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

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.

Figures (4)

• Figure 1: Sample and electrical measurement.a 3D schematic of the SHNO device showing the orientations of the external field and incident X-rays. The external field B had a strength of 255mT, in-plane angle $\varphi=-45°$, and out-of-plane angle $\vartheta=60°$. The X-ray incident angle was normal to the sample plane. b Sample top view. c Power spectral density (PSD) of the AOs as a function of $I_\mathrm{DC}$. The inset shows a cut along the white line of the free running (red diamonds) and injection locked (blue squares) signal at 300µA.
• Figure 2: Experimental and simulated ps-fast magnetization dynamicsa Measured relative magnon amplitude. The highly asymmetrical amplitude suggests the presence of additional magnetic interactions, not considered by the existing literature. b-c Snapshots of the spin wave propagation. Average magnetization ($m_{\mathrm{z}}$) of the marked rectangular areas is shown in the insets to the right and top. We observe a spin wave propagating to the left in the horizontal direction. d-f Simulated magnon amplitude and snapshots. Agreement between the experiment and the simulation was only achieved with the inclusion of grain boundaries and DMI in the CoFeB layer.
• Figure 3: AO modes for different simulation parameters.a Nominal PMA and monocrystalline film, similar to the existing literature. b Reduced PMA. c Reduced PMA with grain boundaries. d Reduced PMA with grain boundaries and DMI. In order to reproduce the asymmetry and SW direction detected in the measurements, grain boundaries and DMI must be considered. $I_\mathrm{DC}$ flows in the direction indicated by the arrow on the left.
• Figure 4: AO modes before and after X-ray irradiation. Power spectral density as function of current. The frequency and amplitude increase after irradiation, hinting at larger values of the PMA and SOT. The modified curvature implies a transformation of the mode geometry. While a rising frequency was consistently observed, the changes in amplitude and curvature varied between samples.