Study of magnetotransport in polycrystalline perovskite manganites

In this letter, a self-consistent effective-medium approximation is presented for the magnetotransport in the polycrystalline manganites. Both intrinsic properties which are achieved through strong-link grain boundaries and the extrinsic properties which are achieved through weak-link grain boundaries are taken into account. It is noted that the manganites are phase separated into a metallic phase and an insulating phase. The conduction of the system is determined by the metallic channels when the metallic phase has percolated. In the polycrystalline cases, the weak-link grain boundaries, spread along the percolating paths, are expected to inhibit the formation of metallic channels on the one hand, and to favour the low-field magnetoresistance on the other hand. The relationship between phase separation and percolation is investigated for various grain boundary conditions. Our calculations agree with experiments well.