Scientists in Japan1 have demonstrated how to generate a pure spin current in magnetic thin films by the simple application of a temperature gradient. This phenomenon exploits the so-called spin Seebeck effect.

In the Seebeck effect, electrical voltage is generated between two ends of a material kept at different temperatures. This effect is the basis of thermocouples, in which two materials exhibiting different degrees of the Seebeck effect are placed in parallel, resulting in the generation of a voltage between them, which is proportional to the temperature gradient.

The experiments conducted by Eiji Saitoh and colleagues is an analogy of a thermocouple, but in the spin domain. That is, like electrons in the two different materials of a thermocouple, the two spins in a magnetic layer have different scattering rates, so that a temperature gradient produces a spin voltage.

The real challenge however, is detecting this tiny spin voltage. For this purpose, the researchers used the inverse spin Hall effect, which is another recently discovered phenomenon consisting of the conversion of a spin current into an electromotive force (EMF) that can be readily measured.

Fig. 1: Experimental set-up for measuring the spin related EMF in the magnetic layers.

In the experiments, the magnetic layer consisted of a permalloy film, and a platinum wire was attached at its end, perpendicular to the permalloy length (Fig.1). The spin current generated by the spin Seebeck effect was transferred to the non magnetic Pt wire, which in turn generated the EMF. A crucial observation was that the sign of the EMF was directly linked to that of the temperature gradient, and its value depended on the position of the Pt wire along the length of the magnet. This is direct proof that the EMF was actually generated by a spin current.

“The thermally induced spin current allows us to transport magnetic information without an electric current over long distances, with potentially new applications such as new energy technology,” say the authors.