Spatial Mapping of Torques within a Spin Hall Nano-oscillator

Time-resolved scanning Kerr microscopy (TRSKM) was used to study precessional magnetization dynamics induced by a radio frequency (RF) current within a Al$_2$O$_3$/Py(5 nm)/Pt(6 nm)/Au(150 nm) spin Hall nano-oscillator structure. The Au layer was formed into two needle-shaped electrical contacts that concentrated the current in the centre of a Py/Pt mesa of 4 $\mu$m diameter. Due to the spin Hall effect, current within the Pt layer drives a spin current into the Py layer, exerting a spin transfer torque (STT). By injecting RF current, and exploiting the phase-sensitivity of TRSKM and the symmetry of the device structure, the STT and Oersted field torques have been separated and spatially mapped. The STT and torque due to the in-plane Oersted field are observed to exhibit minima at the device centre that is ascribed to spreading of RF current that is not observed for DC current. Torques associated with the RF current may destabilise the position of the self-localised bullet mode excited by the DC current, and inhibit injection locking. The present study demonstrates the need to characterise both DC and RF current distributions carefully.