Curie-temperature enhancement in ferromagnetic semiconductor superlattices

Curie temperatures in ferromagnetic semiconductor superlattices are calculated self-consistently within mean-field theory and the effective bond orbital model of the superlattice band structure. Assuming a null valence-band offset and a constant average hole density of ${10}^{20}{\mathrm{cm}}^{\mathrm{-}3},$ the Curie temperature for a GaAs/GaMnAs “digital alloy” superlattice is found to be enhanced over its value in a bulk GaMnAs alloy with the same average Mn fraction. This enhancement results from the formation of a hole potential well, induced by the exchange interactions, in the vicinity of the thin magnetic layer. We further show that much larger enhancements may be possible in structures incorporating an Al(Ga)As nonmagnetic barrier layer having a large valence-band offset relative to GaMnAs. This structure creates a much deeper well that much more effectively confines the hole wave functions to the vicinity of the layers with the high Mn concentrations.