Mössbauer study of Cr-based chalcogenide spinels Fe1-xCuxCr2S4

We report on the magnetic and electronic state of iron in Cu-substituted Cr-based thiospinels of the nominal composition Fe1-xCuxCr2S4 (x 0 0.25; 0.5; 0.75), a class of compounds which recently raised considerable interest in relation with the colossal magnetoresistance effect they display. 57Fe Mössbauer measurements carried out at different temperatures revealed the gradual transformation of iron from the state of Fe2+ (representative for x = 0) to the state of Fe3+ (representative for x>= 0.5), along with a simultaneous increase in the magnetic ordering temperature. In Fe0.75Cu0.25Cr2S4, where Fe2+ and Fe3+ oxidation states were found to coexist, the Mössbauer–Lamb factor characteristic of Fe3+ sites was found to exceed that characteristic of Fe2+ sites. Special attention is paid to the composition with x = 0.5. This compound was found to display a Curie temperature TC 340K; a value which considerably exceeds the recently reported TC value for a single crystal with the same average composition.