~ 1 Solid State Communications, Vol. 85, No. 9, 787-791, 1993. PP. Printed in Great Britain.

0038-1098/9356.00+ .00 Pergamon Press Ltd

MAGNETIC CIRCULAR X-RAY DICHROISM AT HIGH MAGNETIC FIELD AND LOW TEMPERATURE IN FERRIMAGNETIC HnCo2 AND PARAMAGNETIC Ho203

J.Ph. Schill6 c1~, Ph. Sainctavit¢2, 3), Ch. Cartier c2), D. Lefebvre TM, C. Brouder <2,4), J.P. Kappler cl,2~ and G. Krill <2,4~

c1)Institut de Physique et de Chimie des Mat6riaux de Strasbourg, Groupe d'Etude des Mat6riaux M6talliques, Universit6 Louis Pasteur, 67070 Strasbourg, France.

(2)Laboratoire d'Utilisation du Rayonnement Electromagn6tique, B~timent 209 D, 91405 Orsay, France.

C3)Laboratoire de Min6ralogie Cristallographie, Universit6s Paris 6 et 7, 75252 Paris, France. C4)Laboratoire de Physique du Solide de Nancy, 54506 Vandoeuvre-l~s-Nancy, France.

(Received 27 October 1992, in revised form 1st December 1992, by P. Burlet)

This letter presents Magnetic Circular X-Ray Dichroism at the Ho-M 5 edge in the ferrimagnetic HoCo2 and for the first time in a paramagnetic compound: the Ho203 oxide. The latter experiment is possible because of an experimental set-up allowing to work under high magnetic field (up to 7T) and at low temperature, the experimental spectra, isotropic absorption and dichroism, are compared to calculated ones using a fully local multiplet interaction model. The intensity of the Magnetic Circular X-Ray Dichroism being proportional to the average magnetic moment on the probed atom, its value is deduced from our measurements. Finally, we show that the shape of the Magnetic Circular X-Ray Dichroism is independent of the applied magnetic field, its intensity following the behaviour of the magnetic moment.

Magnetic Circular X-ray Dichroism (MCXD) is now widely used for the determination of local magnetic properties of ions in a solid 1.

In the presence of a magnetic field (either external or from exchang.e origin), the absorp- tion cross-section of a given element becomes strongly dependent on the state of polarization of the light and then direct information about the magnetic properties of the ion which has absorbed the photon may be obtained. If circu- larly polarized light is used instead of lin- early one, information may be obtained not only about the magnitude of the local magnetic moment, but also about its direction. In this latter case, however, studies are in principle restricted to ferro- or ferri-magnetic ground states. It is quite obvious that it is not possible to study an antiferroma~netic ground state by using circularly polartzation, since MCXD at the M 4 5 edges of Rare Earth metals is proportional to t~e average magnetic moment on the absorbing atoms. However, there is no reason which may prevent the observation of MCXD effects in a paramagnetic compound, effec- tively in this case the MCXD effect is pro- portional to the paramagnetic moment value which depends directly on the B/T ratio of the magnetic field and the temperature. In this letter, we present the first experimental evi- dence for the existence of such MCXD effects in the paramagnetic Ho203 oxide at low temperature (T = 10 K) and high magnetic field (up to 5T). Our study is performed at the M s edge of Holmium in Ho203 oxide which has developed at the surface of a ferrimagnetic intermetallic compound (HoCo2). We present the results ob- tained on both a "clean" and "contaminated" HoCo z surface and comparison with the case of pure Ho203 powder will be displayed.

The experiments presented in this letter were performed on the beam line behind the new inserted asymmetric hybrid wiggler on the Super-Aco storage ring at LURE (Orsay). The description of the beam line and its main per- formances were detailed in previous papers 2. Particularly, it was demonstrated that the flux delivered in the 1 keV range (typically Ip = 1012 photons/sec/0.1%/mradhor) is about one order of magnitude higher than that of a conventional bending magnet beam line. As the rate of circular polarization (~) is at least as high as for a bending magnet beam line, this beam station is especially dedicated to the Magnetic Circular X-ray Dichroism studies.

The data are recorded with an incident beam selected with a pair of slits, located at 14 m from the wiggler center, delimiting a 0.07 mrad vertical aperture and centred at 0.3 mrad above the orbit plane i.e. the helicity of the photons used is left (or +h). This angular aperture has been chosen in order to get the maximum signal noise ratio in the relevant energy range. The photon beam is monochroma- tized with a constant deviation double crystal monochromator (two independent rotations and one translation).

The absorption spectra presented here were recorded with natural beryl crystals (2d = 15.95 ~) which allow us to cover an en- ergy range from 800 to 1550 eV, i.e. typically the M 4 s edges of the Rare Earth (RE) elements which " involve 3d~/2 5/2 ~ 4f transitions. The energy resolution ot {he monochromator, within the Ho M~, 5 range of energy, is about 0.5 eV. In this case, after the two Bragg reflexions on the beryl crystals, we calculate, on the basis of the beamline optics, the rate of circular polarization z to be around 9% for the con-

787

788 MAGNETIC CIRCULAR X-RAY DICHROISM

sidered energy range3, 4. The Holmium M s absorption spectra were

obtained in an Ultra High Vacuum (UHV) chamber (P = 1.10 -9 mbar) at normal incidence by measuring directly the sample current which is obviously related to the total yield photocur- rent. In order to be able to measure properly a current of a few pA on samples which have to be cooled down to 4 K, electrical isolation (but not thermal) is achieved by a sapphire inter- face between the cryosystem and the sample holder. This type of measurement overcomes the difficulties encountered when the detection is made by channeltron in the presence of a mag- netic field. In our case, the high magnetic field canalizes the photoelectrons and thus improves the stability of the sample current (down to one fA). To take into account the variation of the beam intensity during the experiment, we measure simultaneously, in a separate UHV chamber, the incident flux by collecting the photocurrent (I0) on a 90% transmitting A1 foil (0.7/~m thick). Because of the high absorption of the X-rays at the used energies, it stands to reason that the exper- iments are sensitive to surface effects. In the 1-1.5 keV photon energy range, it was esti- mated 5 that the thickness probed by photocur- rent detection should be typically in the 20- 60 ~ range, and probably not more.

The magnetic field is produced by a split coil superconducting magnet produced by American Magnetics Inc. and fitted by TBT France. The direction of the magnetic field (which is fixed by the sense of the current in the coil) is either parallel or antiparallel to the photon propagation vector (~¢) and its in- tensity may be as large as 7 T. The temperature of the sample holder, which fits in the verti- cal bore of the coil, may be continuously changed from 10 to 300 K. The sample, in a first step mounted in a preparation chamber connected to the main chamber, can be scrapped

Ms

r=10K ' ~ E.., '~' ( .v) ' "

:: + 4T - 4T

~ t V

4 .5 0

0

,.Q <

~q 1.5

E 0 0.0 z

- 1 . 5 1 5 3 5 1 3 4 5 1 3 5 5 1 3 6 5

Energy (eV)

Fig. 1. Ho M s normalized absorption spectra (upper) with left circular polarization light in HoCo 2 at T = 1 0 K in B = + 4 T ( - - ) and B = - 4 T (.), after scraping the sample sur- face. MCXD spectrum (lower) of Ho obtained by difference of the two upper spectra. Inset: comparison of the MCXD signal for 1 T (--) and 4 T (+), scaled for the intensity at 1350 eV.

Vol. 85, No. 9

4.5

o

" ~ 3,0

m ,.o

,~j 1.5

E ~ 0.0 0

Z

I

- 1 . 5 1335 1545 1355 1365

E n e r g y ( e V )

Fig. 2. Ho M s normalized absorption spectra (upper) with left circular polarized light in HoCo z at T = 1 0 K in B = + 5 T ( - - ) and B = -5 T (.), before cleaning the surface sample. MCXD spectrum (lower) obtained by dif- ference of the two upper spectra. The scale factor × 2 refers to the scale of the ab- sorption intensity.

with a diamond file in order to ensure the cleanness of the surface, and then it is trans- ferred (always under UHV conditions) into the sample holder at the center of the supercon- ducting coil.

The polycrystalline compound HoCo 2 was prepared by arc melting followed by annealing under vacuum at 900°C for one week. The charac- teristic MgCu 2 Laves phase structure was checked by X-ray diffraction and the magnetic properties were measured in order to ensure that both the Curie temperature (T¢ = 77 K) and the value of the magnetic moment at saturation

3 ~' " '

O :S

o 2 ~o,o~ I 1 " , L ,.~ ~ l Ex~erlly (eV) " ~ - : H=+5T ,.~ , : H=-ST

N

0 0 " 2;

Energy (eV)

Fig. 3. Ho M s normalized absorption spectra (upper) with left circular polarization light in Ho203 at T = 1 0 K in B = + 5 T ( - - ) and B = - 5 T (.). MXD spectrum (lower) of Ho ob- tained by difference of the two upper spectra. The scale factor × 3 refers to the scale of the absorption intensity. Inset: Comparison of the Ho MCXD spectra of HoCo2 ( - - ) and Ho203 (+), scaled for the inten- sity at 1350 eV.

Vol. 85, No. 9 MAGNETIC CIRCULAR X-RAY DICHROISM 789

(M s = 7.8 as/formula unit) are in agreement with previous results 6 The Ho20 3 sesquioxide powder (99.9% p u r e ) c o m e s from R E Products company. To avoid charge effect, a thick pellet was prepared by compressing a mixture of HoaO3 powder and amorphous carbon. Our magnetic measurements, with a conventional magnetometer, demonstrate that no magnetic ordering occurs down to 2 K, for Ho203 sesquioxide.

In Figs. 1, 2 and 3, we report the M s X-ray Absorption spectra of Holmium in respect- ively HoCo 2 (clean Surface), HoCo 2 (contami- nated Surface) and in the Ho203 sesquioxide respectively. All the spectra were taken at the same temperature T = 10 K and for a left hell- city of the photons. The MCXD effects are studied by changing the sense of the magnetic field which is fully equivalent to changing the helicity of the light. Typical accumulation times are around twenty minutes for one direc- tion of the field, the inductance of the coil is responsible for a dead time, when the field is changed, of about a minute per Tesla. The MCXD spectrum is obtained within an hour.

Only the M edge is presented in this letter because o~ the low absorption cross sections at the M 4 edge and consequently the reduced MCXD. However, we have checked that the expected MCXD is obtained at the M 4 edge.

Since the yield of photoelectron detection depends on the intensity and sign of the applied magnetic field, a normalization is required to compare our experimental spectra. Let ~+ ((r-) be the spectrum obtained with left circularly polarized X-rays and magnetic ofield applied parallel (antiparallel) to k. Let ~ be the spectrum obtained with linearly polarized light (in the orbit plane) and with the same magnetic field (of any sign). Then for samples which have a rotation symmetry C n (n -- 3) around ~, we have cr ° = 1/2 (~+ + ~-) if all spectra are normalized correctly. This con- dition is used ' + - with to normahze ~ and respect to ¢o by fitting a and b to minimize the difference between cr ° and 1/z (a~ ÷+lxr-). With these conventions, t h e . MCXD spectra plotted here are defined as ¢ - - ¢ .

In the following parts of this letter the MCXD effect will be quantitavely described by the ratio of the MCXD amplitude to the maximum absorption obtained with non polarized light at the considered edge.

With such a convention, as shown on the lower parts of Figs. 1, 2 and 3, large MCXD effects are observed at the M s edge. The MCXD effect is 25% for a clean HoCo2 surface, 12% for a contaminated HoCo2 surface and 9% for Ho203.

The analysis of the shape of the MCXD effect shows that it is sample independent (see the comparison of the Ho MCXD in HoCo 2 and Ho203 in the inset to the Fig. 3). Moreover it does not depend on applied magnetic field or temperature (see inset Fig. 1). In all cases the MCXD signals are characterized by a small

low energy contribution followed by a rge negative part centred on the main absorp-

tion structure. Fig. 4 shows the good agreement of shape

between our experimental results and the atomic calculation for optical transitions from the 3d1°4f ~ ground state to the dipole allowed

0.3

0 .~ 0.0

O

- 0 . 3

• _.~ - 0 . 6

O Z - o . 9

Y }

T= 10K t

-~&,~o 13'45 135o ~3'ss 136o Energy (eV)

Fig. 4. Comparison between the experimental MCXD spectrum of Ho in the HoCo 2 compound (-) at 10 K and the atomic calculation MCXD of trivalent Ho ion ( ). The intensities are normalized at 1350 eV.

manifold of the 3d94f n+l multiplets, reported in Ref. [7]. The relative intensities o f the main positive and negative contributions as well as both their width and splitting in en- ergy match in a very satisfying way. Even the small features in the high energies part of the spectra are present and nicely resolved. Since the calculation of Goedkoop 7 was carried out by considering Ho as an isolated ion, we see that non-atomic effects (such as band structure and/or crystal-field effects) contribute weakly to the shape of MCXD spectra of rare earths. However, such contributions obviously influence the magnetic ground state of Ho and we hope that in the future, new calculations will allow to extract from those MCXD studies relevant solid state parameters.

The mgn of the MCXD for our samples is in all cases the same as for pure Ho, which is in agreement with the fact that the magnetic moment on the Ho atoms is parallel to the net magnetization. This is obvious in the case of the oxide. For the intermetallic compound since the magnetic moments on Ho and Co atoms are antiparallel in HoCo2 (as shown by neutron diffraction experimentsS,9), it confirms that

12 ~- H o C o 2

._~ 6 T=IOK

2 4 6

M a g n e t i c F ie ld (T )

Fig. 5. Magnetization at T = 10 K as a function of external magnetic field: full circle = MCXD intensity in clean HoCo2, full s q u a r e - - M C X D intensity in Ho203, open square = MCXD inten- sity in as cast HoCo2, the solid lines = measured magnetic moment of Ho in HoCo2 and Ho203, at T = 10K.

790 MAGNETIC CIRCULAR X-RAY DICHROISM Vol. 85, No. 9

the magnetic moment on Holmium is greater than that on Cobalt. However it should be stressed that the magnetic coupling between two types of atoms in a intermetallic compound can be ob- tained straightforwardly by recording the MCXD spectra at the M 4 or M 5 edge of the Rare Earth and at the L e or I_~ edge of the Transition Metal and comparing their respective signs with those obtained for the corresponding pure metals 10,11 or with the calculated ones.

Theoretical expressions of MCXD at RE M s edges show that the amplitude of the dichroic signal is proportional to the magnetic moment and to the rate of polarization zA2. The ab- solute value of the 4f magnetic moment can be extracted from MCXD measurements We take benefit of the calculated rate of circular polarization obtained from the optical charac- teristics of the beamline used in these exper- iments 4 and of the amplitude of the calculated MCXD spectrum 7A2. For an isolated Ho ion bearing a magnetic moment M4¢ = gJ --- 10 taB, fully circular polarized light leads to a MCXD effect whose amplitude is about 300% of the maximum isotropic absorption.

The reduced MCXD observed (about 25%) is due to the actual rate of circular polarization (around 9%) and to the fact that in HoCo 2 the Ho atom cannot be considered as fully isolated. With all these considerations, in HoCoz, the average moment on Ho is found to be about 9 ~B, at B = 4 T and at T = 10K. AS our magnetic measurements lead to a macroscopic satured moment of 7.8 taJformula unit , the MCXD con- firms an antiferromagnetically coupled Co mag- netic moment. These values are in agreement with those obtained by neutrons measurements 9 giving 9.5 tab on a Ho atom and 1 ta B on a Co atom.

Measurements to determine the spin and orbital contributions to the magnetic moment of Rare Earths or Transition metals using the recent sum rule for spin-orbit split edges developed in Ref. [13] are under progress for our HoCo2 compound.

The dependence of the characteristic Ho MCXD signal has been studied as a function of temperature and magnetic field. The temperature variation ( 1 0 K < T < 80K) in the case of HoCo 2 (which is not reported here) shows that the MCXD integrated intensity follows the vari- ation of the magnetic moment according to the fact that the MCXD cross-section is pro- portional, at least for M4,s. edges of Rare

Earth, to the average magnetic moment carried by the probed atom.

The Fig.5 presents the field dependence of the magnetic moment extracted from the MCXD integrated intensities for the systems under study compared to data obtained on the same sample with a standard magnetometer. As shown in this figure the data match in a satisfactory way for HoCo2, the discrepancies are due to surface effects related to anisotropy with surface roughness. In the case of the contami- nated HoCo 2 surface the magnetic moment is in between the one in HoCo 2 and the one in the oxide. For the pure Ho203 oxide, the moment extracted from the MCXD study is lower than the bulk one: that can be easily explained by the poor thermal conduction of the sample leading to a small temperature difference between the sample holder and the powder pellet. The Brillouin function, characteristic of the mag- netic variation in the paramagnetic state being strongly dependent on the temperature, the magnetic moment is overestimated.

In conclusion, with the accuracy of our experiments, our results of MCXD study in high magnetic field and low temperature show that the shape of the MCXD signal observed at the M4 5 edges of Ho is independent of the environ- melit o f the absorbing atoms, of the magnetic field and the temperature. As predicted by atomic calculations and again taking into ac- count the uncertainties linked to our exper- iments, the MCXD intensity at the M4, s edge is proportional to the average magnetic moment on the probed atom whatever the magnetic state of the atom is. Moreover, the sign of the MCXD signal can give the relative orientation of the sublattice magnetization.

Due to the high absorption cross-sections in the energy range of RE M edges and conse- quently the sensitivity to surface effects as well as the ability to probe little amounts of matter, one can expect that MCXD experiments with RE thin layers or artificial structures using RE under a less absorbing element are feasible. This would be of high interest for the study of magnetic properties of surfaces or thin layers even in the paramagnetic state.

The authors would like to thank G. Schmerber for the preparation and the crys- tallographic characterization of the HoCo2 compound.

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Vol. 85, No. 9 MAGNETIC CIRCULAR X-RAY DICHROISM 791

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