ISSN 0020�1685, Inorganic Materials, 2013, Vol. 49, No. 9, pp. 939–942. © Pleiades Publishing, Ltd., 2013.Original Russian Text © V.V. Parfenov, A.V. Pyataev, I.I. Nig’matullina, Sh.Z. Ibragimov, 2013, published in Neorganicheskie Materialy, 2013, Vol. 49, No. 9, pp. 1008–1011.

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INTRODUCTION

Multicomponent 3d–4f metal oxides offer anextremely great diversity of physical properties, whichmakes them attractive for electronic applications. Thecolossal magnetoresistance of perovskite�structure 3d–4f metal oxides (rare�earth manganites and cobaltites),discovered in the 1990s, is most likely due to magneticphase separation in these materials [1]. Previously [2–5], Mössbauer spectroscopy was used to study the mag�netic microstructure and magnetic and electrical prop�erties of ferromanganites—solid solutions substitutedin both the rare�earth and 3d sublattices:Nd0.65Sr0.35FexMn1 – xO3, La1 – yPbyFexMn1 – xO3, andEu0.65Sr0.35Mn1 – xFexO3. It has been shown that thecompetition between ferro� and antiferromagneticinteractions in these solid solutions is favorable for mag�netic phase separation and formation of clusters thatdiffer in magnetic order from the major part of thematerial (matrix).

The objectives of this work were to synthesizeTm0.65Sr0.35FexMn1 – xO3 solid solutions and studytheir magnetic microstructure and macroscopic mag�netic properties. The choice of thulium was promptedby the fact that this heavy lanthanide has a smallerionic radius than do the other lanthanides. This leadsto significant changes in crystal structure because, atrelatively small ionic radii of the A cations, the АВО3

compounds lie at the stability limit of the perovskitestructure [6]. The heavy lanthanide manganites crys�tallize in hexagonal symmetry (ilmenite structure) [7]and possess properties of magnetoelectric materials(multiferroics). The concentration of iron cations (х =0.3, 0.35, 0.4) in this study has been prompted by thefact that, at such concentrations, the magnetic sub�system of the ferromanganites separates into individ�ual phases (at high iron concentrations, there is noseparation and ferromagnetic order is stable) [5].Phase separation leads to a number of nontrivial mag�

netic and electrical properties. In particular, lead�sub�stituted lanthanum ferromanganites and strontium�substituted europium ferromanganites with iron con�centrations х 0.4 have a high positive magnetoresis�tance near room temperature in relatively weakfields [2].

EXPERIMENTAL

Tm0.65Sr0.35FexMn1 – xO3 (х = 0.3; 0.35; 0.4) sam�ples were prepared by a standard ceramic processingtechnique using stoichiometric amounts of pure� andextrapure�grade Tm2O3, Mn2O3, Fe2O3, and SrCO3 asstarting materials. Powder mixtures were pressed intopellets at 180 MPa with no organic binder. The mixtureswere first sintered in air at a temperature of 1220 K for8 h. Next, the samples were reground and re�pressed.The intermediate and final sintering steps were per�formed at 1320 and 1420 K, respectively, each foranother 8 h. The heating rate was 10 K/min and thecooling rate was 5 K/min. X�ray diffraction character�ization showed that, after the final sintering step, all ofthe samples were single�phase and their crystal latticewas similar to that of ilmenite (FeTiO3, hexagonalstructure).

In magnetic measurements, we used a differentialthermomagnetic analysis (DTMA) system, whichemployed the Faraday method. The magnetic moment(M) of our samples and its temperature derivative(dM/dT) were measured as functions of temperaturebetween 100 and 900 K in a magnetic field of 0.2 T.Magnetization was measured as a function of field atroom temperature in fields of up to 1.5 T. The sensitivityof the DTMA system in magnetic moment measure�ments was 5 × 10–8 A m2 in a field of 0.2 T.

Mössbauer spectra were measured at temperaturesof 300 and 80 K in transmission geometry on anSM2201DR spectrometer. Low temperatures were

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Mössbauer Spectra and Magnetic Propertiesof Tm0.65Sr0.35FexMn1 – xO3 (x = 0.3, 0.35, 0.4)

V. V. Parfenov, A. V. Pyataev, I. I. Nig’matullina, and Sh. Z. IbragimovKazan (Volga Region) Federal University, Kremlevskaya ul. 18, Kazan, 420008 Tatarstan, Russia

e�mail: Viktor.Parfenov@ksu.ruReceived July 12, 2012; in final form, October 12, 2012

Abstract—We report Mössbauer spectroscopy results for Tm0.65Sr0.35FexMn1 – xO3 (x = 0.3–0.4) at 300 and80 K. Like in the case of lighter lanthanide ferromanganites, we observe phase separation of the magnetic sub�system: a magnetic phase shows up in the spectra in the form of a Zeeman sextet and “paramagnetic” dou�blets.

DOI: 10.1134/S0020168513090148

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PARFENOV et al.

ensured by a cold finger cryostat. The spectra were ana�lyzed using standard UnivemMS software.

RESULTS AND DISCUSSION

Mössbauer spectra of Tm0.65Sr0.35Mn1 – xFexO3. Theroom�temperature Mössbauer spectra of the thuliumferromanganites show two paramagnetic doublets(Fig. 1). This differentiates the spectra ofTm0.65Sr0.35FexMn1 – xO3 from those of the light lan�thanide (lanthanum, neodymium, and europium) fer�romanganites, which have only one doublet [2, 4].

All of the samples have roughly equal isomer shifts ofthe paramagnetic doublets, corresponding to trivalentiron, but the doublets differ significantly in quadrupolesplitting. The quadrupole splitting in one of the doubletsapproaches that in perovskite�like ferromanganites, andthe quadrupole splitting in the other doublet is roughly

twice as large (Table 1). Thus, these data strongly sug�gest that the materials contain Fe ions in two structur�ally inequivalent sites. Like in the lighter lanthanide fer�romanganites, we observe phase separation in the mag�netic subsystem: a magnetic phase shows up in thespectra as a Zeeman sextet.

The likely reason for the relatively large linewidth inthe sextet is that we observe a superposition of severalsextets, corresponding to different numbers of nearestneighbor iron and manganese atoms. At our measure�ment accuracy, however, further decomposition of thespectrum into its components would be meaningless.

The Mössbauer spectrum of theTm0.65Sr0.35Fe0.3Mn0.7O3 at T = 80 K ferromanganite ispresented in Fig. 2, and the Mössbauer parameters ofour samples are listed in Table 2.

In contrast to the room�temperature spectra, thelow�temperature spectra are well represented by acombination of two Zeeman sextets and one quadru�pole doublet. The isomer shift (IS) of all the compo�nents is larger than that in the room�temperaturespectra but also corresponds to a trivalent state of theFe cation (Table 2). The relative intensities of thecomponents lead us to conclude that the sextet in theroom�temperature spectra (Table 1) corresponds tosextet 1 in the low�temperature spectra (Table 2). Thisis particularly well seen for the х = 0.3 sample. Webelieve that this component of the spectrum arisesfrom the antiferromagnetically ordered ferromangan�ite matrix. The effective field at nuclei, Нeff, in thismagnetic phase increased with decreasing tempera�ture (from Нeff 32000 kA/m to Нeff 40000 kA/m),whereas the quadrupole splitting QS remained nearzero. In addition to this Zeeman sextet, another sextetis present in the low�temperature spectra (Table 2),with a weaker effective field and a larger quadrupolesplitting. Comparison of the relative intensity of thedoublets in the room�temperature spectra with that of

� �

100

96

92

88

1050–5–10

% t

ran

smis

sion

Source velocity, mm/s

Fig. 1. Mössbauer spectrum of Tm0.65Sr0.35Fe0.3Mn0.7O3(T = 300 K).

Table 1. Mössbauer parameters of Tm0.65Sr0.35FexMn1 – xO3at room temperature (T = 300 K)

x 0.3 0.35 0.4

Sex

tet

IS, mm/s 0.37 0.36 0.38

QS, mm/s 0.08 0.12 0.16

Heff, kA/m 30720 30080 32280

ρrel, % 23.2 15.7 14.1

Dou

blet

1 IS, mm/s 0.31 0.31 0.31

QS, mm/s 1.81 1.83 1.83

ρrel, % 24.8 28.1 24.9

Dou

blet

2 IS, mm/s 0.34 0.34 0.34

QS, mm/s 0.79 0.75 0.77

ρrel, % 51.9 56.1 60.9

100

98

96

1050–5–10

% t

ran

smis

sion

Source velocity, mm/s

94

Fig. 2. Mössbauer spectrum of Tm0.65Sr0.35Fe0.3Mn0.7O3 .

INORGANIC MATERIALS Vol. 49 No. 9 2013

MÖSSBAUER SPECTRA AND MAGNETIC PROPERTIES 941

sextet 2 in the low�temperature spectra leads us toconclude that the sextet results from the splitting of theformer doublet, which had a larger room�temperatureQS of 1.8 mm/s. Following Mostafa et al. [8], wethink that the doublet structures in the Mössbauerspectra correspond to a superparamagnetic state offerromagnetically ordered small clusters. Doublet 1,with a large quadrupole splitting at Т = 300 K, corre�sponds, in our opinion, to fivefold coordination(hexahedron) of the Fe cation. The other doublet inthe room�temperature spectra has a quadrupole split�ting QS 0.8 mm/s, typical of a distorted octahedralcoordination. Because of the difference in the numberof nearest neighbors (five and six), interatomic dis�tances, etc., the hexahedral and octahedral configura�tions will differ in the strength of ferromagnetic inter�action, which will lead to different blocking tempera�tures, characteristic of superparamagnetic systems, atwhich the fluctuation frequency of the magneticmoments of small particles becomes lower than theLarmor frequency of 57Fe (≈ 107 Hz). At lower tem�peratures, the fluctuation frequency decreases andthere is magnetic hyperfine splitting for clusters whosemagnetic fluctuation frequency is lower than the Lar�mor frequency of 57Fe. In general, because of thespread in cluster size, their should be a blocking tem�perature spectrum. Large clusters will exhibit theirmagnetic hyperfine structure at higher temperatures,and the smallest clusters will undergo ferromagneticordering at the lowest temperature.

The Mössbauer spectroscopy results lead us to con�clude that the clusters in which the Fe3+ cations are pre�dominantly in hexagonal coordination, are on thewhole larger than the clusters in which the Fe cationsare in octahedral coordination.

Magnetic properties of the Tm0.65Sr0.35xFexMn1 – xO3ferromanganites. The present magnetization versustemperature and external magnetic field data are wellconsistent with the above Mössbauer spectroscopy

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results. The magnetic moment per formula unit doesnot exceed 0.06μВ at 100 K and 0.1μВ at absolute zero(as evaluated by extrapolating the M(T) curve).

This lends support to the conclusion that the Zee�man sextets observed in the Mössbauer spectra, whosetotal intensity in the low�temperature spectra exceeds50%, correspond to an antiferromagnetic phase ratherthan to a ferromagnetic one. The magnetic field depen�dence of the magnetic moment for our samples (Fig. 3)is typical of magnetization reversal in single�domainsmall particles (the remanent magnetization is almostzero and the coercive force is small) [9].

It is seen that, in fields В ≥ 200 mT, the magnetiza�tion is a linear function of field. Figure 4 illustrates thedecomposition of the magnetization curve forTm0.65Sr0.35Fe0.3Mn0.7O3 into nonlinear (ferromag�netic) and linear (antiferromagnetic) components.

Table 2. Mössbauer parameters of Tm0.65Sr0.35FexMn1 – xO3at room temperature (T = 80 K)

x 0.3 0.35 0.4

Sex

tet1

IS, mm/s 0.50 0.56 0.51

QS, mm/s 0.0 0.0 0.0

Heff, kA/m 39520 39920 40160

ρrel, % 23.4 20.0 26.1

Sex

tet2

IS, mm/s 0.41 0.51 0.51

QS mm/s 0.79 0.52 0.79

Heff, kA/m 27040 27840 28000

ρrel, % 22.5 48.6 35.7

Dou

blet

IS, mm/s 0.39 0.37 0.41

QS, mm/s 0.90 0.90 0.91

ρrel, % 54.0 31.2 38.1

1.81.51.20.90.6

–0.3–0.6–0.9–1.2

15001200

900600–300 0

300

0.3

–1.5–1.8

–600–900

–1200–1500

B, mT

М, А m2/kg

Fig. 3. Magnetic field dependence of magnetization for theTm0.65Sr0.35Fe0.3Mn0.7O sample (T = 295 K).

1.2

1.0

0.8

0.6

0.4

0.2

5004504003503002500B, mТ

М × 10–3, А m2

20015010050

M (1) Mfer (2) Mantifer (3)

1

2

3

Fig. 4. Ferromagnetic and antiferromagnetic componentsof the magnetization of the Tm0.65Sr0.35Fe0.3Mn0.7O3sample (T = 295 K).

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PARFENOV et al.

In our opinion, the linear component of the curverepresents the action of the external field on the mag�netic moments of the ions in the antiferromagneticmatrix, and the nonlinear component is due to the mag�netization of ferromagnetic clusters.

CONCLUSIONS

After the final sintering step, theTm0.65Sr0.35FexMn1 – xO3 samples studied were single�phase. In the range of iron concentrations examined,the samples, as well as the rare�earth manganites, hadthe ilmenite structure, which differs drastically from thecrystal structure of the light lanthanide ferromangan�ites.

Throughout the range of iron concentrations stud�ied, we observed phase separation in the magnetic sub�system. The Mössbauer spectra T = 300 K of oursamples showed a Zeeman sextet of an antiferromag�netic matrix and two superparamagnetic doublets offerromagnetically ordered small clusters. In the 80 Kspectra, the paramagnetic doublet corresponding tohexahedral coordination of Fe cations was split intoa Zeeman sextet.

The present magnetization versus temperature andexternal magnetic field data lend support to the phaseseparation of the magnetic subsystem of the ferroman�ganites into an antiferromagnetically ordered matrixand small ferromagnetic clusters.

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magnetoresistance manganite conductors, Usp. Fiz.Nauk, 1996, vol. 166, no. 8, p. 833.

2. Parfenov, V.V., Bashkirov, Sh.Sh., Valiullin, A.A., andAver’yanov, A.V., Electrical and magnetic properties oflead�substituted lanthanum ferrimanganites, Phys. SolidState, 2000, vol. 42, no. 7, pp. 1310–1312.

3. Bashkirov, Sh.Sh., Parfenov, V.V., and Ihab Abdel�Latif,On the structure and transport mechanism ofNd0.65Sr0.35Mn1 – xFexO3 solid solution (x = 0, 0.4, 0.6,0.8), Arab. J. Nucl. Sci. Appl., 2007, vol. 40, no. 1,pp. 167–174.

4. Bashkirov, Sh.Sh., Boltakova, N.V., Parfenov, V.V., et al.,Mössbauer investigations of magnetic system stratifica�tion in europium and thulium ferromanganites, Bull.Russ. Acad. Sci., Phys., 2010, vol. 74, no. 3, pp. 373–376.

5. Eremina, R.M., Nigmatullina, I.I., Parfenov, V.V., et al.,Study of the magnetic phase separation in theEu0.65Sr0.35Mn1 – xFexO3 ceramics by EPR and Möss�bauer spectroscopy, Phys. Solid State, 2010, vol. 52,no. 11, pp. 2399–2404.

6. Vorob’ev, Yu.P., Men’, A.N., and Fetisov, V.B., Raschet iprognozirovanie svoistv oksidov (Properties of Oxides:Calculation and Prediction), Moscow: Nauka, 1983,p. 12.

7. Smolenskii, G.A. and Chupis, I.E., Magnetoelectricmaterials, Usp. Fiz. Nauk, 1982, vol. 137, no. 3, p. 415.

8. Mostafa, A.G., Abdel�Khalek, E.K., Daoush, W.M.,and Hassaan, M.Y., Structural, magnetic andelectrical transport properties of theLa0.70Sr0.30Mn0.96

57Fe0.04O3 + δ perovskite, HyperfineInteract., 2008, vol. 184, p. 167.

9. Krupi ka, S., Physik der Ferrite und der verwandten mag�netischen Oxide, Prague: Academia, 1973. Translatedunder the title Fizika ferritov i rodstvennykh im magnit�nykh okislov, Moscow: Mir, 1976, vol. 1, p. 109.

Translated by O. Tsarev

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