Ferromagnetism and the metal-insulator transition in the thiospinel Cu(Ir1-xCrx)2S4

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 TC≃377 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)2S4 system. The M-I transition temperature decreases steeply with increasing Cr-composition x and this transition is not detected above x≃0.05. The value of TC decreases with decreasing x from 1.0, then TC disappears below x≃0.20. 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 t2g splits into lower symmetry. Then the low-spin state within the t2g 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