  We study the controlled introduction of defects in GaMnAs by irradiating the
samples with energetic ion beams, which modify the magnetic properties of the
DMS. Our study focuses on the low-carrier-density regime, starting with
as-grown GaMnAs films and decreasing even further the number of carriers,
through a sequence of irradiation doses. We did a systematic study of
magnetization as a function of temperature and of the irradiation ion dose. We
also performed in-situ room temperature resistivity measurements as a function
of the ion dose. We observe that both magnetic and transport properties of the
samples can be experimentally manipulated by controlling the ion-beam
parameters. For highly irradiated samples, the magnetic measurements indicate
the formation of magnetic clusters together with a transition to an insulating
state. The experimental data are compared with mean-field calculations for
magnetization. The independent control of disorder and carrier density in the
calculations allows further insight on the individual role of this two factors
in the ion-beam-induced modification of GaMnAs.
