Manipulation of an unusual anomalous Hall effect in Ga_1-xMn_xSb random alloys

We study the low-field Hall resistivity of Ga_1-xMn_xSb random alloys grown by molecular beam epitaxy (MBE). In a sample showing weakly localized behavior in resistivity vs. temperature the Hall resistance is initially negative at low temperatures and reverses sign twice as a function of temperature up to the Curie temperature, T_c. In a similar sample showing metal-like resistivity vs. temperature we observe a sign change of the Hall resistance induced by gate voltage in a metal-insulator-semiconductor structure. We propose a model to explain this behavior based on recent theoretical work and a two impurity band (spin-up and spin-down) model in the ferromagnetic state.