Dilute magnetic semiconductors

We describe exchange interactions in dilute magnetic semiconductors (DMS) based on ab initio calculations. The electronic structure of DMS is calculated on the basis of the density functional theory using the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA). We show that there are two classes of DMS with very different properties. In systems with localized majority d-states deep in the valence band, the ferromagnetism is induced by Zener's p–d exchange interaction. This interaction is weak, but long ranged. For systems with impurity bands in the gap, the ferromagnetism is driven by Zener's double exchange mechanism. This interaction is very strong, but short ranged. Sophisticated Monte Carlo methods show that, for small concentrations, the percolation effect should be included to estimate the Curie temperatures of DMS. In particular, the ferromagnetism is strongly suppressed in double exchange systems due to the absence of percolation for low concentrations.