Exploring Optical Nuclear Polarization in Ga_1-xMn_xAs Films

Gallium-71 optically-pumped NMR (OPNMR) measurements were performed on thin films (<1 µm thickness) of the dilute magnetic semiconductor Ga_1-xMn_xAs (where x= 0, 0.01 and 0.05), a candidate material for spintronics applications. The optical irradiation time dependence of the OPNMR signal and dark nuclear spin relaxation were measured to investigate the effects of Mn on the spin polarization dynamics of optical nuclear polarization. The action spectra (OPNMR signal vs. photon energy) acquired in bulk GaAs and a Ga_.99Mn_.01As film are compared to theoretical simulations based on the electronic band structure calculations. Incorporation of Mn produces strong oscillations in the OPNMR signal amplitude, which are in qualitative agreement with theory. In addition, the Mn sites were characterized by X-band EPR spectroscopy at 5 K. The results are interpreted in terms of a modified optical pumping model and spectral simulations.