  The magnetoresistance (MR) in polycrystalline colossal magnetoresistive
compounds follows a behavior different from single crystals below the
ferromagnetic transition temperature. This difference is usually attributed to
spin polarized tunneling at the grain boundaries of the polycrystalline sample.
Here we derive a theoretical expression for the contribution of spin polarized
tunneling to the magnetoresistance in granular ferromagnetic systems under the
mean field approximation. We apply this model to our experimental data on the
half metallic ferromagnet La0.7Sr0.3MnO3, and find that the theoretical
predictions agree quite well with the observed dependence of the spin polarized
MR on the spontaneous magnetization.
