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Tuesday, 05 December 2017

Thermal gradients shown to enhance spin transport in graphene

ICN2 researchers have demonstrated that the application of a thermal gradient in spintronic devices can cause spin signal to increase as a result of a novel thermoelectric phenomenon predicted and subsequently observed in graphene. Specifically, the enhanced spin signal is two orders of magnitude larger than anything previously reported for thermal effects in metals. Published in Nature Nanotechnology, these findings push at the frontier of graphene spintronics technologies.

Scientists of the ICN2 Physics and Engineering of Nanodevices Group, led by ICREA Prof. Sergio O. Valenzuela, have contributed to the literature on spin caloritronics with a focus on the effect of thermal gradients on spins in graphene. The paper titled “Thermoelectric spin voltage in graphene” was published this week in Nature Nanotechnology, with lead author Juan F. Sierra.

Spin caloritronics is an emerging field that studies the interaction of spin and heat currents in different materials, where spin is an intrinsic property of electrons which, like charge, can be used to store and transport information. Researchers are looking at different ways to generate spin currents and exploit them in a future generation of electronic devices. However, sustaining them over the kind of distances needed in practice is a challenge. Heat currents offer a possible solution.

In this paper ICN2 researchers turn their attention to graphene. Boasting a wealth of properties that make it uniquely able to transport spin efficiently over long distances, this material is already the focus of much attention in spintronics. And given that graphene is known to present large thermoelectric effects and extraordinarily long carrier cooling times, the application of heat currents promised interesting results. It did not disappoint.

Using a precise experimental setup, the researchers were able to independently control spin and heat currents in graphene. They observed that the presence of a thermal gradient significantly enhances the spin signal, and that it does so around the charge neutrality point. Overall, graphene’s baseline spin signal was increased by around thirty percent upon application of a heat current, giving a total signal two orders of magnitude greater than anything previously reported for thermal effects in metals.

Such a large thermoelectric spin signal is the combined consequence of graphene’s large Seebeck coefficient, which governs the scale of the thermoelectric response; the fact that this coefficient varies strongly with the Fermi level; and the presence of hot carriers. Indeed, it is these hot electrons that cause thermal gradients on a scale that allow observation of this thermoelectric effect on spin.

These results represent unprecedented advances in our understanding of spin caloritronics, holding promise for technological advances in the form of devices able to control and sustain spin currents over useful distances through the application of a heat current.

Article reference:

J.F. Sierra, I. Neumann, J. Cuppens, B. Raes, M.V. Costache, and S.O. Valenzuela. Thermoelectric spin voltage in graphene. Nature Nanotechnology (2017). DOI: 10.1038/s41565-017-0015-9