Spin current generation by acousto-electric evanescent wave

We show both experimentally and theoretically that the electromagnetic fields associated with surface acoustic waves (SAWs) dominate the charge current response of a ferromagnet/heavy metal multilayer deposited on top of a LiNbO3 substrate under SAW excitation. Despite travelling much slower than the light in vacuum, the SAW-induced evanescent electric field turns out to have a transverse component that remains unscreened by conduction electrons and permeates through the film stack, causing a sizable ac electric current. The ac spin current generated through the spin-Hall effect combined with a magnetoelastic oscillation of magnetization leads to a rectified dc voltage signal of order 10 μV. The transverse electric field is accompanied by a magnetic field, which is calculated to be as large as 1 μT. We clarify the relation between the validity of electrostatic approximation and the presence of non-negligible transverse fields. The work reveals the electromagnetic origin of acoustic spin-Hall effect and sheds light on the potential relevance of piezoelectricity in spintronics applications.