Orbital ordering to orbital glass transition in spinel FeCr2−xAlxS4 (0 ⩽ x ⩽ 0.2)

Abstract

The polycrystalline (PC) sample of FeCr2S4 displays orbital ordering around TOO ∼ 9 K, while single crystal sample shows orbital glass. In this paper, with the substitution of Al for Cr, a step by step transition from the orbital ordering to the orbital glass is reported in FeCr2−xAlxS4 (0 ⩽ x ⩽ 0.2). For PC FeCr2S4, the onset of long-range orbital order at TOO is evidenced by the appearance of a step-like transition in the temperature dependence of the magnetization (M(T)), a small kink at about 5.5 T below 9 K in the isotherms' magnetic field dependence of the magnetization (M(H)) curves as well as a λ-type anomaly in specific heat. With increasing Al content, the TOO decreases gradually. For the samples with x ⩾ 0.1, the orbital ordering is replaced by orbital glass, where the specific heat obeys a T2-dependence. The calculated residual orbital entropy consistently increases with x, implying the progressive freezing of the orbital moments and the coexistence of orbital ordering and orbital glass in the middle doping level.