Most electronics devices are driven by charge carrying currents: the heating of a coil, the charging of a capacitor, or the generation of radio waves from an antenna. In the emerging field of ‘spintronics’, however, it is spin—the quantized angular momentum intrinsic to all electrons—that govern device operation. Spin carrying currents can switch small-scale magnets—the type that could be used for high-density data storage—more efficiently than charge carrying currents and do not dissipate as much heat in the process. But, how do you create a purely spin carrying current?

Fig. 1: By driving a current through the ferromagnetic electrode on the left side of this ‘non-local spin valve’, a spin-current is generated that can flip the nanoscale magnetic electrode on the right.© 2008 NPG

Tao Yang, Takashi Kimura and Yoshichika Otani at the Advanced Science Insitute of RIKEN in Wako, Japan1 are exploring ways to do this with a device called the ‘non-local spin-valve’. This is a thin metal wire made of copper that has two ferromagnetic electrodes sitting on top of it (Fig. 1). When a current flows between the wire and one ferromagnetic electrode, it generates a spin current between the wire and the other ferromagnetic electrode. In principle, this spin current can flip the magnetization of the second ferromagnetic electrode. And, since the charge current is flowing in a different part of the device, it does not heat the flipping magnet.

The challenge is injecting the spin current into the second ferromagnetic electrode with a great enough efficiency to actually flip it. The researchers realized they could increase this efficiency by making better interfaces between the ferromagnetic electrodes and the metal wire, and devised a single-step fabrication procedure to prevent the copper wire from oxidizing before the ferromagnetic electrode was deposited.

The spin-injection efficiency of their devices was greater by an order of magnitude than previous reports and the group was able to switch the magnetization of one of the electrodes without directly running a current through it.

“Magnetization control using a pure spin current could lead to the realization of very advanced electronic devices,” says Otani. “We believe, for example, that it will be possible to achieve different types of transistors—which have no analogues in current electronics—based only on electron spin.”