Phonon anomalies and charge dynamics in Fe_{1-x}Cu_{x}Cr_{2}

A detailed investigation of phonon excitations and charge carrier dynamics in single crystals of Fe_{1-x}Cu_{x}Cr_{2}S_{4} (x = 0, 0.2, 0.4, 0.5) has been performed by using infrared spectroscopy. In FeCr_{2}S_{4} the phonon eigenmodes are strongly affecte

a r X i v :c o n d -m a t /0506052v 1 [c o n d -m a t .s t r -e l ] 2 J u n 2005

Phonon anomalies and charge dynamics in Fe 1−x Cu x Cr 2S 4single crystals

T.Rudolf,1K.Pucher,1F.Mayr,1D.Samusi,2V.Tsurkan,1,2R.Tidecks,3J.Deisenhofer,1and A.Loidl 1

1

EP V,Center for Electronic Correlation and Magnetism,University of Augsburg,86135Augsburg,Germany

2

Institute of Applied Physics,Academy of Sciences of Moldova,MD 2028,Chisinau,Republic of Moldova

3

Institute of Physics,University of Augsburg,86135Augsburg,Germany

(Dated:February 2,2008)

A detailed investigation of phonon excitations and charge carrier dynamics in single crystals of Fe 1−x Cu x Cr 2S 4(x =0,0.2,0.4,0.5)has been performed by using infrared spectroscopy.In FeCr 2S 4the phonon eigenmodes are strongly affected by the onset of magnetic order.Despite enhanced screening effects,a continuous evolution of the phonon excitations can be observed in the doped compounds with x =0.2(metallic)and x =0.4,0.5(bad metals),but the effect of magnetic ordering on the phonons is strongly reduced compared to x =0.The Drude-like charge-carrier contribution to the optical conductivity in the doped samples indicates that the colossal magneto-resistance effect results from the suppression of spin-disorder scattering.

PACS numbers:72.15.-v,72.20.-i,78.30.-j,75.30.Vn

Keywords:infrared spectroscopy,optical spectra,chalcogenide spinel,colossal magnetoresistance

I.INTRODUCTION

The discovery of colossal magnetoresistance (CMR)in perovskite-type manganites has attracted considerable attention.1,2,3,4,5Double-exchange (DE)mechanism,6,7strong electron-phonon coupling,7phase separation scenarios 8or a Griffiths singularity 9were suggested to clarify the origin of the CMR effect,but a conclusive mi-croscopic model has not yet been established.Ever since,the occurrence of CMR effects has been reported for vari-ous other classes of materials,such as pyrochlores,10rare-earths based compounds like GdI 2,11and ternary chalco-genide spinels A Cr 2S 4.12These CMR materials have been classified in terms of spin-disorder scattering and a universal dependence of the magnetoresistence vs.carrier density has been suggested on theoretical grounds.13,14Ramirez et al.drew attention to the spinel system Fe 1−x Cu x Cr 2S 4in 1997.12In polycrystalline FeCr 2S 4with T C =170K,the CMR effect reaches values compa-rable to those observed in perovskite oxides.The substi-tution of Fe by Cu increases T C to temperatures above room temperature,and the CMR effect remains relatively strong (∼7%).12In addition,solid solutions of the ferri-magnetic semiconductor FeCr 2S 4and the metallic ferro-magnet CuCr 2S 4show a number of puzzling properties:From the very beginning,a controversial discussion has been arising whether the Cu ions are mono-or divalent for x ≥0.5.15,16,17For x <0.5it was established that only monovalent and hence diamagnetic (d 10)Cu exists in the mixed crystals.18Moreover,Fe 1−x Cu x Cr 2S 4shows two metal-to-insulator transitions as a function of x ,as the room-temperature resistivity reveals two minima at x =0.2and x =1and concomitantly the Seebeck co-efficient changes sign two times.15,19Additionally,band-structure calculations predicted that the Fe 1−x Cu x Cr 2S 4system should exhibit a half-metallic nature.17,20,21

Recent experimental investigations of Fe 1−x Cu x Cr 2S 4single crystals indicated a strong dependence of their

magnetic and magneto-transport properties on hy-drostatic pressure suggesting a strong magneto-elastic coupling.22,23Measurements on the ac susceptibility in pure FeCr 2S 4exhibited a cusp in the low-field magneti-zation and the onset of magnetic irreversibilities at 60K was explained by domain-reorientation processes.24Later on,ultrasonic studies indicated an anomaly in the tem-perature dependence of the shear modulus close to 60K,and it was suggested that the onset of orbital order in-duces a structural distortion at this temperature.25This result,however,is hardly compatible with the observa-tion that orbital order is established in polycrystals close to 10K,while an orbital glass state is found in single crystals.26

Optical spectroscopy simultaneously probes the lattice and electronic degrees of freedom and is,therefore,ide-ally suited to investigate structural phase transitions and to clarify the importance of electron-phonon coupling for the CMR effect.27Earlier infrared (IR)studies in polycrystalline FeCr 2S 4reported,in accordance with the crystal-lattice symmetry F d 3m ,the existence of four IR-active phonons,which strongly depend on temperature near and below T C .28,29We performed measurements of the optical properties of single crystals of Fe 1−x Cu x Cr 2S 4(x =0,0.2,0.4and 0.5)to shed light on the interplay of structural and electronic properties in these compounds.Since the optical properties of the samples with x =0.4and x =0.5were found to be very similar,we only show and discuss the corresponding data for x =0.5in the following.

II.EXPERIMENTAL DETAILS

Single crystals of Fe 1−x Cu x Cr 2S 4were grown using a chemical transport-reaction method with chlorine as transport agent and the ternary polycrystals as start-ing material.Details of the sample preparation are de-