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