β-detected \NMR\ Study of Semi-Insulating GaAs

A beta-detected \{NMR\} experiment is carried out on a semi-insulating GaAs crystal using a low energy spin polarized 8Li+ beam to investigate the magnetic properties as a function of temperature and depth. The range of depths probed, from ∼137 nm down to ∼17 nm, coincides with the region of electronic band-bending due to the surface. We find a resonance broadened by the host nuclear dipoles, consistent with an earlier report[6], but using a pulsed radio frequency scheme, we are able to make higher resolution measurements. We find a small negative shift of the resonance (relative to an MgO reference) at room temperature that is independent of depth. Remarkably, as the temperature is reduced below about 150 K, the resonance shifts negatively and becomes depth dependent, with a larger negative shift deeper into the sample. This shift saturates below 10 K, i.e. it does not follow a Curie's law that might be expected for dilute magnetic impurities. Possible origins are discussed.