 90 100

Log(

echo

inte

nsity

) [ar

b. u

nits

]

Pulse spacing [μs]

x=0.62 AFI Mn4+

x=0.62 FMI Mn4+

x=0.62 FM DE x=0.75 AFI at 0 T x=0.75 AFI at 1 T x=0.75 AFI at 2.5 T

Fig. 7.11 Spin echo decay curves at the resonant frequencies of all the observed lines for the compounds with x=0.62 (at no applied field) and x=0.75 (at no applied field and at 1 T and 2.5 T) at 4.2 K. The axis of echo intensity is in logarithmic scale.


100

Frequency

[MHz] T2 [μs] ΔT2

[μs] x=0.5 330 20.1 0.3 375 24.2 0.6

x=0.62 286 29.4 1.9 330 24.0 0.6 380 33.3 1.1

x=0.68 290 40.9 1.4 380 30.6 1.1

x=0.75 290 (at 0 T) 58.6 0.8 290 (at 1 T) 62.2 1.3 290 (at 2.5 T) 69.9 1.9

x=0.8 288 (at 0 T) 56.8 3.2

Table 7.2. Values of the spin-spin relaxation times, T2 and their uncertainties, ΔT2, for AFI Mn4+ line (at the frequencies of 290 MHz), FMI Mn4+ line (330 MHz) and the FM DE line (at the frequencies of 380 MHz).

Table 7.2 presents the spin-spin relaxation times of all the observed

resonant lines. Values of the T2 of the DE line for x=0.5, x=0.62 and x=0.68 are comparable to the T2 of the DE line for domains in the La0.75Sr0.25MnO3 compound at 4.2 K and at no applied field. This suggests that the main contribution to the observed DE line comes from nuclei in domains, rather than from domain walls. As was already mentioned in the chapter describing the NMR technique (chapter 3), the spin-spin relaxation of the nuclei in the FMM regions with effective DE interaction is due to fluctuations of the hyperfine fields related to the fast hopping of the electrons holes [Savosta 1999]. The spin-spin relaxation time of the FMI Mn4+ nuclei is shorter than that of the DE line. In the case of FMI state the mechanism of the spin-spin relaxation is different than in the case of the FMM state and is due to fluctuations of the hyperfine fields and of the electric field gradient (EFG) related to the motion of the Jahn-Teller polarons [Savosta 2003]. In the model proposed by Savosta et al., in [Savosta 2003] in the FMI state the charge carriers can be represented as the Jahn-Teller small polarons and their movement is accompanied by a lattice excitation leading to a fluctuation of the EFG. The T2 of nuclei in the FMI state was found to be shorter than T2 of nuclei in the FMM state in several cubic


101

perovskite manganites, for example in La0.9Ca0.1MnO3 [Algarabel 2003] or in La1-δMnO3 [Savosta 2003]. The spin-spin relaxation times of the AFI lines are longer than for nuclei in the ferromagnetic regions (see table 7.2). For x=0.75 and x=0.8 compounds the T2 from AFI state is two times longer than T2 from FM states (insulating or metallic) in x=0.5, x=0.62 and x=0.68 compounds.

For the x=0.75 compound measurements of the T2 at the applied field were also carried out (see Fig. 7.11 and table 7.2). The results indicate that the T2 increases by 20% at 2.5 T comparing to T2 at zero Tesla. Similarly to the T2 increase in the applied field in the FM state (see chapter 5) it can be attributed to the field dependence of the spin-spin relaxation resulting from the Suhl-Nakamura interaction. The Suhl-Nakamura interaction was found to play important role also in antiferromagnets, for example in MnF2 [Yasuoka 1969]. An increase of the T2 in the AFI state with the applied field was also found in cubic manganese perovskites [Allodi 1997].

Chapter 8: Conclusions 102

8. Conclusions The 55Mn and 139La NMR study of “cubic” and bilayered perovskites doped with Sr or Ca, which exhibit ferromagnetic insulating/metallic or antiferromagnetic insulating behaviour, lead to the following findings:

• The phase segregation is observed in all compounds studied and does not depend on the dimensionality of the compounds and the type of dopant (Sr or Ca). It is also observed on both sides of the phase diagram, below x=0.15 Sr doping (in “cubic” perovskites) and above x=0.5 (in bilayered perovskites). It was observed in both the 55Mn spectra and the 139La spectra of low doped “cubic” compounds.

• Temperature measurements of La0.875Sr0.125MnO3 compound reveal that ferromagnetic metallic regions exist above the bulk TC indicating existence of ferromagnetic polarons above this temperature. They also reveal that the magnetic coupling in the FMI regions is weaker than in the FMM ones.

• The Mn4+ signals range from below 300 MHz in the case of Mn4+ ions ordered antiferromagnetically, to 330 MHz for Mn4+ ions in FMI regions.

• Multi-line signals are observed for Mn3+ ions, indicating their considerable hyperfine field anisotropy. They range from 400 MHz to 550 MHz. In bilayered perovskites (for compounds with 5.035.0 ≤≤ x ) the occupation of 3d eg orbitals was derived from the hyperfine field anisotropy and a higher occupation of the 223 rz − orbital was concluded.

• The frequency swept spectra and the frequency dependence of the spin-spin relaxation time of the La0.85Sr0.15MnO3 compound at 4.2 K and at 77 K show the influence of the Suhl-Nakamura interaction between 55Mn nuclear moments within the DE-driven metallic-like regions. These regions in La0.85Sr0.15MnO3 are found to be larger than 10 nm while in La0.875Sr0.125MnO3 they are smaller.

• Nanoparticle powders of La0.75Sr0.25MnO3 show a degradation of magnetic properties with decreasing of the average grain size. Signals from ferromagnetic metallic grain interiors and ferromagnetic insulating outer layers of grains are identified.

• In heavily doped bilayered perovskites with 68.05.0 ≤≤ x signals from both antiferromagnetic and ferromagnetic regions are observed, also in the compound with no long range magnetic order (x=0.68). For compounds with x=0.75 and x=0.8 only signals from antiferromagnetic regions are observed. Also magnetisation measurements show no ferromagnetic behaviour of these two compounds.

Chapter 9: References

103

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10. List of author's publications 10.1 Publications related to thesis

1. J. Przewoźnik, Cz. Kapusta, J. Żukrowski, K. Krop, M. Sikora, D. Rybicki, D. Zając, B. Sobanek, C.J. Oates, P.C. Riedi “On the strength of the double exchange and superexchange interactions in La0.67Ca0.33 Mn1-

yFeyO3 – an NMR and Mössbauer study”, Physica Status Solidi B 243, 259 (2006)

2. D. Rybicki, M. Sikora, Cz. Kapusta, P.C. Riedi, Z. Jirak, K. Knizek, M. Marysko, E. Pollert and P. Veverka, “The 55Mn NMR study of the La0.75Sr0.25MnO3 nanoparticles”, Physica Status Solidi C 3, 155 (2006)

3. D. Rybicki, Cz.Kapusta, P.C.Riedi, C.J. Oates, M.Sikora, D. Zając, J.M. De Teresa, C. Marquina M.R. Ibarra, „A 55Mn NMR study of La0.33Nd0.33Ca0.34MnO3 with 16O and 18O”, Acta Phys. Polon. A 105, 183 (2004)

4. C.J.Oates, Cz.Kapusta, P.C.Riedi, M.Sikora, D.Zajac, D.Rybicki, C.Martin, C.Yaicle, A.Maignan „An NMR study of Pr0.5Ca0.5Mn0.97Ga0.03O3”, Acta Physica Polonica A 105, 189 (2004)

5. Cz. Kapusta, D. Rybicki, P.C. Riedi, C.J. Oates, D. Zając, M. Sikora, C. Marquina, M.R. Ibarra, „NMR study of layered manganite La1.4Sr1.6Mn2O7”, Journal of Magnetism and Magnetic Materials, 272-276, 1759 (2004)

10.2 Other publications

1. M. Sikora, D. Zajac, Cz. Kapusta, M. Borowiec, C.J. Oates,

V. Procházka, D. Rybicki, J.M. De Teresa, C. Marquina and M.R. Ibarra, „Evidence of unquenched Re orbital magnetic moment in AA'FeReO6 double perovskites”, Applied Physics Letters 89, 62509 (2006)

2. M. Sikora, Cz. Kapusta, K. Knížek, Z. Jirák, C. Autret, M.Borowiec, C. J.

Oates, V. Procházka, D. Rybicki, and D. Zając „X-ray absorption near-edge spectroscopy study of Mn and Co valence states in LaMn1−xCoxO3, x=0–1”, Physical Review B 73, 94426 (2006)

3. M. Sikora, C. Kapusta, K. Knížek, Z. Jirák, C. Autret, M. Borowiec, C.J.

Oates, V. Procházka, D. Rybicki and D. Zajac “XANES study of LaMn1-

xCoxO3 series”, Hasylab Annual Report, DESY, Hamburg (2005)

Chapter 10: List of author’s publications

111

4. C.J. Oates, M. Borowiec, C. Kapusta, D. Rybicki, M. Sikora, W. Szczerba, R. Ruiz-Bustos, P.D. Battle, M.J. Rosseinsky and E. Welter, “XAS study of Ru doped n=1,2 Ruddlesden-Popper manganites” Hasylab Annual Report, DESY, Hamburg (2004)

5. M. Sikora, D. Zając, M. Borowiec, C. J. Oates, V. Prochazka, D. Rybicki, Cz. Kapusta ”XMCD study of rhenium orbital moment in magnetoresistive double perovskites” Hasylab Annual Report, DESY, Hamburg (2004)

6. D. Zając, Cz. Kapusta, P.C. Riedi, M. Sikora, C.J. Oates, D. Rybicki, J.M. De Teresa, D. Serrate, C. Marquina, M.R. Ibarra, „NMR study of (Sr,Ba,La)2Fe1+yMo1-yO6 double perovskites”, Acta Physica Polonica A 106, 759 (2004)

7. D. Zając, Cz. Kapusta, P.C. Riedi, M. Sikora, C.J. Oates, D. Rybicki, D. Serrate, J.M. De Teresa M.R. Ibarra, „NMR and X-MCD study of Sr1-

3xBa1+xLa2xFeMoO6”, Journal of Magnetism and Magnetic Materials, 272-276, 1756 (2004)

8. M. Sikora, Cz. Kapusta, D. Zając, W. Tokarz, C.J. Oates, M. Borowiec, D. Rybicki, E. Goering, P. Fisher, G. Shütz, J.M. De Teresa, M.R. Ibarra, „XMCD magnetometry of CMR perovskites La0.67-yReyCa0.33MnO3”, Journal of Magnetism and Magnetic Materials, 272-276 (2004) 2148

9. D. Zając, M. Sikora, C.J. Oates, M. Borowiec, D. Rybicki, Cz. Kapusta, J. Blasco, J.M. De Teresa, C. Marquina, M.R. Ibarra, „X-MCD study of double perovskites (Sr,Ba,Ca)2FeReO6” Hasylab Annual Report 2003, DESY, Hamburg (2003)

10. D. Zając, Cz. Kapusta, M. Sikora, D. Rybicki, M. Borowiec, E. Welter, „XANES and X-MCD study of double perovskites (Sr,Ba,La)2FeMoO6”, Hasylab Annual Report 2002, DESY, Hamburg (2002)

11. M.Sikora, Cz.Kapusta, D.Zając, M.Borowiec, D.Rybicki, J.Żukrowski, M.Kapusta, P.Fischer, E.Goering, G.Schütz, E.Welter, „X-MCD magnetometry of mixed valence manganites (La,Re)0.67Ca0.33MnO3 (RE=Nd,Tb)”, Hasylab Annual Report 2001, DESY, Hamburg (2001)