Spin Hall effect

Spin Hall effects are a collection of relativistic spin-orbit coupling phenomena in which electrical currents can generate transverse spin currents and vice versa. Although first observed only a decade ago, these effects are already ubiquitous within spintronics as standard spin-current generators and detectors. Here we review the experimental and theoretical results that have established this sub-field of spintronics. We focus on the results that have converged to give us a clear understanding of the phenomena and how they have evolved from a qualitative to a more quantitative measurement of spin-currents and their associated spin-accumulation. Within the experimental framework, we review optical, transport, and magnetization-dynamics based measurements and link them to both phenomenological and microscopic theories of the effect. Within the theoretical framework, we review the basic mechanisms in both the extrinsic and intrinsic regime which are linked to the mechanisms present in their closely related phenomenon in ferromagnets, the anomalous Hall effect. We also review the connection to the phenomenological treatment based on spin-diffusion equations applicable to certain regimes, as well as the spin-pumping theory of spin-generation which has proven important in the measurements of the spin Hall angle. We further connect the spin-current generating spin Hall effect to the inverse spin galvanic effect, which often accompanies the SHE, in which an electrical current induces a non-equilibrium spin polarization. These effects share common microscopic origins and can exhibit similar symmetries when present in ferromagnetic/non-magnetic structures through their induced current-driven spin torques. Although we give a short chronological overview, the main body is structured from a pedagogical point of view, focusing on well-established and accepted physics.