Effects of La/sup 3+/ substitution on the magnetic properties of double perovskites A/sub 2/FeMoO/sub 6/ (A=Ca and Ba)

We have studied effects of the partial substitution of La/sup 3+/ for A/sup 2+/ on the magnetic properties of double perovskites A/sub 2/FeMoO/sub 6/ (A=Ca and Ba). Polycrystalline A/sub 2-x/La/sub x/FeMoO/sub 6/ (0/spl les/x/spl les/0.3) samples have been prepared by the conventional solid-state reaction in a stream of 5% H/sub 2//Ar gas. The X-ray data indicate that A=Ca is monoclinic with the space group P2/sub 1//n and A=Ba is cubic with the space group Fm3m. The substitution of La/sup 3+/ for A/sup 2+/ results in a cell volume increase for A=Ca and a cell volume reduction for A=Ba. Saturation magnetization, which is 3.5 /spl mu//sub B//f.u. for undoped A=Ca (x=0) and 3.9 /spl mu//sub 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 La/sup 3+/ for Ba/sup 2+/ considerably enhances the Curie temperature T/sub c/ from 316 K for x=0 to 334 K for x=0.2. However, the T/sub c/ in Ca/sub 2-x/La/sub x/FeMoO/sub 6/ is not enhanced significantly with increasing x.