Study of field dependence of magnetoresistance of polycrystalline perovskite manganites

The field dependence of magnetoresistance in polycrystalline perovskite manganites is studied. It is found that there are two kinds of conduction channels. One is insulating channel, in which the spin-polarized tunneling affects the magnetotransport, the other is metallic, where the spin-dependent scattering dominates. The increase of metallic channels in the high external magnetic field, which is due to the reorientation of disorder spins at the interface, is assumed determinant to the continuous decrease of resistance. Within our theoretic model, the magnetoresistance of the systems with different grain size and different intergrain connectivity can be well explained. Our calculations agree with the experimental data well.