Spin Hall Nano-Antenna

TL;DR

This work tackles the challenge of creating electrically small antennas capable of microwaves without relying on electromagnetic resonance. It introduces the Spin Hall nano-antenna (SHNA), which uses AC current through a heavy-metal nanostrip to generate alternating spin-orbit torque and excite confined spin waves in magnetostrictive nanomagnets on LiNbO3, emitting photons at the drive frequency $f$. The study demonstrates sub-wavelength radiation with strongly anisotropic patterns arising from internal spin-wave modes, while higher-frequency non-drive modes are suppressed by ensemble averaging. The findings point to a new class of spintronic antennas with potential for embedded, underwater, wearable, or biomedical applications and for dual electromagnetic/acoustic functionality on piezoelectric substrates.

Abstract

The spin Hall effect is a celebrated phenomenon in spintronics and magnetism that has found numerous applications in digital electronics (memory and logic), but very 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 (alternating electric/magnetic fields) into the surrounding medium. It can also radiate an acoustic wave in 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 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), which radiate electromagnetic waves (photons) in space with the same frequency as the ac current. Despite being much smaller than the radiated wavelength, the SHNA surprisingly does not act as a point source which would radiate isotropically. Instead, there is clear directionality (anisotropy) in the radiation pattern, which is frequency-dependent. This is due to the (frequency-dependent) intrinsic anisotropy in the confined spin wave patterns generated within the nanomagnets, which effectively endows the "point source" with internal anisotropy.

Figures (6)

• Figure 1: (a) A spin Hall nano-oscillator (SHNO) acts as a time-varying resistor that delivers a time-varying current to a load resistor connected in series with it when both are powered by a dc voltage source. (b) Schematic of a SHNO device actuated by passing an alternating current through a heavy metal (HM) or topological insulator (TI) and no external magnetic field present. (c) Schematic of a spin Hall nano antenna (SHNA) where, again, no external magnetic field is needed.
• Figure 2: (a) Schematic of the spin Hall nano-antenna device. (b) Scanning electron micrograph of the device showing the various structural dimensions).
• Figure 3: (a) The pump and probe laser spots are approximately 1 $\mu$m in diameter and hence cover two nanomagnets at a time. Hence the spin waves are always sampled from two nanomagnets. (b) Kerr oscillations in the nanomagnets plotted in the time domain. (c) Fast Fourier transform of the Kerr oscillations showing the peaks in the spin wave spectra.
• Figure 4: Electromagnetic radiation spectrum of the real and control samples when the frequency of the ac current pumped into the Pt nanostrip is set to 3 GHz and the input power is 15 dbm. The distance between the receiving horn antenna and the sample in this case is 81 cm which is 8 times the wavelength, ensuring that we are measuring the far-field radiation.
• Figure 5: The radiation pattern at different frequencies in the plane of the nanomagnets. The patterns are shown for both the real sample and the control sample, as well as for both horizontal and vertical polarizations.