Abstract
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 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. [S0031-9007(99)09282-0].