Theory of Ferromagnetism in Diluted Magnetic Semiconductors

Carrier-induced ferromagnetism has been observed in several (III,Mn)V semiconductors. We review the theoretical picture of these ferromagnetic semiconductors that emerges from a model with kinetic-exchange coupling between localized Mn spins and valence-band carriers. We discuss the applicability of this model, the validity of a mean-field approximation for its interaction term widely used in the literature, and validity limits for the simpler RKKY model in which only Mn spins appear explicitly. Our conclusions are based in part on our analysis of the dependence of the system's elementary spin excitations on carrier density and exchange-coupling strength. The analogy between this system and spin-model ferrimagnets is explored. Finally, we list several extensions of this model that can be important in realistic modeling of specific materials.