Electron-mediated ferromagnetism and negative s-d exchange splitting in semiconductors

Several approaches are proposed to enhance the spin splitting at the conduction band edge and consequently induce free-electron-mediated stabilization of ferromagnetic semiconductors. We show that kinetic s-d coupling can be introduced through chemical ordering and/or strain. We find that quantum confinement has a large effect on the spin splitting at the conduction band edge. It can effectively reverse the sign of the conduction band splitting, thus, shedding light on the recent puzzling experimental observation that s-d exchange splitting is negative in semiconductor superlattices. Finally, we show that, using rare-earth elements as magnetic dopants, the symmetry-allowed s-f coupling can lead to a large splitting at the conduction band edge.