Strong angle dependence of auto-oscillating spin wave multi-modes in constricted Py/Pt bilayers

Spin-orbit torque driven auto-oscillators (AO) in constricted bilayers can be highly coherent and tunable, even at room temperature. However, the dissipation mechanisms of AO confined by a constriction remain unclear. We have studied the dependence of several AO modes on the angle between the current flow and an in-plane applied magnetic field in a 0.6 μm x 1 μm Py/Pt bilayer. We observe considerable changes in the size, number, and distribution of modes as the angle varies. The spatial profiles of the modes are identified using micromagnetic simulation, and classified into three types: edge, center-like, and their combination. We also characterize the AO modes in terms of resonance frequency, linewidth and power. Beyond a threshold current, the power in the lowest order mode saturates and its linewidth increases. However, this coincides with enhanced excitation of higher order modes such that the power summed over all modes continues to grow with current. We achieve ~ 8 MHz minimum linewidth and ~ 0.5 pW maximum power at 77 K from the lowest order mode at an angle of 65 degree, though below 120 K, the improvement of the mode for the narrower linewidth and the stronger power with decreasing temperature is reduced.