Colossal hopping magnetoresistance of GaAs/ErAs nanocomposites

A theory of bound magnetic polaron (BMP) hopping, driven by thermodynamic fluctuations of the local magnetization, has been developed. It is based on a two-site model of BMP's. The BMP hopping probability rate was calculated in the framework of the 'Golden Rule' approach by using the Ginzburg-Landau effective Hamiltonian method. The theory explains the main features of hopping resistivity observed in a variety of experiments in dilute magnetic semiconductors and magnetic nanocomposites, namely: (a) negative giant magnetoresistance, the scale of which is governed by a magnetic polaron localization volume, and (b) low magnetic field positive magnetoresistance, which usually preceeds negative magnetoresistance.