Enhanced ferromagnetic interactions in electron doped Nd<inf>x</inf>Sr <inf>2-x</inf>FeMoO<inf>6</inf> double perovskites

We report on the structural, magnetic and magnetotransport effects promoted by Nd3+ substitution in NdxSr2-xFeMoO 6. In spite of the fact that the ionic radius of Nd3+ is smaller than that of Sr2+, the unit cell volume remains constant across the series. We also show that the incorporation of Nd3+ induces a substantial rising of the Curie temperature from Tc = 400 K for x = 0 to Tc = 440 K for x = 0.6. On the basis of the structural data we argue that this enhancement is due to the injection of itinerant carriers into the conduction band. Nd doping promotes the appearance of antisite (AS) defects in the Fe-Mo sublattice. It turns out that the concentration of AS is mainly controlled by the donor character of the substituting ions rather than by their ionic radii. Antisites are found to reduce the saturation magnetization and the magnetoresistance. This observation, which sharply contrasts with expectations based on electronic rigid band models, indicates that the half-metallic character of double perovskites may be unstable in the presence of antisites. This is in agreement with some recent proposals and the implications of these findings are discussed.