Ferromagnetism and the metal-insulator transition in the thiospinel Cu(Ir1-xCrx)2S4 -: art. no. 115106

A thiospinel CuIr2S4 exhibits a temperature-induced metal-insulator (M-I) transition at 230 K with a simultaneous spin-dimerization and charge-ordering transition although a three-dimensional system. On the other hand, CuCr2S4 has the same spinel structure without any structural transformations. CuCr2S4 remains metallic and is ferromagnetic with the Curie temperature T(C)similar or equal to377 K. In order to see the effect of substituting Cr for Ir on the M-I transition, we have carried out a systematic experimental study of electrical and magnetic properties of Cu(Ir1-xCrx)(2)S-4 system. The M-I transition temperature decreases steeply with increasing Cr-composition x and this transition is not detected above xsimilar or equal to0.05. The value of T-C decreases with decreasing x from 1.0, then T-C disappears below xsimilar or equal to0.20. The ferromagnetic state suggests the non-collinear spin alignment. In the intermediate composition range over x=0.08 to 0.20, the B-site undergoes a local crystal distortion around 180 K, where the energy level t(2g) splits into lower symmetry. Then the low-spin state within the t(2g) subspace is realized for Cr3+ ion with s=1/2. The magnetic state of Cr3+ ion indicates a crossover from high temperature s=3/2 to low temperature s=1/2 state around 180 K.