Curie temperatures (T_C) of the diluted magnetic semiconductors (DMS) of (Ga,Mn)N and (Ga,Cr)N are evaluated from first-principles with the local spin density approximation by using the Korringa-Kohn-Rostoker method combined with the coherent potential approximation (KKR-CPA). The ferromagnetic states are stable in V- and Cr-doped GaN, but Fe-, Co- and Ni-doped GaN show the spin-glass states. In Mn-doped GaN, T_C depends on the concentration of Mn due to the competition between the ferromagnetic double-exchange and antiferromagnetic super-exchange interactions. High-T_C is expected in those systems if 5 at% concentrations of V, Cr, and Mn in GaN are doped. For (Ga,V)N and acceptor doped (Ga, Cr)N, ferromagnetic super-exchange interaction is responsible for the ferromagnetism, on the other hand, the ferromagnetic double-exchange interaction is dominant for high-
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