TEMPERATURE-DEPENDENCE OF CONDUCTIVITY AND ISOTOPE EFFECT IN THE COLOSSAL MAGNETORESISTANCE

The diffusion of electrons in a disordered metal, by means of the pert urbation approach, is considered in the presence of a magnetic field. It is shown that the multiple backward scattering of electrons on the randomly oriented or distributed magnetic moments of atoms significant ly reduces the diffusion of electrons. Thus, in the diffusive but near ly localized electron system localized magnetic moments appear and a t ransition into the ferromagnetic phase occurs. The isotope effect in T -C, the Curie temperature, is obtained, as first observed in La1-x(Ca , Sr)(x)MnO3+y experiments bq Zhao and coworkers. Both a relatively we ak classical magnetic field (omega(c) tau(0) < 1, where omega(c) is th e cyclotron frequency, and tau(0) is the elastic collision time) and a molecular field, appearing as result of the spin polarization or spon taneous magnetization in the system, are found to be able to completel y suppress the interference effect in the diffusion, It is shown that the giant magnetoresistance is extremely large near the mobility edge. The effect of magnetic field on the magnetic susceptibility is also c onsidered. The application of the results to a layered 2D case is disc ussed as well.