Effective-medium theory of the giant magnetoresistance in magnetic granular samples and dopedLaMnO3perovskites

We present a self-consistent effective-medium theory for the giant magnetoresistance of magnetic granular samples and doped ${\mathrm{LaMnO}}_3$ perovskites on the basis of the resistor network model. Most of the characteristic features of the field and temperature-dependent magnetoresistance can be qualitatively explained in the present theory. The calculations compared with the previous studies within the resistor network model are improved and the results are in better agreement with experiments.