The effect of nonmagnetic ion substitution for the Fe Cr 2 − x M x S 4 ( M = Ga , In ) byMössbauer spectroscopyBae Soon Son, Sam Jin Kim, Koan Sik Joo, and Chul Sung Kim Citation: Journal of Applied Physics 99, 08F715 (2006); doi: 10.1063/1.2177425 View online: http://dx.doi.org/10.1063/1.2177425 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/99/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Magnetic properties of the ferrimagnetic FeCr 2 − x M x S 4 ( M = In , Al ) J. Appl. Phys. 107, 09A501 (2010); 10.1063/1.3337662 The effect of manganese ions in a Mn Cr 1.98 57 Fe 0.02 O 4 by Mössbauer spectroscopy J. Appl. Phys. 103, 07E313 (2008); 10.1063/1.2838468 Mössbauer studies of the magnetic phase transition in Fe 1 − x Zn x Cr 2 S 4 J. Appl. Phys. 103, 07B728 (2008); 10.1063/1.2838013 Mössbauer spectroscopy and neutron diffraction studies of the ferrimagnetic semiconductor on Ga-substitutedFeGa x Cr 2 − x S 4 J. Appl. Phys. 97, 10D322 (2005); 10.1063/1.1854051 Anomalous magnetic properties of the ferrimagnetic semiconductor on Ga-doped sulphur spinel J. Appl. Phys. 95, 6828 (2004); 10.1063/1.1676095

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The effect of nonmagnetic ion substitution for the FeCr2−xMxS4 „M=Ga, In…by Mössbauer spectroscopy

Bae Soon Son and Sam Jin KimDepartment of Physics, Kookmin University, Seoul 136-702, Korea

Koan Sik JooDepartment of Physics, Myongji University, Yongin, Kyungki 449-728, Korea

Chul Sung Kima�

Department of Physics, Kookmin University, Seoul 136-702, Korea

�Presented on 1 November 2005; published online 3 May 2006�

The Mössbauer spectroscopy has been studied for the sulphur spinel FeCr2−xMxS4 �M =Ga, In� atvarious temperatures, from 4.2 K to room temperature. The spectra consist of two doublets at roomtemperature, which show that the Ga and In ions are partially occupied to the tetrahedral �A� site.It is found that the Ga and In ions stimulate the asymmetric charge distribution of Fe ions in the Asite. The electric quadrupole splittings �∆EQ� of the A and B sites in the Mössbauer spectra ofFeCr2−xGaxS4 �x=0.3� are 0.83 and 2.94 mm/s, respectively, while those for the FeCr2−xInxS4 �x=0.3� are 0.54 and 1.54 mm/s, respectively. The ∆EQ for the Ga doped samples are larger than thatfor the In doped samples, in spite of the larger ionic radius for In ions. We suggest that strongercovalence associated with the smaller bond length includes a large asymmetric charge distribution.© 2006 American Institute of Physics. �DOI: 10.1063/1.2177425�

I. INTRODUCTION

The discovery of the colossal magnetoresistance �CMR�phenomenon in the sulphur spinel has attracted considerableattention, because of the potential for technological applica-tions and the interesting physical properties of materials.1,2

The existence of local structural distortions was suggestedthat the quadrupole splitting and low-temperature anomaly ofthe electric field gradient induced on the tetrahedral coordi-nated Fe2+ ions.3,4 These features were attributed to a strongcoupling between the Jahn-Teller �JT� active ferrous ionswhich allows a distortion of the Fe2+S4

2− tetrahedrons5 andwere explained in the frame work of static and dynamic JTeffects.6 These systems have been revisited more recently,showing several unique properties such as their metal-to-insulator transition at high pressure, their ability to form bothmetallic and insulating spin glasses,7–10 geometrical frustra-tion of the spin, relaxor ferroelectricity, and colossal magne-tocapactive effect in sulphur spinel.11–13 Therefore, it is nec-essary to study the properties of the various compounds inthe sulphur spinel.

In this paper, we report the results of the Mössbauerexperiments and compare them with those of x-ray and mag-netization for the nonmagnetic Ga and In doped sulphur spi-nel compounds.

II. EXPERIMENT

The synthesis of the FeCr2−xMxS4 �M =Ga, In; x=0.1,0.3� was accomplished by the direct reaction of thehigh-purity elements Fe, Cr, Ga, In, and S in an evacuatedquartz tube. The structure of the samples was examined us-

ing x-ray diffractometer �XRD� with Cu K� radiation andanalyzed by the Rietveld refinement. The magnetization forthe samples was obtained using vibrating samples magneto-meter �VSM�. The Mössbauer spectra were recorded using aconventional spectrometer of electromechanical type with a57Co source in a rhodium matrix.

III. RESULTS AND DISCUSSION

The crystal structure of the FeCr2−xMxS4 �M =Ga, In; x=0.1,0.3� is found to be a cubic spinel. Figures 1 and 2 showthe results of x-ray diffraction refinement for the Ga and Indoped samples at room temperature, respectively. The solidlines represent the fits of a Rietveld analysis. The spectrashown in Figs. 1 and 2 demonstrate the absence of any im-purity phases. The determined crystal symmetry of thesamples is a cubic spinel structure Fd3m �Fe, Ga, In �8a�, Cr,Ga, In �16d�, and S �32e� �u ,u ,u��. The lattice constants a0,anion parameter u, and bond lengths d are listed in Table I.

The Néel temperature �TN�, which is defined as the tem-perature of the maximum slope in dM /dT, increases from180 to 188 K with the increase of nonmagnetic Ga substitu-tion from the x=0.1–0.3. It can be attributed to a ferrimag-netic structure in these samples. According to the x-ray dif-fraction analysis, Ga ions enter into both the B and A sites,though the majority of the Ga ions are in the B site. Simul-taneously the same amounts of Fe ions migrate from the A tothe B site. It leads to enhancement of superexchange interac-tion and TN.14 for FeCr2−xInxS4, TN decrease from173 to 160 K with increasing x. The decrease in TN inFeCr2−xInxS4 can be understood as the consequence of reduc-tion of superexchange interaction. The x-ray diffractionstudy does not show any difference in the crystal and mag-

a�Author to whom correspondence should be addressed; electronic mail:cskim@phys.kookmin.ac.kr

JOURNAL OF APPLIED PHYSICS 99, 08F715 �2006�

0021-8979/2006/99�8�/08F715/3/$23.00 © 2006 American Institute of Physics99, 08F715-1

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netic structures between the Ga doped samples and the Indoped samples. Therefore, it is necessary to understand themicroscope interaction behaviors.

Figures 3 shows the Mössbauer spectra for theFeCr2−xMxS4 �M =Ga, In; x=0.1,0.3� at room temperature.Both spectra for the Ga doped samples consist of two dou-blets at room temperature. One corresponds to the iron ionsat the A sites �inner doublet�, the other corresponds to theiron ions at the B sites �outer doublet�. The Mössbauer spec-

trum for the In doped FeCr1.7In0.3S4 has the resemblance ofFeCr2−xGaxS4. It is noticeable that the nonmagnetic Ga andIn ions stimulate asymmetric charge distortion of Fe ions inthe A sites, while a single line pattern is shown for theFeCr1.9In0.1S4. The quadrupole doublets are enhanced withincrease of Ga and In ions, as shown in Fig. 3. The electricquadrupole splittings of the A and B sites in the Mössbauerspectra of the samples FeCr2−xGaxS4 �x=0.3� are 0.83 and2.94 mm/s, respectively. Those for the FeCr2−xInxS4 �x=0.3� are 0.54 and 1.54 mm/s, respectively. Generally it hasbeen known that the crystal symmetry of the octahedral siteis slightly tilted. Hence, one can guess a large ∆EQ associ-ated with the B sites. It is noticeable that the ∆EQ for the Gadoped samples are larger than that of the corresponding Indoped samples, in spite of the larger ionic radius for the Inions. In order to resolve this point, we have obtained theionic bond lengths for the both samples. The bond lengths ofCr–S, for the Ga and In doped samples �x=0.3� are found tobe 2.41 and 2.43 Å from the XRD refinements, respectively.We suggest that a stronger covalence associated with thesmaller bond length includes a large asymmetric charge dis-

FIG. 1. Refined x-ray diffraction patterns of the FeCr2−xGaxS4 �x=0.1 and0.3� at room temperature.

FIG. 2. Refined x-ray diffraction patterns of the FeCr2−xInxS4 �x=0.1 and0.3� at room temperature.

TABLE I. Results of refinement parameters of x-ray diffraction onFeCr2−xMxS4 �M =Ga and In; X=0.1 and 0.3� �Fd3m: Fe, M, �8a�; Fe, MCr�16d�; S �32e�u ,u ,u���.

x

FeCr2−xGaxS4 FeCr2−xInxS4

0.1 0.3 0.1 0.3

a0 �� 10.0070 9.9962 10.0294 10.0920u �S� 0.7401 0.7409 0.7401 0.7401

dFe–S �Å� 2.3380 2.3218 2.3203 2.3577dM–S �Å� 2.4070 2.4115 2.4228 2.4274dCr–S �Å� 2.4070 2.4115 2.4228 2.4274

FIG. 3. Mössbauer spectra of FeCr2−xMxS4 �M =Ga,In; x=0.1,0.3� at roomtemperature. �a� FeCr1.9Ga0.1S4, �b� FeCr1.7Ga0.3S4, �c� FeCr1.9In0.1S4, and�d� FeCr1.7In0.3S4.

08F715-2 Son et al. J. Appl. Phys. 99, 08F715 �2006�

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tribution. Finally, it gives a large quadrupole interaction,where ∆EQ remains even at room temperature. Although thepresence of ∆EQ still persists at room temperature, there isno crystallographic distortion. This is also in accord with thex-ray diffraction refinement results. Figures 4 shows theMössbauer spectra for the FeCr2−xMxS4 �M =Ga, In; x=0.1,0.3� at 4.2 K. The spectra at 4.2 K were fit to the eightLorentzians by diagonalizing magnetic hyperfine and quad-rupole interaction matrices. In this process, the Mössbauerabsorption lines are shown with the superposed or distortedline shapes of eight-lines, according to the correspondingtransition probabilities. The large asymmetrical line broaden-ing of the Mössbauer absorption lines is shown for thesamples at 4.2 K. It is interpreted to be the dynamic Jahn-Teller effect. Since the Fe2+ ion, with 3d6 electrons leads tothe Jahn-Teller distortions of tetrahedral symmetry, the en-ergy splitting will occur due to the Jahn-Teller effect at theFe sites resulting in the ground state,15 which corresponds toa degenerate orbital doublet Eg state of Fe2+.16 The charge

state of Fe ions is ferrous �Fe2+� as characterized by isomershift �, 1.03 mm/s for x=0.1; 0.84 mm/s for x=0.3 on Gadoped samples, 0.86 mm/s for x=0.1; and 0.86 mm/s forx=0.3 on In doped samples. The ferrous character is alsoseen from the large ∆EQ at room temperature, deduced fromthe doublets of Mössbauer spectra as shown in Fig. 3. Sincethat the electric quadrupole splitting of the Fe2+ ion is largerthan that of Fe3+ due to the orbital contribution, the possibil-ity of Fe3+ can be ruled out.17,18 From the isomer shift and∆EQ values, we concluded that there are no other iron spe-cies than Fe2+ in these samples. The Mössbauer spectra showthat the Fe2+–S–Cr3+ exchange interaction is dominant forthe FeCr2−xMxS4 �M =Ga, In; x=0.1,0.3�.

ACKNOWLEDGMENT

This work was supported by the Korea Research Foun-dation Grant No. �KRF-2005-070-C00050�.

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FIG. 4. Mössbauer spectra of FeCr2−xMxS4 �M =Ga,In; x=0.1,0.3� at4.2 K. �a� FeCr1.9Ga0.1S4, �b� FeCr1.7Ga0.3S4, �c� FeCr1.9In0.1S4, and �d�FeCr1.7In0.3S4.

08F715-3 Son et al. J. Appl. Phys. 99, 08F715 �2006�

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