When an electrical current travels along a conducting bar, a Lorentz force due to an applied perpendicular magnetic field deflects positive charges to one side and negative charges to the other. This charge accumulation creates an electric field transverse to the current flow, resulting in a voltage difference (Hall voltage) and a corresponding Hall resistance.

There is also an anomalous Hall effect in ferromagnetic materials for which relativistic effects lead to an additional contribution to the resistance. Spin‑orbit coupling can give rise to skew scattering, caused by the inherent right‑left asymmetry experienced by electrons in a ferromagnet, in which the spins are polarized. The spin Hall effect may have a similar origin.

Now, Takeshi Seki and co‑workers at the Institute for Materials Research, Tohoku University, Sendai, CREST, Japan Science and Technology Agency, Tokyo, the National Institute of Advanced Industrial Science and Technology, Tsukuba, and Graduate School of Engineering, Tohoku University, Sendai, Japan report the largest spin Hall effect observed to­‑date.1

Fig. 1: A schematic diagram showing the FePt injector (magnetized along M) and Au Hall cross (with voltage difference V).

In addition to the anomalous Hall effect that gives rise to a local Hall effect, the researchers also measured the spin Hall effect and the inverse spin Hall effect—which is the transverse charge current induced by the spin current. The authors used a multi‑terminal device with a magnetic FePt spin injector and a non‑magnetic gold Hall cross (Fig. 1).

In one configuration, polarized spin current was injected by the FePt injector, which was deflected in the gold Hall cross, leading to the inverse spin Hall effect. Alternatively, the injector acts as a detector for the spin‑dependent electrochemical potentials in gold as a result of the direct spin Hall effect.

The resulting (inverse) spin Hall resistance of 2.9 mW was ten times greater than previous reports, which was attributed to skew‑scattering effects in gold. As for the implications of this giant spin Hall effect, the authors believe that it may lead to, “new spintronic devices, such as a magnetic noise‑free reader for data storage and a spin Hall effect‑based logic circuit.”