  Ion irradiation of semiconductors is a well understood method to tune the
carrier concentration in a controlled manner. We show that the ferromagnetism
in GaMnAs films, known to be hole-mediated, can be modified by He ion
irradiation. The coercivity can be increased by more than three times. The
magnetization, Curie temperature and the saturation field along the
out-of-plane hard axis all decrease as the fluence increases. The electrical
and structural characterization of the irradiated GaMnAs layers indicates that
the controlled tailoring of magnetism results from a compensation of holes by
generated electrical defects.
