Spin Hall Nano-Antenna.

The spin Hall effect is a well-known phenomenon in spintronics that has found numerous applications in digital electronics (memory and logic), but relatively few in analog electronics. Practically the only analog application in widespread use is the spin Hall nano-oscillator (SHNO) that delivers a high frequency alternating current or voltage to a load. Here, we report its analogue - a spin Hall nano-antenna (SHNA) that radiates a high frequency electromagnetic wave into the surrounding medium. It can also radiate an acoustic wave into an underlying substrate if the nanomagnets are made of a magnetostrictive material. That makes it a dual electromagnetic/acoustic antenna. The SHNA is made of an array of ledged magnetostrictive nanomagnets deposited on a substrate, with a heavy metal nanostrip underlying/overlying the ledges. An alternating charge current passed through the nanostrip generates an alternating spin-orbit torque (SOT) in the nanomagnets via the spin Hall effect, which makes their magnetizations oscillate in time with the frequency of the current, producing confined spin waves (magnons) in the nanomagnets. These spin waves radiate electromagnetic waves (photons) in space owing to the transfer of energy from magnons to photons, thereby implementing a transmitting antenna. Curiously, although the SHNA is much smaller in size than the radiated electromagnetic wavelength and hence qualifies as a "point source", it does not radiate isotropically owing to the anisotropy of the (frequency-dependent) spin wave patterns that form within the odd-shaped nanomagnets, which endows the "point source" with internal anisotropy. The same device can also act as a receiving antenna. Incident electromagnetic radiation generates polychromatic spin waves in the nanomagnets, which pump spin into the heavy metal nanostrips at their own frequencies, giving rise to a polychromatic alternating voltage across the latter owing to the ac inverse spin Hall effect. This implements the receiver. The transmitting/receiving gain and radiation efficiency of the SHNA far exceed those of a conventional antenna of the same size, thereby enabling extreme antenna miniaturization.