Disorder-induced orbital glass state in FeCr ₂ S ₄

Abstract

The effect of disorder on the orbital state in the spinel FeCr2S4 has been investigated with the substitution of Cr by Al, Ga, and Fe, respectively. For polycrystalline FeCr2S4, being related to the orbital-ordering transition around 9 K, the temperature dependence of magnetization shows a step-like transition, and the specific heat displays a well-defined λ-type anomaly correspondingly. However for single-crystal and doped FeCr2S4 samples, the step-like transition in magnetization disappears, and the λ-type anomaly of specific heat is replaced by a broad hump. Moreover, the specific heat obeys a T2-dependence at temperatures below 2 K, suggesting the formation of the orbital glass state in these samples. Consistently with different orbital states, the resistivity at low temperature can be better fitted with a thermally activated model for the polycrystalline FeCr2S4 sample, and better described by Mott's variable-range hopping expression for the others. All these results imply that the disorder induces orbital glass state in FeCr2S4.