Roles of Extended and Localized States in the Magnetic and Transport Properties of GaMnAs Alloys

We investigate the spin-polarized transport of holes in GaMnAs/GaAs heterostructures with Mn concentration 5% in which a ferromagnetic order is known to exist. Disorder enters the problem both in the random distribution of Mn centers and impurities, and also in the orientation of the magnetic moments at the Mn sites. We take into account the existence of an impurity band determining the extended or localized nature of the states at and near the Fermi level. Magnetic order and resistivity are inter-related due to the influence of the spin polarization of the impurity band and the effect of the Zeeman splitting on the mobility edge. We obtain two channels of transport, one provided by the extended states above the mobility edge, when this lies below the Fermi level, the other coming out of the thermal excitation of the localized states through a hopping mechanism. We discuss the competition between these two channels in determining, once having obtained the dependence of the resistance on temperature, the metallic or non-metallic nature of the sample.