Direct Observation of Propagating Spin Waves in a Spin Hall Nano-Oscillator.

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 (TR-STXM). Our measurements reveal that the magnon amplitude is strongest at the two constriction edges, with a pronounced asymmetry favoring one edge, and that the emitted spin waves (SWs) 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 measurement, indicating that the CoFeB/MgO interface may be more susceptible to x-ray-induced modifications than previously recognized, challenging its presumed radiation hardness.