Circuit model for spin-bottleneck resistance in magnetic-tunnel-junction devices

Spin-bottlenecks are created in magnetic-tunnel-junction devices by spatial inhomogeneity in the relative resistances for up and down spins. We propose a simple electrical circuit model for these devices which incorporates spin-bottleneck effects and can be used to calculate their overall resistance and magnetoresistance. The model permits a simple understanding of the dependence of device magnetoresistance on spin diffusion lengths, tunneling magnetoresistance, and majority and minority spin resistivities in the ferromagnetic electrodes. The circuit model is in a good quantitative agreement with detailed transport calculations.Comment: 4 pages, 3 figures, submitted to Phys. Rev.

Resistance spikes and domain wall loops in Ising quantum Hall ferromagnets

We explain the recent observation of resistance spikes and hysteretic transport properties in Ising quantum Hall ferromagnets in terms of the unique physics of their domain walls. Self-consistent RPA/Hartree-Fock theory is applied to microscopically determine properties of the ground state and domain-wall excitations. In these systems domain wall loops support one-dimensional electron systems with an effective mass comparable to the bare electron mass and may carry charge. Our theory is able to account quantitatively for the experimental Ising critical temperature and to explain characteristics of the resistive hysteresis loops.Comment: 4 pages, 3 figure

Theory of Weak Localization in Ferromagnetic (Ga,Mn)As

We study quantum interference corrections to the conductivity in (Ga,Mn)As ferromagnetic semiconductors using a model with disordered valence band holes coupled to localized Mn moments through a p-d kinetic-exchange interaction. We find that at Mn concentrations above 1% quantum interference corrections lead to negative magnetoresistance, i.e. to weak localization (WL) rather than weak antilocalization (WAL). Our work highlights key qualitative differences between (Ga,Mn)As and previously studied toy model systems, and pinpoints the mechanism by which exchange splitting in the ferromagnetic state converts valence band WAL into WL. We comment on recent experimental studies and theoretical analyses of low-temperature magnetoresistance in (Ga,Mn)As which have been variously interpreted as implying both WL and WAL and as requiring an impurity-band interpretation of transport in metallic (Ga,Mn)As.Comment: 16 pages, 10 figures; submitted to Phys. Rev.

Non-vanishing spin Hall currents in disordered spin-orbit coupling systems

Spin currents that flow perpendicular to the electric field direction are generic in metals and doped semiconductors with spin-orbit coupling. It has recently been argued that the spin Hall conductivity can be dominated by an intrinsic contribution which follows from Bloch state distortion in the presence of an electric field. Here we report on an numerical demonstration of the robustness of this effect in the presence of disorder scattering for the case of a two-dimensional electron-gas with Rashba spin-orbit interactions (R2DES).Comment: 4 pages, 3 figure

Boltzmann theory of engineered anisotropic magnetoresistance in (Ga,Mn)As

We report on a theoretical study of dc transport coefficients in (Ga,Mn)As diluted magnetic semiconductor ferromagnets that accounts for quasiparticle scattering from ionized Mn$^{2+}$ acceptors with a local moment $S=5/2$ and from non-magnetic compensating defects. In metallic samples Boltzmann transport theory with Golden rule scattering rates accounts for the principle trends of the measured difference between resistances for magnetizations parallel and perpendicular to the current. We predict that the sign and magnitude of the anisotropic magnetoresistance can be changed by strain engineering or by altering chemical composition.Comment: 4 pages, 2 figure

Anisotropic magnetoresistance of spin-orbit coupled carriers scattered from polarized magnetic impurities

Anisotropic magnetoresistance (AMR) is a relativistic magnetotransport phenomenon arising from combined effects of spin-orbit coupling and broken symmetry of a ferromagnetically ordered state of the system. In this work we focus on one realization of the AMR in which spin-orbit coupling enters via specific spin-textures on the carrier Fermi surfaces and ferromagnetism via elastic scattering of carriers from polarized magnetic impurities. We report detailed heuristic examination, using model spin-orbit coupled systems, of the emergence of positive AMR (maximum resistivity for magnetization along current), negative AMR (minimum resistivity for magnetization along current), and of the crystalline AMR (resistivity depends on the absolute orientation of the magnetization and current vectors with respect to the crystal axes) components. We emphasize potential qualitative differences between pure magnetic and combined electro-magnetic impurity potentials, between short-range and long-range impurities, and between spin-1/2 and higher spin-state carriers. Conclusions based on our heuristic analysis are supported by exact solutions to the integral form of the Boltzmann transport equation in archetypical two-dimensional electron systems with Rashba and Dresselhaus spin-orbit interactions and in the three-dimensional spherical Kohn-Littinger model. We include comments on the relation of our microscopic calculations to standard phenomenology of the full angular dependence of the AMR, and on the relevance of our study to realistic, two-dimensional conduction-band carrier systems and to anisotropic transport in the valence band of diluted magnetic semiconductors.Comment: 15 pages, Kohn-Littinger model adde