Carrier-mediated ferromagnetic interactions in structures of magnetic semiconductors

Various mechanisms that control the magnitude of the ferromagnetic interaction in semiconductors and their quantum structures are described. Two alternative approaches, the Ruderman-Kittel-Kasuya-Yosida (RKKY) and self-consistent models are recalled and their equivalence is demonstrated in the mean field approximation (MFA). It is shown how the long-range nature of the RKKY interactions in semiconductors, by making the thermal fluctuations of magnetization irrelevant, stabilizes an ordered phase and results in the validity of the MFA, even in the disordered magnetic systems of reduced dimensionality. The role of confinement and of the associated modifications in the density-of-states is examined and shown to be important. It is pointed out that disorder and carrier-carrier interactions may have a profound influence on the spin-spin coupling, particularly near the metal-to-insulator transition (MIT). The corresponding effects are analyzed in terms of the disordered Fermiliquid and Hubbard models, developed for the description of doped semiconductors on the metallic side of the MIT. Outstanding properties of 1D systems, resulting from peculiarities of their DOS, tendency toward spin-density formation and possible charge-spin separation are mentioned. The question concerning the role of non-scalar spin-spin interactions driven by spin-orbit coupling, notably in strained systems, is addressed. Finally, the important issue of control of magnetic properties by electrostatic gate potentials or illumination is presented. (C) 1999 Elsevier Science S.A. All rights reserved.