Frequency Stability of Spin-Hall Nano-Oscillators with Realistic Grain Structure

TL;DR

This study investigates how realistic grain structures affect the frequency stability of spin-Hall nano-oscillators, combining measurements and micromagnetic simulations to understand device variability and oscillation modes.

Contribution

It introduces a novel micromagnetic simulation method to systematically analyze frequency instability caused by grain structures in SHNOs.

Findings

Device-to-device frequency variability of ~270 MHz observed experimentally.

Simulations show ~100 MHz variability due to grain effects.

Double-mode oscillations linked to partial decoupling at nano-constriction edges.

Abstract

Nano-constriction spin-Hall nano-oscillators (NC-SHNOs) are one of the most promising alternatives among the microwave spintronics devices. They can provide highly coherent and widely tuneable microwave signals and can be fabricated at low temperatures, which makes them compatible with back-end-of-the-line CMOS processing. For its applications, the frequency stability of each device is crucial, in particular for synchronization of oscillator arrays. In this work, we focus on the influence of a realistic grain structure on the SHNO frequency stability using both measurements as well as micromagnetic simulations. Grains in the thin ferromagnetic metal films can influence the output characteristic of the SHNO since the exchange coupling is reduced locally at the grain boundaries. This work provides a novel micromagnetic simulation method, for systematic investigation of frequency instability or variability from device-to-device. Experimentally, a device-to-device frequency variability of ~270 MHz was found and in some extreme cases, a double-mode oscillation was observed in the high current operation range. This oscillation behavior was reproduced in simulations, where the inclusion of grains resulted in a frequency variability of ~100 MHz and double-mode oscillations for some particular configurations. The double modes are consistent with a partial decoupling, or non-coherent operation, of two oscillating regions located at the nano-constriction edges.