Direct observation of propagating spin waves in a spin-Hall nano-oscillator

TL;DR

This study provides the first direct imaging of magnetization dynamics in a spin-Hall nano-oscillator, revealing anisotropic spin wave propagation and the influence of material interfaces and interactions.

Contribution

It introduces time-resolved STXM imaging of SHNOs, uncovering detailed magnetization behaviors and the effects of grain boundaries and DMI on spin wave propagation.

Findings

Magnon amplitude is strongest at constriction edges with asymmetry.

Spin waves propagate anisotropically within the device.

Magnetodynamics change due to possible X-ray induced modifications.

Abstract

Constriction-based spin Hall nano-oscillators (SHNOs) show great promise for application as highly tunable microwave sources with straightforward scalability toward large coupled networks. However, details of the magnetization dynamics within SHNOs have thus far not been addressed experimentally, due to the minute time and length scales involved. In this work, we present direct imaging of the magnetization dynamics within a single CoFeB-based SHNO using time-resolved scanning transmission X-ray microscopy (STXM). Our measurements reveal that the magnon amplitude is the strongest at the two constriction edges, with a pronounced assymetry favoring one edge, and that emitted spin waves exhibit strongly anisotropic propagation. Micromagnetic simulations suggest that grain boundaries and the Dzyaloshinskii-Moriya interaction (DMI) play a key role in both effects. Furthermore, the magnetodynamics changed during the measurement, indicating that the CoFeB/MgO interface may be more susceptible to X-ray induced modifications than previously recognized, challenging its presumed radiation hardness.