Prospects for high temperature ferromagnetism in (Ga,Mn)As semiconductors

We report on a comprehensive combined experimental and theoretical study of Curie temperature trends in (Ga,Mn)As ferromagnetic semiconductors. Broad agreement between theoretical expectations and measured data allows us to conclude that $T_c$ in high-quality metallic samples increases linearly with the number of uncompensated local moments on ${\mathrm{Mn}}_{\mathrm{Ga}}$ acceptors, with no sign of saturation. Room temperature ferromagnetism is expected for a 10% concentration of these local moments. Our magnetotransport and magnetization data are consistent with the picture in which Mn impurities incorporated during growth at interstitial ${\mathrm{Mn}}_{\mathrm{I}}$ positions act as double-donors and compensate neighboring ${\mathrm{Mn}}_{\mathrm{Ga}}$ local moments because of strong near-neighbor ${\mathrm{Mn}}_{\mathrm{Ga}}{\mathrm{Mn}}_{\mathrm{I}}$ antiferromagnetic coupling. These defects can be efficiently removed by post-growth annealing. Our analysis suggests that there is no fundamental obstacle to substitutional ${\mathrm{Mn}}_{\mathrm{Ga}}$ doping in high-quality materials beyond our current maximum level of 6.8%, although this achievement will require further advances in growth condition control. Modest charge compensation does not limit the maximum Curie temperature possible in ferromagnetic semiconductors based on (Ga,Mn)As.