Effects of La3+ substitution on the magnetic properties of double perovskites A2FeMoO6 (A=Ca and Ba)

We have studied effects of the partial substitution of La³⁺ for A²⁺ on the magnetic properties of double perovskites A₂FeMoO₆ (A=Ca and Ba). Polycrystalline A_2-xLa_xFeMoO₆ (0≤x≤0.3) samples have been prepared by the conventional solid-state reaction in a stream of 5% H₂/Ar gas. The X-ray data indicate that A=Ca is monoclinic with the space group P2₁/n and A=Ba is cubic with the space group Fm3m. The substitution of La³⁺ for A²⁺ results in a cell volume increase for A=Ca and a cell volume reduction for A=Ba. Saturation magnetization, which is 3.5 μ_B/f.u. for undoped A=Ca (x=0) and 3.9 μ_B/f.u. for undoped A=Ba (x=0), decreases with increasing x. This decrease of magnetization arises from the reduction of moment associated with the electron doping and the disorder at the Fe and Mo sites. The partial substitution of La³⁺ for Ba²⁺ considerably enhances the Curie temperature T_c from 316 K for x=0 to 334 K for x=0.2. However, the T_c in Ca_2-xLa_xFeMoO₆ is not enhanced significantly with increasing x.