A current that sorts electrons by their . In materials with strong – platinum, tungsten, some layered tellurides – electrons flowing along a strip are pushed sideways according to their spin, spin-up to one edge and spin-down to the other, so the edges become magnetised without any magnet or magnetic field. It also works in reverse: a flow of spin turns into a voltage. Together these let electronics create and detect spin currents, and a spin current poured into a neighbouring magnet can flip it – a way to write magnetic memory.
As the site uses it
The polarisation is usually described by the spin-current (inverse Dzyaloshinskii–Moriya) mechanism for spiral magnets, so models need the full spin texture, not only ordering temperatures.
TaIrTe4 is a Weyl semimetal whose low symmetry lets thin flakes rectify radio waves and detect mid-infrared light without any bias, and lets its spin currents flip magnets that point out of the plane without a magnetic field.
Spin–orbit coupling sends electrons of opposite spin to opposite edges of a current-carrying strip, with no magnetic field. The inverse effect turns a spin current into a voltage, and a spin current poured into a magnet exerts a torque that can flip it – from WTe2, even a magnet pointing out of the plane, without a field.
Sorting spins without a magnet
In the ordinary a magnetic field bends moving charges to one side of a conductor. In the spin Hall effect there is no field: spin–orbit coupling, the link between an electron’s spin and its motion, deflects electrons with spin pointing one way to one side and those with the opposite spin to the other. Equal numbers go each way, so no charge builds up and no voltage appears – but a spin current flows across the strip, and its edges end up carrying opposite spin.
It was first seen directly in 2004, when microscopy imaged spin polarisation pointing out of the plane, with opposite signs at the two edges of a gallium arsenide channel carrying a current, at zero magnetic field (Kato and colleagues, 2004). Charge cannot vanish but spin can, so the polarisation builds up only near the edges, within the distance over which spins lose their direction.
Writing and reading spin
Run backwards, the effect turns a spin current into a charge current across it – the inverse spin Hall effect, which converts spins pumped in from a magnet, or launched by a laser pulse, into a voltage that can be measured. emitters work this way. The fraction converted is the spin Hall angle; in it has been tuned up to about 2%, comparable to platinum (Sinova and colleagues, 2015), and heavy metals such as platinum, tantalum and tungsten are the usual spin sources.
The practical prize is the spin–orbit torque. A current through a heavy-metal layer pushes a spin current into a magnet on top, and the spins it delivers exert a torque that can flip the magnet – the writing step of spin–orbit-torque memory. The catch is symmetry: in such stacks the torque that does the work must lie in the plane, which suits magnets lying in the plane but not the perpendicular ones that dense memories need, unless an extra magnetic field breaks the tie.
Layered spin sources
Layered crystals with low symmetry relax that rule. The surface of WTe2 has a single and no twofold rotation, and a current along its low-symmetry axis produces an out-of-plane antidamping torque on a permalloy layer above it, which a current along the high-symmetry axis does not (MacNeill and colleagues, 2017). The same kind of torque has switched a perpendicularly magnetised magnet deterministically, with no field applied (Kao and colleagues, 2022). and other layers with strong spin–orbit coupling are studied as spin sources for the same reason.
The quantum spin Hall effect, despite the name, is a different thing: in a topological insulator electrons of opposite spin run in opposite directions along each edge without scattering, and the conductance is quantised.
For specialists
The generation of a transverse spin current by a charge current through spin–orbit coupling – intrinsic, from the of the bands, or extrinsic, from skew and side-jump scattering – and its inverse, the conversion of a spin current into a transverse charge current. Unlike the Hall effect it needs no magnetic field; charge is conserved but spin is not, so spin accumulates only near the edges, first imaged by Kerr rotation in GaAs.
The conversion efficiency, the spin Hall angle, has been tuned up to about 0.02 in semiconductors, comparable to platinum. In heavy-metal/ symmetry confines the antidamping spin–orbit torque to the plane; WTe2, with a single mirror plane, also gives an out-of-plane antidamping torque, enough to switch a perpendicularly magnetised van der Waals magnet without an applied field. The quantised quantum spin Hall effect belongs to topological insulators.
switching in provides a powerful route to reconfiguring quantum states and their associated transport responses. Here, we propose a ferroelectric heterobilayer in which the stepwise reversal of polarization in the two monolayers enables selective control of spin…
We demonstrate that twisted and vector optical beams can spatially separate two-dimensional electrons with opposite spin projections. Spin accumulation arises from diverging spin currents induced by structured light together with spin relaxation and diffusion. We derive analytical expressions for the resulting spin…
A longitudinal spin current arises for an electric field along the lattice direction, while a transverse pure spin current emerges along the lattice angle bisector.