Carrier States and Ferromagnetism in Diluted Magnetic Semiconductors

TL;DR

This paper uses a theoretical model to analyze carrier states and ferromagnetism in diluted magnetic semiconductors, revealing how impurity bands and exchange interactions lead to ferromagnetic behavior.

Contribution

It introduces a systematic study of carrier states in DMSs using the dynamical coherent potential approximation, highlighting the role of impurity bands and nonmagnetic potentials in ferromagnetism.

Findings

Magnetic impurity bands split from the host band with strong exchange interactions.

Carriers in impurity bands strongly couple with localized spins.

Ferromagnetism arises via a double-exchange-like mechanism facilitated by impurity states.

Abstract

Applying the dynamical coherent potential approximation to a simple model, we have systematically studied the carrier states in $A_{1-x}$Mn$_xB$-type diluted magnetic semiconductors (DMS's). The model calculation was performed for three typical cases of DMS's: The cases with strong and moderate exchange interactions in the absence of nonmagnetic potentials, and the case with strong attractive nonmagnetic potentials 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 {\it double-exchange (DE)-like} mechanism. We have investigated the condition for the DE-like mechanism to operate 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 Ga$_{1-x}$Mn$_x$As. The result shows that the ferromagnetism is caused through the DE-like mechanism by the carriers in the bandtail originating from the impurity states.