Oxygen-induced grain boundary effects on magnetotransport properties of Sr₂FeMoO_6+δ

The magnetic, electric, and structural properties of polycrystalline samples of Sr₂FeMoO_6+δ with controlled oxygen content have been investigated for nominal values δ=0 and 0.04. We found that the magnetization and lattice parameters are the same in both samples, while the electric properties are drastically affected. This behavior shows that the difference in the oxygen contents is localized in a region near the surface of the grains, thus changing the transport properties of the grain boundaries (gb's). The sample with δ=0.04 presents a resistivity (ρ) that exceeds in more than two orders of magnitude the ρ for δ=0. Also the low-field magnetoresistance is enhanced in ≈40% for δ=0.04 and the voltage-current characteristics present appreciable nonlinearities. We discuss these results in terms of a reinforced tunneling barrier due to the formation of an impurity phase at the gb's.