Browsing by Subject "Spin"
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Item Electrical detection of the spin Hall effect in ferromagnet-semiconductor heterostructures.(2010-06) Garlid, Eric ScottThis thesis describes the effects of spin-orbit coupling on electron transport in Fe/InGaAs heterostructures. Spin-orbit coupling is a relativistic phenomenon that couples the spin of an electron to its momentum by means of a momentum-dependent effective magnetic field. The spin-orbit coupling in bulk InGaAs is determined by measuring the spin Hall effect. In the spin Hall effect, an applied charge current induces a spin current due to spin-orbit coupling. The spin current flows in a direction that is perpendicular to the charge current, with a spin orientation that is perpendicular to the flow direction of both the charge current and the spin current. The spin Hall effect leads to an out-of-plane spin accumulation that is opposite in sign at opposite edges of the channel. Lateral Fe/InGaAs devices are fabricated using standard semiconductor processing techniques. The interface between the Fe and InGaAs is a highly doped Schottky tunnel barrier for efficient electrical spin injection and detection. Measurements of the spin valve and Hanle effect are performed in the non-local geometry to confirm that the Fe electrodes are sensitive to spin polarization in the InGaAs channel and its dephasing by precession in an applied magnetic field. The spin accumulation due to the spin Hall effect is identified through the observation of a Hanle effect in the Hall voltage measured by pairs of ferromagnetic contacts at the channel edges. The data are fit using a model which includes spin diffusion, precession, and relaxation. We use the parameters determined from the fit to calculate the spin Hall conductivity. We find that the magnitude of the spin Hall conductivity is in agreement with models of the extrinsic SHE due to ionized impurity scattering. By analyzing the dependence of the spin Hall signal on channel conductivity we determine the contributions of both skew and side jump scattering to the total spin Hall conductivity. We calculate that the spin-orbit coupling parameter is larger than predicted by standard k ยท p perturbation theory.Item Hyperfine effects in ferromagnet-semiconductor heterostructure(2010-04) Chan, Mun KeatThis 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.