Self-Oscillations in a Nanogap Spin Hall Nano-Oscillator with a Perpendicularly Magnetized External Film

Abstract

We study spin-current-induced magnetodynamics in a planar nanogap spin Hall nano-oscillator (SHNO) based on $\mathrm{Pt}/[\mathrm{Co}/\mathrm{Ni}{]}_{4}$ with a large perpendicular magnetic anisotropy (PMA) at in-plane field geometry. Two distinct dynamic modes are observed in the spin-torque ferromagnetic resonance (FMR) and the generated microwave spectra. The primary mode has a frequency above the FMR frequency and a significant blueshift of the frequency with the increasing current $I$, consistent with the propagating spin-wave mode. The secondary higher-frequency mode with a much weaker power intensity, discrete frequency jump, and a slight redshift of frequency is only observed at low fields below the saturation field of 2 kOe, suggesting that it is likely related to the emerged domain region with in-plane magnetization due to grain-boundary defects, which is further supported by the frequency enhancement with reducing temperature. The primary propagating mode decreases its frequency and broadens its minimum linewidth by reducing the temperature, indicating that the frequency-drift instability dominates its dynamical decoherence due to the boundary-pinning effect. Furthermore, the micromagnetic simulation reproduces the experimentally observed propagating spin-wave and in-plane domain modes, and provides their spatial characteristics. Our demonstrated propagating spin waves in a SHNO with an out-of-plane magnetization extended bilayer can facilitate long-distance mutual synchronization in the SHNO network arrays for enhancing coherence and power as spin-wave sources in magnon-based devices or spin-based neuromorphic computing.