Acoustic spin pumping as the origin of the long-range spin Seebeck effect

TL;DR

This paper reveals that the long-range spin Seebeck effect in metals is mediated by phonons, demonstrated through acoustic spin pumping, opening new avenues for phonon-based spintronic devices.

Contribution

It introduces the mechanism of acoustic spin pumping as the origin of the long-range spin Seebeck effect in magnetic metals, highlighting phonons' role in spin transport.

Findings

Long-range spin voltage is due to phonons in magnetic metals.

Spin-phonon coupling is evidenced by non-local voltage signals.

Sound waves can directly induce spin currents, demonstrating acoustic spin pumping.

Abstract

The spin Seebeck effect (SSE) is known as the generation of 'spin voltage' in a magnet as a result of a temperature gradient. Spin voltage stands for the potential for spins, which drives a spin current. The SSE is of crucial importance in spintronics and energy-conversion technology, since it enables simple and versatile generation of spin currents from heat. The SSE has been observed in a variety of materials ranging from magnetic metals and semiconductors to magnetic insulators. However, the mechanism, the long-range nature, of the SSE in metals is still to be clarified. Here we found that, using a Ni81Fe19/Pt bilayer wire on an insulating sapphire plate, the long-range spin voltage induced by the SSE in magnetic metals is due to phonons. Under a temperature gradient in the sapphire, surprisingly, the voltage generated in the Pt layer is shown to reflect the wire position, although the wire is isolated both electrically and magnetically. This non-local voltage is direct evidence that the SSE is attributed to the coupling of spins and phonons. We demonstrate this coupling by directly injecting sound waves, which realizes the acoustic spin pumping. Our finding opens the door to "acoustic spintronics" in which phonons are exploited for constructing spin-based devices.