  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.
