Anisotropy of the Bound Magnetic Polaron States and Colossal Hoppig Magnetoresistance in Dilute Magnetic Semiconductors

A theory of positive and negative colossal magnetoresistance (CMR) in dilute magnetic semiconductors (DMS) is presented. The phenomenon is described in the framework of bound magnetic polaron (BMP) hopping, driven by thermodynamic fluctuations of the local magnetization. The latter is caused by fluctuating spins of the magnetic atoms which strongly interact with the spin of a trapped electron or a hole within the localization volume. The authors' previous approach to the BMP hopping problem1,2, based on the Ginzburg-Landau effective Hamiltonian method, has been generalized in order to take into account the anisotropy of the BMP states. This effect can be attributed to the anisotropy of the local exchange field and/or to the field-induced anisotropy of the magnetic susceptibility. In DMS the latter is shown to affect the position and the shape of the maximum in the magnetic-field dependence of CMR.