Structural and doping effects in the half-metallic double perovskite A₂CrWO₆ (A=Sr, Ba, and Ca)

Abstract

The structural, transport, magnetic, and optical properties of the double perovskite A₂CrWO₆ with A=Sr,Ba,Ca have been studied. By varying the alkaline earth ion on the A site, the influence of steric effects on the Curie temperature T_Cand the saturation magnetization has been determined. A maximum T_C=458K was found for Sr₂CrWO₆ having an almost undistorted perovskite structure with a tolerance factor f≃1. For Ca₂CrWO₆ and Ba₂CrWO₆ structural changes result in a strong reduction of T_C. Our study strongly suggests that for the double perovskites in general an optimum T_C is achieved only for f≃1, that is, for an undistorted perovskite structure. Electron doping in Sr₂CrWO₆ by a partial substitution of Sr²⁺ by La³⁺ was found to reduce both T_C and the saturation magnetization M_s. The reduction of M_s could be attributed both to band structure effects and the Cr/W antisites induced by doping. Band structure calculations for Sr₂CrWO₆ predict an energy gap in the spin-up band, but a finite density of states for the spin-down band. The predictions of the band structure calculation are consistent with our optical measurements. Our experimental results support the presence of a kinetic energy driven mechanism in A₂CrWO₆, where ferromagnetism is stabilized by a hybridization of states of the nonmagnetic W site positioned in between the high spin Cr sites.