WAVE WIRE: A collective excitation of magnetic moments [represented here by blue arrows], known as a spin wave, can pass through an insulator to link electric processes on either side.
Jens Böning via Wikimedia Commons

An electric insulator, in the simplest terms, blocks the flow of electric current. So it would be a bit counterintuitive, to say the least, if a current on one side of an insulator could produce voltage on the other.

But that is precisely what a group of Japanese researchers has found, as detailed in a study in the March 11 issue of Nature. The electric current induces a collective excitation in the magnetic insulator that can travel relatively long distances before unloading its momentum to generate a voltage when it reaches an electric conductor. (Scientific American is part of Nature Publishing Group.)

Although insulators are impervious to the movement of electric current, an electron is more than a simple charge carrier. It also features a quantum-mechanical property known as spin, which can be thought of as describing the pointing of its axis, like that of a spinning top, as well as the orientation of its magnetic field. A wave of spin can ripple through a magnetic insulator as a disruption in the ordered pointing of the material's magnetic moments for relatively long distances.

In the new research, physicist Eiji Saitoh of Tohoku University in Japan and his colleagues laid two platinum conductors on a layer of yttrium iron garnet (Y₃Fe₅O₁₂), a magnetic insulator in which spin waves can travel centimeters. Separated by one millimeter, the platinum films were sufficiently distant to preclude the quantum-mechanical phenomenon known as tunneling, in which a particle slips through a barrier to the other side.

The researchers showed that at the interface between the magnetic insulator and the conductor, the two materials can exchange angular momentum from spin. The spin of conduction electrons in platinum sets off a lateral spin wave in the yttrium iron garnet below, which then transfers spin into the other platinum film a millimeter away. Such an exchange is made possible by a pair of phenomena in the conductor known as the spin Hall effect and the inverse spin Hall effect, which convert charge currents to spin currents and vice versa. So a spin wave in an insulator acts as a signal that can be encoded from and decoded into electric charge in a conductor on either side. "In the experiments, electric signal is carried by an electron's spin instead of electric current," Saitoh says.

Arne Brataas, a physicist at the Norwegian University of Science and Technology, calls the new work an "impressive combination of...independently interesting phenomena." The researchers, Brataas says, "experimentally demonstrate how electric signals can be converted into pure spin signals, transported through an insulating medium where no charges flow, and finally converted back into electric signals." He adds that the research might find use in searching for a way to transmit signals across macroscopic distances in spin-based electronics, or spintronics.

Burkard Hillebrands, a physicist at the Technical University of Kaiserslautern in Germany, also praised the researchers for marrying previously disparate phenomena. Hillebrands says that spin waves have been studied extensively, but that instigating and detecting them via the spin Hall effect and its inverse "is a major breakthrough."