The Effects of Granular Magnetic Anisotropy Variation on Nanowire Spin Hall Oscillators

Spin Hall oscillators (SHOs) are nanoscale devices consisting of a ferromagnet and a heavy metal. Above a threshold current density, negative magnetic damping from spin torque injected by the heavy metal compensates for the positive damping in the ferromagnet, which can create sustained auto-oscillations in the magnetization. SHOs have applications as compact tunable microwave sources, spin-wave generators, and as the active components of neuromorphic computing systems. In order for SHOs to be practical for applications, the excited frequency modes must be predictably tunable. However, experimental data shows large variability from device to device. Recent computational studies indicate that this variation can be caused by reduced intergrain exchange coupling across grain boundaries. Here, we use Mumax3 micromagnetic simulations to investigate the effect of random perpendicular magnetic anisotropy (PMA) variation in addition to reduced intergrain exchange. We find that moderate PMA variations can lead to multimode operation and reduce SHO performance.