Exchange interactions in diluted magnetic semiconductors

On the basis of ab initio calculations for Mn-doped GaN, GaP, GaAs and GaSb, we discuss the origin of ferromagnetism in diluted magnetic semiconductors. The calculations use the Korringa–Kohn–Rostoker method in connection with the coherent potential approximation to describe the substitutional and moment disorder. By mapping the total energy results onto a Heisenberg model, the Curie temperature T C is estimated in the mean field approximation (MFA). If impurity bands are formed in the gap, as is the case for (Ga, Mn)N, double exchange dominates leading to a characteristic dependence of T C MFA as a function of the Mn concentration c . On the other hand, if the d states are localized, as in (Ga, Mn)Sb, Zener’s p–d exchange prevails, resulting in a linear c dependence of T C MFA . (Ga, Mn)As is an intermediate case, showing a -like behaviour in the local density approximation (LDA), but a nearly linear c dependence, if the more accurate LDA + U method with U = 4 eV is used.