Phonon anomalies and charge dynamics in Fe_{1-x}Cu_{x}Cr_{2}(7)

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

7

Finally,we want to draw attention to the tempera-ture dependence of the relaxation rateγ(lower panel of Fig.6).In the magnetically ordered state below T C,the relaxation rates become significantly reduced,e.g.the re-duction amounts to almost50%for x=0.2.We recall that the plasma frequency has been kept constant for each compound as a function of temperature.This indi-cates that the increase of the conductivity just below the magnetic ordering temperature results from the freezing-out of disorder scattering and not from a change of the carrier density via band-structure changes at the onset of ferrimagnetic order.Taking into account the classifi-cation of chalcogenide spinels A Cr2S4as systems where CMR originates from spin-disorder scattering,13the ob-served reduction of the relaxation rate below T C has to be regarded as direct evidence of such a scenario:In externalfields the onset of ferrimagnetic order shifts to higher temperatures.Concomitantly,a reduction of the scattering rate and the anomalous increase of the con-ductivity arise.As a consequence,maximal CMR effects will show up just below T C as a function of an external magneticfield.A similar scenario has been reported for GdI2,where the magnetic and magneto-transport prop-erties have been described successfully in terms of spin-fluctuations and their suppression by external magnetic fields in the vicinity of T C.44,45We would like to point out,that at low temperatures the relaxation rates for x=0.2and x=0.5are of the same order of magnitude ∼104cm−1,indicating a similar level of disorder for the Cu doped compounds.

IV.SUMMARY

In summary,we investigated the optical properties of Fe1−x Cu x Cr2S4single crystals for Cu concentrations x=0,0.2,0.4and0.5.Phonon excitations and dynamic conductivity for x=0.4are very similar to the results for Fe0.5Cu0.5Cr2S4and were not discussed separately. The phonon excitations were measured as a function of temperature between5K and room temperature.Pure FeCr2S4shows clear anomalies in the eigenfrequencies at the transition from the paramagnetic to the ferromag-netic state,which can be explained by spin-phonon cou-pling.Concerning the complex behavior of the damping constants,spinfluctuations in the vicinity of T C may de-scribe many of the anomalous changes,but further the-oretical studies are necessary to corroborate this inter-pretation.The influence of magnetic order on the eigen-modes is reduced with increasing x,and the appearance of a new phonon mode close to350cm−1is attributed to an impurity mode rather than to a symmetry reduction due to A-site order.

Morover,the charge dynamics of Fe1−x Cu x Cr2S4were investigated.FeCr2S4is an insulator,but becomes metal-lic when slightly doped with Cu.The conductivity of the free charge carriers can be described by a normal Drude-type behavior.The dc conductivity for x=0.2is en-hanced by a factor of four in comparison to x=0.5.The temperature dependence of the optically derived dc con-ductivity for both doped compounds is is in good agree-ment with resistivity measurements,but localization ef-fects at lowest temperatures appear weaker in the op-tical measurements.The corresponding behavior of the scattering rate,which shows a strong decrease below the ferrimagnetic phase transition,evidences the freezing-out of disorder scattering below T C.In accordance with the proposed classification of the ternary chalcogenide spinels as spin-disorder magnetoresistive materials,the reduc-tion of the relaxation rate corroborates such a scenario and makes clear that spin-disorder has to be considered a necessary ingredient towards a theoretical description of this fascinating class of materials.

Acknowledgments

It is a pleasure to thank H.-A.Krug von Nidda, J.Hemberger,and Ch.Hartinger for fruitful discus-sions.This work was partly supported by the DFG via the Sonderforschungsbereich484(Augsburg),by the BMBF/VDI via the Contract No.EKM/13N6917/0,by the U.S.Civilian Research&Development Foundation (CRDF)and by the Moldavian Research&Development Association(MRDA)via Grant No.MP2-3047.

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