ferromagnetic semiconductor films*

Motivated by the fact that holes in the Ga_1-xMn_xAs family of ferromagnetic semiconductors play a key role in determining their ferromagnetic properties, we have measured hole concentrations in a series of three Ga_1-xMn_xAs_1-yP_y alloys grown by MBE with varying amounts of phosphorus and fixed Mn. This was carried out by anomalous Hall Effect measurements on a series of samples (10%<y<22%), both annealed and as grown. By comparing the total hole concentrations obtained from the number of acceptors and compensating donors to the Hall carrier density, we find that the number of itinerant holes is significantly less that the total hole concentration. This indicates that a sizable fraction of those holes become localized with increasing P concentration, reaching a high resistivity regime for y=21% at low temperature. We also find that the Curie temperature described by the Zener model is determined by the itinerant holes rather than by the total hole concentrationOur results indicate that that ferromagnetism in Ga_1-xMnxAs_1-yP_y vanishes when the total hole concentration falls below a certain Mott-like threshold, suggesting that the holes reside in the acceptor impurity band created by the Mn acceptors.