  We report on in-situ resistivity measurements on GaMnAs during post-growth
annealing in different atmospheres. A drop in the resistivity is observed when
the GaMnAs is exposed to oxygen, which indicates that the passivation of Mn
interstitials (Mn_I) at the free surface occurs through oxidation. The presence
of oxygen can therefore be an important annealing condition for the
optimization of GaMnAs thin films, all the more since the oxidation appears to
be limited to the sample surface. Annealing in an oxygen-free atmosphere leads
to an increase in the resistivity indicating a second annealing mechanism
besides the out-diffusion of Mn_I. According to our magnetization and Hall
effect data, this mechanism reduces the amount of magnetically and electrically
active Mn atoms.
