An Unconventional Ultra-Sub-Wavelength Receiving Nano-Antenna Activated by ac Spin Pumping and the ac Inverse Spin Hall Effect

We report an extreme sub-wavelength unconventional receiving antenna. It consists of an array of nanomagnets connected to heavy metal nanostrips. Incident electromagnetic (EM) radiation generates intrinsic and extrinsic 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 a receiving nano-antenna. We demonstrate its operation at two different EM wave frequencies of 1.5 GHz and 2.4 GHz - the latter being the Bluetooth and Wi-Fi frequency. We measure the receiving gain at 2.4 GHz to be approximately -9 db. The free space radiated wavelength "lambda" at 2.4 GHz is 12.5 cm while the antenna area A is merely 160 micron^2, making the ratio A/lambda^2 = 0.97x10^-8. This antenna's receiving gain should be very poor because of the tiny size. Yet the measured gain is more than 4000 times larger than the theoretical limit for a conventional antenna of this size at this wavelength because of the unconventional operating principle.

A Spintronic Nano-Antenna Activated by Spin Injection from a Three-Dimensional Topological Insulator

A charge current flowing through a three-dimensional topological insulator (3D-TI) can inject a spin current into a ferromagnet placed on the surface of the 3D-TI. Here, we report leveraging this mechanism to implement a nano-antenna that radiates an electromagnetic wave (1-10 GHz) into the surrounding medium efficiently despite being orders of magnitude smaller than the radiated free space wavelength. An alternating charge current of 1-10 GHz frequency is injected into a thin film of the 3D-TI Bi2Se3, resulting in the injection of an alternating spin current (of the same frequency) into a periodic array of cobalt nanomagnets deposited on the surface of the 3D-TI. This causes the magnetizations of the nanomagnets to oscillate in time and radiate electromagnetic waves in space, thereby implementing a nano-antenna. Because it is so much smaller than the free space wavelength, the nano-antenna is effectively a "point source" and yet it radiates anisotropically because of internal anisotropy. One can change the anisotropic radiation pattern by changing the direction of the injected alternating charge current, which implements beam steering. This would normally not have been possible in a conventional extreme sub-wavelength antenna.