Carrier states and ferromagnetism in diluted magnetic semiconductors

Applying the dynamical coherent potential approximation to a simple model, we have systematically studied the carrier states in A(1-x)Mn(x)B-type diluted magnetic semiconductors (DMS's). The model calculation was performed for three typical cases of DMS's: cases with strong and moderate exchange interactions in the absence of nonmagnetic potential, and a case with strong attractive nonmagnetic potential in addition to moderate exchange interaction. When the exchange interaction is sufficiently strong, magnetic impurity bands split from the host band. Carriers in the magnetic impurity band mainly stay at magnetic sites, and coupling between the carrier spin and the localized spin is very strong. The hopping of the carriers among the magnetic sites causes ferromagnetism through a double-exchange (DE)-like mechanism. We have investigated the conditions for the operation of DE-like mechanism in DMS's. The result reveals that the nonmagnetic attractive potential at the magnetic site assists the formation of the magnetic impurity band and makes the DE-like mechanism operative by substantially enhancing the effect of the exchange interaction. Using conventional parameters, we have studied the carrier states in Ga1-xMnxAs. The result shows that the ferromagnetism is caused by the DE-like mechanism via carriers in the band tail originating from the impurity states.