Spin dependent electron transport in nanostructures

Abstract

Spin-electronic devices, exploiting the spin degree of freedom of the current carrying particles, are currently a topic of great interest. In parallel with experimental developments, theoretical studies in this field have been mainly focused on the coherent transport regime characteristics of these devices. However, spin dephasing processes are still a fundamental concern [1-6]. The Landauer transmission formalism has been the widely used method in the coherent transport regime [7]. Recently this formalism has been adapted to incorporate spin scattering processes by introducing random disorder directly into the conducting medium and subsequently solving the disordered transport problem over a large ensemble of disorder distributions [8-10]. Although proposed to be a way of incorporating spin scattering processes, what this approach basically offers is an averaged way of adding random coherent scatterings (similar to the scatterings from boundaries) into the transport problem. Certainly such a treatment of spin-dephasing processes misses the incoherent and inelastic nature of the scattering processes. As a result, a rigorous way of treating the spin scattering processes is still needed [10-12]. The objective of this thesis is to present a quantum transport model based on non-equilibrium Green\u27s function (NEGF) formalism providing a unified approach to incorporate spin scattering processes using generalized interaction Hamiltonians. Here, the NEGF formalism is presented for both coherent and incoherent transport regimes without going into derivational details. Subsequently, spin scattering operators are derived for the specific case of electron-impurity exchange interactions and the model is applied to clarify the experimental measurements [5]. Device characteristics of magnetic tunnel junctions (MTJs) with embedded magnetic impurity layers are studied as a function of tunnel junction thicknesses and barrier heights for varying impurity concentrations in comparison with experimental data. For MTJs with embedded magnetic impurity layers, this model is able to capture and explain three distinctive experimental features reported in the literature regarding the dependence of the junction magneto-resistances (JMRs) on (1) barrier thickness, (2) barrier heights and (3) the concentrations of magnetic impurities [5,6,29,46]. Although in this dissertation our treatment was restricted to the electron-impurity spin exchange interactions, the NEGF model presented here allows one to incorporate other spin exchange scattering processes involving nuclear hyperfine, Bir-Aranov-Pikus (electron-hole) and electron-magnon interactions. This model is general and can be used to analyze and design a variety of spintronic devices beyond the large cross-section multilayer devices explored in this work