Hyperfine effects in ferromagnet-semiconductor heterostructure

Abstract

This thesis describes the effect of hyperfine interactions on non-equilibrium electron spins in Fe/GaAs heterostructures. Nuclei in bulk GaAs are dynamically polarized by a non-equilibrium electron spin population injected through an Fe/GaAs Schottky tunnel barrier. The polarized nuclei in turn exert a large hyperfine field upon the electron spins, resulting in rapid electron spin precession. Electrical measurements of the steady state electron spin polarization as a function of applied magnetic field for various injector biases and temperatures allow us to extract the electron spin lifetime, Knight shift, and nuclear field parameters in bulk GaAs. We successfully model electron spin dynamics using a coupled electron-nuclear drift diffusion equation. We confirmed the strong hyperfine coupling between electron and nuclear spins by performing nuclear magnetic resonance measurements on Fe/GaAs devices in applied fields of only a few hundred Oe. Resonant frequencies of different isotopes in the GaAs channel were detected. In addition to exerting a hyperfine field on the electron spins, we also observe a hyperfine induced spin-dependent Hall effect measured across the spin-polarized region of a GaAs channel. Application of a transverse magnetic field results in a modulation of the Hall voltage consistent with spin de-phasing. This signal changes sign when the magnetization of the Fe contact is switched, indicating sensitivity to electron spin direction. The observed spin-dependent Hall signal is approximately two orders of magnitude larger than that expected from previous measurements of the spin Hall effect in n-GaAs, which was attributed to spinorbit coupling and impurity scattering. This suggests that a different mechanism is active in our system. We demonstrate full suppression of the spin-dependent Hall signal by eliminating nuclear polarization through a field cycling procedure. Additionally, while the electron spin accumulation, detected by a spin sensitive Fe contact, persists up to 200 K, the spin-dependent Hall signal is not observed above 120 K, in coincidence with the disappearance of the nuclear spin polarization due to delocalization of donor electrons. We conclude that the observed spindependent Hall signal is coupled to the nuclear spin polarization. This is the first observation of a hyperfine-induced spin Hall effect.