Carrier-mediated stabilization of ferromagnetism in semiconductors: holes and electrons

Band structure models are proposed to help understand carrier-induced ferromagnetism in diluted magnetic semiconductors. We describe both hole- and electron-mediated ferromagnetism. For hole-mediated ferromagnetism, we show that there are two distinct mechanisms related to the stabilization of ferromagnetism. The difference between them is related to the position of the impurity d levels with respect to the valence band edge. If the impurity states are in the band gap, ferromagnetism can be explained by double exchange, which is related to the direct coupling between the impurity levels, whereas if the filled impurity states are below the VBM, ferromagnetism can be explained by the Zener model, which is related to the coupling between the impurity d levels and the host valence p states. In both cases, it is necessary to have holes, either free or localized, to stabilize the ferromagnetism. Our model successfully explains the ground state magnetic configuration of CdMnTe, GaMnAs, ZnMnO, and GaMnN. An extension of our model can also successfully explain the intriguing behavior of GaMnN. We show that at low Mn concentration, its ground state is FM, although AFM can be stabilized by increasing the Mn concentration, applying pressure, or by compensating the holes. For electron-mediated ferromagnetism, we point out that it is necessary first to increase the exchange splitting of the conduction band. This can be done by reducing the symmetry of the system, by quantum confinement, or by changing the magnetic impurities from transition metals to rare earths. Our results also clarify some issues related to experimental works, such as the negative exchange splitting of the conduction band in GaMnAs quantum wells and the stabilization of ferromagnetism in GaGdN. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.