Mössbauer studies of dynamic Jahn-Teller relaxation on the Cu-substituted sulfurspinelSam Jin Kim, Bae Soon Son, Bo Wha Lee, and Chul Sung Kim Citation: Journal of Applied Physics 95, 6837 (2004); doi: 10.1063/1.1687551 View online: http://dx.doi.org/10.1063/1.1687551 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/95/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A study of spin canting in Li3Fe2(PO4)3 with Mössbauer spectroscopy under 5T J. Appl. Phys. 115, 17E126 (2014); 10.1063/1.4864747 Mössbauer study of magnetic structure of cation-deficient iron sulfide Fe 0.92 S J. Appl. Phys. 105, 07D535 (2009); 10.1063/1.3072752 Mössbauer studies for spinel-type A Cr 2 S 4 ( A = Cd and Fe) J. Appl. Phys. 105, 07E505 (2009); 10.1063/1.3072380 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 Magnetic and structural properties of ultrafine Ni–Zn–Cu ferrite grown by a sol–gel method J. Appl. Phys. 87, 6241 (2000); 10.1063/1.372667

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

136.165.238.131 On: Fri, 19 Dec 2014 14:58:33

Mossbauer studies of dynamic Jahn-Teller relaxation on the Cu-substitutedsulfur spinel

Sam Jin Kim and Bae Soon SonDepartment of Physics, Kookmin University, Seoul 136-702, Korea

Bo Wha LeeDepartment of Physics, Kookmin University, Seoul 136-702, Korea and Department of Physics, HankukUniversity of Foreign Studies, Yongin, Kyungki 449-791, Korea

Chul Sung Kima)

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

~Presented on 7 January 2004!

Samples of Fe12xCuxCr2S4 (x50.0, 0.1, 0.3, and 0.5! have been studied with Mo¨ssbauerspectroscopy, x-ray diffraction, magnetization, and magnetoresistance. A cusp-like anomaly isobserved for the samplex50.1 in the both field-cooled and zero-field-cooled magnetization curvesnear 130 K under an applied fieldH5100 Oe. The charge state of iron ions are ferrous for thesamplesx50.0 and x50.1, whereas they are ferric for the samplesx50.3 and x50.5. TheMossbauer spectra for the samplex50.1 show asymmetric line broadening, and it is considered tobe dynamic Jahn-Teller relaxation. The unusual reduction of magnetic hyperfine field below 110 Kcan be interpreted in terms of cancellation effect between the mutually opposite orbital current fieldHL and Fermi contact fieldHC. © 2004 American Institute of Physics.@DOI: 10.1063/1.1687551#

I. INTRODUCTION

Very large negative magnetoresistance~MR! has beenreported in sulfur spinel compounds (Fe12xCuxCr2S4 ; x50, 0.5!.1 Various kinds of experiments, electron spinresonance,2 photoelectron spectroscopy,3 and band calcula-tion on the compounds of Fe12xCuxCr2S4 have been re-ported extensively. Lotgeringet al.4 developed a model con-sidering the monovalent Cu1 ion, while Goodenough5

postulated a divalent Cu21 for the concentration range 0.5,x<1.0. Recently Palmer and Greaves6 and Fritschet al.7

reported the triple exchange model and suggested the coex-istence of iron ions Fe21 and Fe31 in the tetrahedral sites. Aferrous character of the iron ion in FeCr2S4 and a ferriccharacter of the superexchange interaction mechanism inFe0.5Cu0.5Cr2S4 have earlier been reported.8,9 However, thestudy on the intermediate compound of the copper dopedsample Fe12xCuxCr2S4 (0,x,0.5) is still unresolved.Therefore, it is essential to determine the valence state ofiron ions in various sulfur spinel compounds to understandthe underlying mechanism properly. Here, we present theresults of Mossbauer experiments and compare them withthose of x-ray, MR, and vibrating sample magnetometer~VSM! magnetization for the sulfur spinel compounds ofFe12xCuxCr2S4 (x50.0, 0.1, 0.3, and 0.5!. The temperaturedependence of Mo¨ssbauer spectra for the samplex50.1 ispresented with a special emphasis on dynamic Jahn-Tellerrelaxation.

II. EXPERIMENT

Fe12xCuxCr2S4 (x50.0, 0.1, 0.3, and 0.5! were preparedby the direct reaction of the high-purity elements Fe, Cr, Cu,and S in an evacuated quartz tube. The crystal structureswere analyzed by Rietveld refinement of x-ray diffraction.The Mossbauer spectra were recorded using a conventionalspectrometer with a57Co source in a rhodium matrix. MRand magnetization were measured with Van Der Pauwmethod and VSM, respectively.

III. RESULTS AND DISCUSSION

The crystal of Fe12xCuxCr2S4 (x50.0, 0.1, 0.3, and 0.5!is found to be a cubic spinel structure. Figure 1 presents theobserved, calculated peak profile, Bragg positions of the

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

FIG. 1. Refined x-ray diffraction patterns of Fe0.9Cu0.1Cr2S4 at 295 K. Tickmarks show the Bragg positions.

JOURNAL OF APPLIED PHYSICS VOLUME 95, NUMBER 11 1 JUNE 2004

68370021-8979/2004/95(11)/6837/3/$22.00 © 2004 American Institute of Physics

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

136.165.238.131 On: Fri, 19 Dec 2014 14:58:33

x-ray diffraction pattern for the samplex50.1. The deter-mined crystal symmetry of the samplex50.1 at room tem-perature is a normal cubic spinel structureFd3m @Fe, Cu(8a); Cr (16d); S (32e) (u,u,u)] with a lattice constanta059.9880(2) Å. The final oxygen parameteru, Bragg fac-tor RB , andRF are 0.7413%~2!, 4.28%, and 4.16%, respec-tively. There is no indication of a static crystal distortion inthis sample.

Figure 2 shows the temperature dependence of the zero-field-cooled~ZFC! and field-cooled~FC! magnetization forthe samplex50.1 under an applied field of 100 Oe and thesaturated magnetization curve under an applied field of 5kOe, respectively. The magnetization exhibits the large irre-versibility between the ZFC and FC curves. The magnetiza-tion under 100 Oe increases abruptly with decreasing tem-perature near the region of paramagnetic-to-ferrimagneticphase transition. A cusp-like anomaly is observed in both FCand ZFC curves near 130 K and gradually disappears withincreasing applied field. It moves toward the lower tempera-ture region with increase of the magnetic field, and finally itshows the convex type maximum at 110 K, under an appliedfield of 5 kOe. We note that the unusual increase of magne-tization persists up to 110 K. These abnormal phenomena aresimilar to the temperature dependence of the magnetic hy-perfine fields in Mo¨ssbauer analysis.

Resistivity was measured at the temperature range from30 to 300 K, which showed a similar behavior as in Ref. 7.Resistivity of the samplex50.1 showed the semiconductorbehavior in the rangeT,110 K, while metallic behavior inT.110 K. Generally, it is known that the metal-insulatortransition occurs with ferromagnetic-paramagnetic transition.The resistivity of FeCr2S4 exhibits a local maximum slightlyabove the Ne´el temperatureTN (;170 K), which corre-sponds to the metal-semiconductor transition, and an acti-vated type is shown aboveTN . However, the resistivity ofFe0.9Cu0.1Cr2S4 showed only metal-metal transition nearTN

(;202 K). Related to the conduction mechanism ofFe12xCuxCr2S4 (x<0.5), Palmer and Greaves6 and Fritschet al.7 proposed the triple exchange model, in which the elec-trical conduction is established via electron hopping between

Fe21 and Fe31. In order to clarify the conduction mecha-nism and to investigate the valence states of the ions inFe0.9Cu0.1Cr2S4, the Mossbauer spectra were obtained.

Figure 3 shows the Mo¨ssbauer spectra of the samplesFe12xCuxCr2S4 (x50.0, 0.1, 0.3, and 0.5! at 13 K. The spec-tra for the samplesx50.0 and 0.5 are similar to the previousreports. The large asymmetrical line broadenings are shownfor the samplesx50.0 and 0.1, whereas the symmetrical lineshapes are shown in the samplesx50.3 and 0.5. The mag-netic hyperfine fieldsHhf and electric quadrupole splittingsnEQ, for the samplesx50.0, 0.1, 0.3, and 0.5 at 13 K, were185, 170, 325, and 378 kOe; and 1.12, 2.23, 0.26, and 0.0mm/s, respectively.Hhf of the samplex50.1 correspond tothe typical value of the ferrous ion in Fe-Cr based sulfurspinel compounds, while that ofx50.3 correspond to theferric ion. It is noticeable that the degree of asymmetricalline broadening is severe up tox50.1, and then it weakensover x50.3. Considering Cu21 are cooperative Jahn-Tellerions, one can guess the distorted line shape with increasingCu ions. However, the nearly symmetrical sextet pattern isshown for the samplex50.3 and finally the sharp sextet is

FIG. 2. Temperature dependence of magnetizationM of Fe0.9Cu0.1Cr2S4

under 5 kOe external field, zero-field cooled~ZFC!, and field cooled~FC!curves under 100 Oe. FIG. 3. Mossbauer spectra of Fe12xCuxCr2S4 (x50.0, 0.1, 0.3, and 0.5! at

13 K.

FIG. 4. Mossbauer spectra of Fe0.9Cu0.1Cr2S4 at various temperaturesT.

6838 J. Appl. Phys., Vol. 95, No. 11, Part 2, 1 June 2004 Kim et al.

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

136.165.238.131 On: Fri, 19 Dec 2014 14:58:33

shown for the samplex50.5. Also Hhf of the samplex50.3 is quite large compared to that of the samplex50.1.Reminding that the ferrous ions have a large quadrupole shiftdue to the large orbital angular contribution and the ferric ionhas a small quadrupole shift, we carefully conclude that thecharge state of the iron ions in the samplesx50.1 and 0.3are ferrous and ferric, respectively. Simultaneously, the val-ance state of copper ions of the samplesx50.1 and 0.3 aredivalent and monovalent, respectively, from the charge neu-trality. Lotgering et al.4 and Fritschet al.7 suggested thatelectronic properties are determined by Fe21-Fe31 exchangein copper doped sulfur spinel compounds. However, we can-not find any evidence of the ferric character forFe0.9Cu0.1Cr2S4 in Mossbauer spectra. Therefore, it is neces-sary to examine the detailed magnetic interaction behaviorsof Fe0.9Cu0.1Cr2S4 .

In order to study the temperature dependence of the mi-croscopic interaction of the samplex50.1, the Mossbauerspectra have been investigated at various temperatures from13 K to room temperature. Some of the representative spec-tra for Fe0.9Cu0.1Cr2S4 are presented in Fig. 4. The Mo¨ss-bauer spectra were analyzed with using the eight Lorentzianlines fitting method. The results ofHhf , nEQ, and isomershift d for Fe0.9Cu0.1Cr2S4 are listed in Table 1d at roomtemperature is 0.54 mm/s relative to Fe metal, which meansthat the charge state of Fe ions is ferrous in character.nEQ

rapidly decreases with increasing temperature. The asym-metrical line broadening is apparent in Fig. 4 and it is inter-preted to be dynamic Jahn-Teller relaxation.10 We also notethat an unusual increase ofHhf is observed below 110 K astemperature increases. It is noticeable that this point is nearthe temperature, which presents the highest value in magne-tization. An increase of magnetization with increasing tem-perature has been interpreted to be indicative of spin frustra-tion or even a spin glass phase in Mn perovskites. On theother hand, this anomaly can be attributed to spin-reorientation transition in a ferrimagnetic compound. How-ever, the neutron diffraction data on FeCr2S4 did not showany symptom of a diffused line broadening or superstructurepeaks from spin reorientation in the temperature range from10 K to room temperature.8 Therefore, one can remove theabove possibilities. We suggest that the unusual increasing ofHhf below 110 K may be explained in terms of a cancellationeffect between the mutually oppositeHL andHC.11 HereHL

and HC are the orbital current field and the Fermi-contactfield, respectively.

This work has been supported by the KOSEF~Grant No.R02-2003-000-10046-0!.

1A. P. Ramirez, R. J. Cava, and J. Krajewski, Nature~London! 386, 156~1997!.

2Z. Yang, S. Tan, Z. Chen, and Y. Zang, Phys. Rev. B62, 13 872~2000!.3J. S. Kang, S. J. Kim, C. S. Kim, C. G. Olson, and B. I. Min, Phys. Rev.B 63, 144412~2001!.

4F. K. Lotgering, A. M. van Diepen, and J. F. Olijhohoek, Solid StateCommun.11, 1417~1972!.

5J. B. Goodenough, J. Phys. Chem. Solids30, 261 ~1969!.6H. M. Palmer and C. Greaves, J. Mater. Chem.9, 637 ~1999!.7V. Fritsch, J. Deisenhofer, R. Fichtl, J. Hemberger, H.-A. Krug von Nidda,M. Mucksch, M. Nicklas, D. Samusi, J. D. Thompson, R. Tidecks, V.Tsurkan, and A. Loidl, Phys. Rev. B67, 144419~2003!.

8S. J. Kim, W. C. Kim, and Chul Sung Kim, J. Appl. Phys.91, 7935~2002!.9H. N. Ok, K. S. Baek, H. S. Lee, and C. S. Kim, Phys. Rev. B41, 62~1990!.

10J. A. Tjon and M. Blume, Phys. Rev.165, 446 ~1968!.11H. N. Ok and J. G. Mullen, Phys. Rev.168, 563 ~1968!.

TABLE I. Magnetic hyperfine fieldHhf , quadrupole splittingDEQ , andisomer shiftd at various temperaturesT for Fe0.9Cu0.1Cr2S4. d is relative toiron metal.

T ~K! Hhf ~kOe! DEQ ~mm/s! d ~mm/s!

13 157 2.23 0.8280 217 1.29 0.69

100 218 0.14 0.68140 187 0.05 0.66180 113 0.01 0.63202 0 0.0 0.62

6839J. Appl. Phys., Vol. 95, No. 11, Part 2, 1 June 2004 Kim et al.

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

136.165.238.131 On: Fri, 19 Dec 2014 14:58:33