Super-harmonic injection locking of nano-contact spin-torque vortex oscillators

TL;DR

This study demonstrates super-harmonic injection locking in nano-contact spin-torque vortex oscillators, revealing frequency locking up to the fourth harmonic and spatial dynamics differences, with implications for nanoscale frequency dividers and oscillator networks.

Contribution

First demonstration of super-harmonic injection locking in NC-STVOs, showing frequency locking up to the fourth harmonic and spatial dynamics differences via TRSKM.

Findings

Frequency locking observed up to the 4th harmonic.

Locking efficiency decreases with harmonic number.

Spatial magnetization dynamics differ between fundamental and harmonic states.

Abstract

Super-harmonic injection locking of single nano-contact (NC) spin-torque vortex oscillators (STVOs) subject to a small microwave current has been explored. Frequency locking was observed up to the fourth harmonic of the STVO fundamental frequency $f_{0}$ in microwave magneto-electronic measurements. The large frequency tunability of the STVO with respect to $f_{0}$ allowed the device to be locked to multiple sub-harmonics of the microwave frequency $f_{RF}$, or to the same sub-harmonic over a wide range of $f_{RF}$ by tuning the DC current. In general, analysis of the locking range, linewidth, and amplitude showed that the locking efficiency decreased as the harmonic number increased, as expected for harmonic synchronization of a non-linear oscillator. Time-resolved scanning Kerr microscopy (TRSKM) revealed significant differences in the spatial character of the magnetization dynamics of states locked to the fundamental and harmonic frequencies, suggesting significant differences in the core trajectories within the same device. Super-harmonic injection locking of a NC-STVO may open up possibilities for devices such as nanoscale frequency dividers, while differences in the core trajectory may allow mutual synchronisation to be achieved in multi-oscillator networks by tuning the spatial character of the dynamics within shared magnetic layers.