The Spin Hall Effect (SHE) is a transport phenomenon predicted by Russian physicists M.I. Dyakonov and V.I. Perel in 1971. It consists of the appearance of spin accumulation on the lateral surfaces of an electric current-carrying sample, the signs of the spin directions being opposite on the opposing boundaries. In a cylindrical wire, the current-induced surface spins will wind around the wire. When the current direction is reversed, the directions of spin orientation is also reversed.
The term "Spin Hall Effect" was introduced by Hirsch in 1999. Indeed, it is somewhat similar to the classical Hall effect, where charges of opposite sign appear on the opposing lateral surfaces to compensate for the Lorentz force, acting on electrons in an applied magnetic field. However, no magnetic field is needed for SHE. On the contrary, if a strong enough magnetic field is applied in the direction perpendicular to the orientation of the spins at the surfaces, SHE will disappear because of the spin precession around the direction of the magnetic field.
Experimentally, the Spin Hall Effect was observed in semiconductors more than 30 years after the original prediction. The spin accumulation induces circular polarization of the emitted light, as well as the Faraday (or Kerr) polarization rotation of the transmitted (or reflected) light, which allows to monitor SHE by optical means.
The origin of SHE is in the spin-orbit interaction, which leads to the coupling of spin and charge currents: an electrical current induces a transverse spin current (a flow of spins) and vice versa. One can intuitively understand this effect by using the analogy between an electron and a spinning tennis ball, which deviates from its straight path in air in a direction depending on the sense of rotation (the Magnus effect).
The Inverse Spin Hall Effect, an electrical current induced by a spin flow due to a space-dependent spin polarization, was first observed in 1984. More recently, the existence of both direct and inverse effects was demonstrated not only in semiconductors, but also in metals.
The SHE belongs to the same family as the anomalous Hall effect, known for a long time in ferromagnets, which also originates from spin-orbit interaction.
The SHE can be used to manipulate electron spins electrically. For example, in combination with the electric stirring effect, the SHE leads to spin polarization in a localized conducting region.
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