Impact of ferromagnetic grains on nanowire spin torque oscillators

Spin-orbit torques in bilayers of ferromagnetic and nonmagnetic materials hold promise for energy efficient nanodevices. Particularly, spin torque oscillators provide for compact tunable microwave sources and have potential application in neuromorphic computing. In this presentation, we investigate the impact of granular media on spin torque oscillator dynamics. We demonstrate that nominally identical wires can exhibit drastically different oscillatory behavior - differing in emission spectra and critical currents. Spin torque ferromagnetic resonance measurements reveal vastly different mode structures for the devices. Using micromagnetic simulations, we qualitatively reproduce the experimentally observed mode structure by accounting for granularity in the ferromagnetic layer that includes weakened intergrain exchange coupling and variations in perpendicular magnetic anisotropy. Our results show that disorder in ferromagnetic materials can significantly impact spin torque oscillator performance.