Ferromagnetism and spinodal decomposition in dilute magnetic nitride semiconductors.

We propose a materials design for dilute magnetic semiconductors (DMS) based on first-principles calculations by using the Korringa-Kohn-Rostoker coherent potential approximation method. We develop an accurate method for calculations of the Curie temperature (T(C)) of DMS and show that the mean field approximation completely fails to predict T(C) for DMS, in particular for wide gap nitride DMS where the exchange interaction is short ranged. The T(C) calculated for homogeneous DMS by using the present method agrees very well with available experimental values. For more realistic material design, we simulate spinodal nanodecomposition by applying the Monte Carlo method to the Ising model with ab initio chemical pair interactions between magnetic impurities in DMS. It is found that by controlling the dimensionality of the decomposition, various characteristic phases occur in DMS. It is suggested that superparamagnetic blocking phenomena should be important for understanding the magnetism of wide gap DMS.