The interplay between disorder and spin polarization in a GaMnAs thin layer results into spin-polarized impurity hole bands. A figure of merit is defined to label the hole state as being extended or localized. The calculation leads to a phase diagram determining the metallic or non-metallic character of the sample. It is shown that samples with the highest figures of merit have a ratio between the extended hole density and the Mn concentration near 0.2, in agreement with the ratio of 0.1-0.25 known to occur among samples produced with the highest Curie temperatures. Both the non-metal-to-metal and the metal-to-non-metal transitions experimentally observed in the ferromagnetic regime are obtained, as the Mn concentration increases. An explanation is given for the occurrence of a maximal Curie temperature in ferromagnetic GaMnAs samples.
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