Spin-related Hall effects in Ferromagnet-Semiconductor Heterostructures

Abstract

This thesis examines a series of spin transport phenomena in Fe/n-GaAs heterostructures with the spin-related Hall effects being the main focus. Spin and charge transport measurements are performed on Fe/n-GaAs devices fabricated with standard semiconductor processing techniques. Spin polarization is injected into the n-GaAs channel by applying a forward bias current through the Schottky tunnel barrier at the Fe/n-GaAs interface. The injected electron spin polarization can be transferred to the nuclei of GaAs through the hyperfine interaction, which is known as dynamic nuclear polarization (DNP). The induced hyperfine field can in turn affect the electron spin transport. In the inverse spin Hall measurement, the spin Hall voltages are probed along the Hall arm fabricated locally under the spin injector Fe contact. An out-of-plane magnetic field is applied to precess the spin polarization for a Hanle measurement. Measured spin Hall voltage signals are analyzed with the D’yakonov-Perel phenomenological model and spin drift and diffusion equations to extract the spin Hall angle which characterizes the spin-to-charge conversion efficiency. We find that the effective spin Hall angle values obtained in dc ISHE measurements are 1 to 2 orders of magnitude larger than the prediction of extrinsic scattering theory and other spin-related Hall effects measurements on n-GaAs with similar doping. We credit the enhancement of the spin Hall voltage signals to the DNP-induced hyperfine field. We develop a pulsed-current (pc) measurement technique to suppress the influence of the hyperfine field by utilizing the vast difference between the nuclear spin polarization time and the electron spin lifetime to effectively obtain the needed electrical signals before the nuclei in the n-GaAs channel polarize and generate a strong hyperfine field. The spin Hall voltages and spin Hall angles decrease by one to two orders of magnitude in pc ISHE measurements and are in good agreement with the calculation of the extrinsic scattering theory and other spin-related Hall experiments. However, the microscopic mechanism of how the hyperfine field enhances the spin-to-charge conversion is still elusive and needs more investigation.