Oxygen-induced grain boundary effects on magnetotransport properties of (formula presented)

The magnetic, electric, and structural properties of polycrystalline samples of (formula presented) with controlled oxygen content have been investigated for nominal values (formula presented) and (formula presented) 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 (formula presented) presents a resistivity (formula presented) that exceeds in more than two orders of magnitude the (formula presented) for (formula presented) Also the low-field magnetoresistance is enhanced in (formula presented) for (formula presented) 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.