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 $\frac{1}{3}$, irrespective of ${T}_{C}$ or composition. These experimental findings are consistent with second-order tunneling through interfacial spin sites at the grain boundary.
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