Microscopic theory of magnetoresistance in granular materials

Based on a simple model of spin-dependent scattering, an improved microscopic approach is proposed to describe the magnetoresistance in granular magnetic systems. The feature is the explicit account of correlated scattering of conduction electrons between granule pairs and of transitions between spin-up and spin-down states. Besides the usual term, proportional to the square of the magnetization, an extra conribution to the magnetoresistance ratio $\Delta \rho /\rho$ is obtained, proportional to the correlator $〈\cos {\theta }_{12}〉$, where ${\theta }_{12}$ is the angle between magnetic moments of neighbor granules. The field dependence of $\Delta \rho /\rho$, together with its dependencies on granule concentration, granule mean size, granule size distribution, temperature and annealing effects, are considered, leading to a reasonable agreement with available experimental data.