Magnetoimpurity theory of resistivity and magnetoresistance for degenerate ferromagnetic semiconductors of the LaMnO3 type

A theory for resistivity and magnetoresistance of LaMnO3-type degenerate ferromagnetic semiconductors is developed. The peak of resistivity not far from the Curie point T-C, which is absent in perfect crystals, is explained by a magnetization-dependent impurity scattering when the potential of ionized impurity is the sum of the usual electrostatic potential and an exchange potential caused by a difference in the local magnetizations close to the impurity and far from it. This exchange potential is a maximum in the vicinity of T-C, which causes a decrease in the mobility and an increase in the number of states in the band-tail slates: that is, decrease in the number of delocalized charge carriers. The latter may be so considerable that it may manifest itself not only in the resistivity peak but also in the transition from the high-conducting state to the insulating state. A magnetic field suppresses the exchange potential and for this reason causes a giant magnetoresistance.