  Hydrogenation and posthydrogenation annealings have been used as a very
efficient tool to tune the hole density over a wide range, at fixed magnetic
moment concentration, in thin GaMnAs layers. Reduction of the hole density
resulted in strong modifications of their ferromagnetic properties. In
particular, we observed in magnetotransport experiments the decrease of the
Curie temperature, along with modifications of the magnetic anisotropy, a
behavior consistent with the mean-field theory.
