Scattering by atomic spins and magnetoresistance in dilute magnetic semiconductors

We studied electrical transport in magnetic semiconductors, which is determined by scattering of free carriers off localized magnetic moments. We calculated the scattering time and the mobility of the majority- and minority-spin carriers with both the effects of thermal spin fluctuations and of spatial disorder of magnetic atoms taken into account. We discuss the role of the above effects on magnetoresistance of nondegenerate semiconductors where magnetic impurities are electrically active or neutral. The application of the external magnetic field suppresses the thermodynamic spin fluctuations thus promoting negative magnetoresistance. Simultaneously, scattering off the built-in spatial fluctuations of the atomic spin concentrations may increase with the magnetic field. The latter effect is due to the growth of the magnitude of random local Zeeman splittings with the magnetic field. It promotes positive magnetoresistance. The enhancement of spin-dependent scattering by the external magnetic field is especially large in the materials with electrically active magnetic impurities.