Carrier Induced Ferromagnetism in Concentrated and Diluted Local-Moment Systems

Abstract

For modeling the electronic and magnetic properties of concentrated and diluted magnetic semiconductors we use the s-f model (ferromagnetic Kondo-lattice model), which traces back the characteristic properties of such materials to an interband exchange coupling between itinerant conduction electrons and localized magnetic moments. For the electronic part an interpolation scheme between a maximum number of rigorously tractable limiting cases is desirable, since the parameter dependence of the selfenergy cannot be calculated exactly. Two possible implementations, which allow the evaluation of temperature-dependent quasiparticle densities of states and spectral densities hi different parameter regimes, axe presented. For constructing the magnetic phase diagram again two approaches are proposed. A modified RKKY theory maps the interband exchange to ail effective Heisenberg model, the exchange integrals of which turn out to be functionals of the electronic selfenergy acquiring therewith a distinct; temperature and carrier concentration dependence. A second method derives the magnetic phase diagram from the paramagnetic susceptibility of the itinerant conduction electrons avoiding therewith a mapping of the s-f exchange onto an effective Heisenberg model. The latter procedure allows to inspect the influence of the moment disorder in diluted systems on magnetic stability by adding a CPA-type concept; to the theory. For almost all moment concentrations x ferromagnetism is possible, however, only for carrier concentrations n distinctly smaller than x. This can be understood by inspecting the respective quasiparticle density of states. The charge carrier compensation in real magnetic semiconductors such as Ga1-xMnxAs seems to be a necessary condition for getting carrier induced ferromagnetism.