Structure, transport and magnetic properties of the double perovskite (Ca_2-2xSr_2x)FeMoO₆

Abstract

In this paper we report the results of a detailed investigation of the double perovskite (Ca_2-2xSr_2x)FeMoO₆ system. Chemical size effects on structural, electrical, and magnetic properties caused by the substitution of isovalent, larger Sr ions into the smaller Ca sites, resulting in (Ca_2-2xSr_2x)FeMoO₆, have been examined. The compounds crystallize in the monoclinic space group P2₁/n for 0.0≤x<0.2, the orthorhombic space group Pbnm for 0.2≤x<0.4, and the tetragonal space group I4/m and I4/mmm for x≥0.4. Examination of the resistivity of all compounds reveals a metallic behavior which is well described by a Tⁿ dependence except for x=1.0. These n values change from 1 to 2 as T decreases lower than T_c. This is indicative of a variation in the transport mechanism at T_c. The ferrimagnetic transition temperature T_c increases with increasing x from 318 (x=0.0) to 393 K (x=1.0). For all samples, the saturation magnetization at 82 K obeys M_s≤3.5 μ_B/(formula unit), compared to a theoretical spin-only moment of 4 μ_B/(formula unit) for a perfectly ordered compound.