Metal-Insulator Transition and Ferromagnetism in Diluted Magnetic Semiconductors

We have investigated the interplay between the metal-insulator transition and ferromagnetism in $({\rm III}_{1-x},{\rm Mn}_x){\rm V}$ ferromagnetic semiconductors. Our study is based on a model in which $S=5/2$ Mn local moments are exchange-coupled to band electrons that interact via Coulomb interactions which each other, with ionized Mn acceptors, and with the antisite defects present in these materials. We find quasiparticle participation ratios that are consistent with a metal-insulator transition that occurs in the ferromagnetic state near $x \sim 0.01$. By evaluating the distribution of mean-field exchange coupling strengths at Mn moment sites, we provide evidence in favor of the applicability of small polaron and hole gas models on insulating and metallic sides of the phase transition respectively.