In spintronics devices, information is transported and stored in the ‘spin’ of an electron. For spintronics to take off as a technology, however, scientists must first find efficient means of generating and powering a spin current in a circuit.

A team of scientists lead by Eiji Saitoh from the Institute for Materials Research at Tohoku University in Japan has now shown that temperature gradients can be used to drive a spin current in insulators.1 The finding provides new clues on how heat can generate a spin current — a versatile and potentially energy-efficient effect for powering spintronics devices.

Fig. 1: A temperature gradient (∇T) induces a spin voltage (V) in the magnetic insulator LaY2Fe5O12.From Ref. 1. © 2010 E. Saitoh

The team’s finding is related to the well-known thermoelectric effect, which causes electrons to move from the hot to cold end of a metal strip. The accumulation of electrons at the hot end of the metal generates a voltage that can, in principle, power a circuit.

Two years ago, Saitoh and his team discovered that a temperature gradient across a magnetic metal strip produces a spin voltage — the driving force for a spin current. Saitoh and others were unsure, though, if it was the motion of electrons or some other mechanism that produced the spin voltage.

The team therefore decided to look for the same effect in a magnetic insulator in which conducting electrons would not play a role. In this study, they placed a thin strip of platinum metal on either end of a rectangular film of the magnetic insulator LaY2Fe5O12 and applied a temperature gradient between the two ends (see image). The researchers found that a spin voltage was generated in the film, inducing a spin current that flowed across the LaY2Fe5O12–platinum interface and which in turn generated an electrical current in the platinum strips.

The observation of a heat-generated spin voltage in insulators as well as metals means that the effect is likely tied to the motion of ‘spin waves’, explains Ken-ichi Uchida of the research group. He also notes that the generation of current in millimeter-long devices in their experiments “upsets the conventional wisdom” that spin currents can only travel a few micrometers before dissipating.