Experimental probing of the interplay between ferromagnetism and localization in (Ga,Mn)As

The question of whether the Anderson-Mott localization enhances or reduces magnetic correlations is central to the physics of magnetic alloys. Particularly intriguing is the case of (Ga,Mn)As and related magnetic semiconductors, for which diverging theoretical scenarios have been proposed. Here, by direct magnetization measurements we demonstrate how magnetism evolves when the density of carriers mediating the spin-spin coupling is diminished by the gate electric field in metal-insulator-semiconductor structures of (Ga,Mn)As. Our findings show that the channel depletion results in a monotonic decrease of the Curie temperature, with no evidence for the maximum expected within the impurity-band models. We find that the transition from the ferromagnetic to the paramagnetic state proceeds by means of the emergence of a superparamagnetic-like spin arrangement. This implies that carrier localization leads to a phase separation into ferromagnetic and non-magnetic regions, which we attribute to critical fluctuations in the local density of states, specific to the Anderson-Mott quantum transition.