Experimental Measurements by Antilocalization of the Interactions between Two-Dimensional Electron Systems and Magnetic Surface Species

Abstract

Low-temperature weak-localization (WL) and antilocalization (AL) magnetotransport measurements are sensitive to electron interference, and thus can be used as a probe of quantum states. The spin-dependent interactions between controllable surface magnetism and itinerant electrons in a non-magnetic host provide insight for spin-based technologies, magnetic data storage and quantum information processing. This dissertation studies two different host systems, an In0.53Ga0.47As quantum well at a distance from the surface of a heterostructure, and an accumulation layer on an InAs surface. Both the systems are two-dimensional electron systems (2DESs), and possess prominent Rashba spin-orbit interaction caused by structural inversion asymmetry, which meets the prerequisites for AL. The surface local moments influence the surrounding electrons in two ways, increasing their spin-orbit scattering, and inducing magnetic spin-flip scattering, which carries information about magnetic interactions. The two effects modify the AL signals in opposing directions: the spin-flip scattering of electrons shrinks the signal, and requires a close proximity to the species, whereas the increase of spin-orbit scattering broadens and increases the signal. Accordingly, we only observe an increase in spin-orbit scattering in the study of the interactions between ferromagnetic Co0.6Fe0.4 nanopillars and the relatively distant InGaAs quantum well. With these CoFe nanopillars, a decrease in spin decoherence time is observed, attributed to the spatially varying magnetic field from the local moments. A good agreement between the data and a theoretical calculation suggests that the CoFe nanopillars also generate an appreciable average magnetic field normal to the surface, of value ∼ 35 G. We also performed a series of comparative AL measurements to experimentally investigate the interactions and spin-exchange between InAs surface accumulation electrons and local magnetic moments of rare earth ions Sm3+, Gd3+, Ho3+, of transition metal ions Ni2+, Co2+, and Fe3+, and of Ni2+-, Co2+-, and Fe3+-phthalocyanines deposited on the surface. The deposited species generate magnetic scattering with magnitude dependent on their electron configurations and effective moments. Particularly for Fe3+, the significant spin-flip scattering due to the outermost 3d shell and the fairly high magnetic moments modifies the AL signal into a WL signal. Experiments indicate a temperature-independent magnetic spin-flip scattering for most of the species except for Ho3+ and Co2+. Ho3+ yields electron spin-flip rates proportional to the square root of temperature, resulting from transitions between closely spaced energy levels of spin-orbit multiplets. In the case of Co2+, either a spin crossover or a spin-glass system forms, and hence spin-flip rates transit between two saturation regions as temperature varies. Concerning the spin-orbit scattering rate, we observe an increase for all the species, and the increase is correlated with the effective electric fields produced by the species. In both 2DESs, the inelastic time is inversely proportional to temperature, consistent with phase decoherence via the Nyquist mechanism. Our method provides a controlled way to probe the quantum spin interactions of 2DESs, either in a quantum well, or on the surface of InAs.