Intergrain Magnetoresistance via Second-Order Tunneling in Perovskite Manganites

The intergrain magnetoresistance (IMR) observed in polycrystalline half-metallic ferromagnets at temperatures below the Curie temperature $T_C$ was investigated. The systematics of the IMR, as a function of field and temperature for ferromagnetic manganites with varying $T_C$, shows that the high field magnetoconductivity (MC), rather than the magnetoresistance, is linear with magnetic field and the slope monotonically decreases with increasing $T_C$. The low field MC value in the low temperature limit is universally close to $\frac{1}{3}$, irrespective of $T_C$ or composition. These experimental findings are consistent with second-order tunneling through interfacial spin sites at the grain boundary.