  A novel on-chip extreme sub-wavelength "acoustic antenna" whose radiation
efficiency is ~50 times larger than the theoretical limit for a resonantly
driven antenna is demonstrated. The antenna is composed of magnetostrictive
nanomagnets deposited on a piezoelectric substrate. The nanomagnets are
partially in contact with a heavy metal (Pt) nanostrip. Passage of alternating
current through the nanostrip exerts alternating spin-orbit torque on the
nanomagnets and periodically rotates their magnetizations. During the rotation,
the magnetostrictive nanomagnets expand and contract, thereby setting up
alternating tensile and compressive strain in the piezoelectric substrate
underneath. This leads to the generation of a surface acoustic wave in the
substrate and makes the nanomagnet assembly act as an acoustic antenna. The
measured radiation efficiency of this acoustic antenna at the detected
frequency is ~1%, while the wavelength to antenna dimension ratio is ~ 67:1.
For a standard antenna driven at acoustic resonance, the efficiency would have
been limited to ~ (1/67)^2 = 0.02%. It was possible to beat that limit (by ~50
times) via actuating the antenna not by acoustic resonance, but by using a
completely different mechanism involving spin-orbit torque originating from the
giant spin Hall effect in Pt.
