Interplay of localization and magnetism in (Ga,Mn)As and (In,Mn)As

We examine the role of localization on the hole-mediated ferromagnetism in dilute ferromagnetic semiconductors by combining results of electrical and magnetic studies for Mn-implanted GaAs and InAs. In both materials upon increasing the Mn concentration, a change from an insulating (hopping) regime of conductivity to a metallic-like is accompanied by a gradual build-up of a long-range magnetic coupling and a monotonic increase of the Curie temperature. For the least conducting sample no (global) Curie temperature can be identified, although the observed slow dynamics (superparamagnetic-like) confirms the presence of a ferromagnetic coupling acting only over limited (mesoscopic) distances. We tentatively characterize the strength of this local coupling by T*, the largest temperature at which dynamic effects are observable by conventional SQUID magnetometry. Our findings strongly advocate for the heterogeneous model of electronic states at the localization boundary and point to the crucial role of weakly localized holes in mediating efficient spin-spin interactions between diluted spins even on the insulator side of the metal-to-insulator transition. Interestingly, T* at low carrier densities becomes larger in (In,Mn)As compared to (Ga,Mn)As. This constitutes a new experimental support for the suggestion [2] that larger p-d coupling results in higher values of TC only in the regime, where hole localization effects are not crucial.