Injection locking of multiple auto-oscillation modes in a tapered nanowire spin Hall oscillator

TL;DR

This paper experimentally demonstrates the injection locking of multiple auto-oscillation modes in a tapered nanowire spin Hall oscillator, revealing localized spin-wave bullets at distinct frequencies and their controllable synchronization.

Contribution

It provides the first direct experimental confirmation of multiple localized spin-wave bullets in tapered SHO and their selective synchronization, advancing multi-mode spintronic oscillator design.

Findings

Two localized spin-wave bullets oscillate at 5.2 GHz and 5.45 GHz.

Bullets can be individually synchronized to external microwave signals.

Synchronization reduces linewidth and enhances oscillation amplitude.

Abstract

Spin Hall oscillators (SHO) are promising candidates for the generation, detection and amplification of high frequency signals, that are tunable through a wide range of operating frequencies. They offer to be read out electrically, magnetically and optically in combination with a simple bilayer design. Here, we experimentally study the spatial dependence and spectral properties of auto-oscillations in SHO devices based on Pt(7 nm)/\ Ni$_{\mathrm{80}}$Fe$_{\mathrm{20}}$(5 nm) tapered nanowires. Using Brillouin light scattering microscopy, we observe two individual self-localized spin-wave bullets that oscillate at two distinct frequencies (5.2 GHz and 5.45 GHz) and are localized at different positions separated by about 750 nm within the SHO. This state of a tapered SHO has been predicted by a Ginzburg-Landau auto-oscillator model, but not yet been directly confirmed experimentally. We demonstrate that the observed bullets can be individually synchronized to external microwave signals, leading to a frequency entrainment, linewidth reduction and increase in oscillation amplitude for the bullet that is selected by the microwave frequency. At the same time, the amplitude of other parasitic modes decreases, which promotes the single-mode operation of the SHO. Finally, the synchronization of the spin-wave bullets is studied as a function of the microwave power. We believe that our findings promote the realization of extended spin Hall oscillators accomodating several distinct spin-wave bullets, that jointly cover an extended range of tunability.