Spin-Orbit-Torque Efficiency and Current-Driven Coherent Magnetic Dynamics in a Pt/Ni/Py Trilayer-Based Spin Hall Nano-Oscillator

We experimentally study the current-induced effective spin-orbit torque (SOT) efficiency and the spectral characteristics of current-driven coherent magnetic dynamics in spin Hall nano-oscillators (SHNOs) based on the Pt/Ni/Py trilayer. We determine that the Pt/Ni/Py trilayer structure has an effective dampinglike torque efficiency ??DL ??? 0.055, comparable with the Pt/Co and Pt/Fe bilayer systems with a strong interfacial spin-orbit coupling and magnetic proximity effect. Furthermore, the microwave-generation spectra show that the Pt/Ni/Py-based SHNOs exhibit a single nonlinear self-localized bullet mode with a frequency below fFMR and a significant current-dependent frequency red shift due to its strong nonlinear effect, which is very similar to that in Pt/Py-based SHNOs, at low oblique angles ?? ??? 50???. In contrast, at high oblique angles ?? ??? 55???, the spectra show two oscillating peaks with very similar field-, current-, and temperature-dependent behaviors, which suggests the spatial coexistence of two same-type nonlinear self-localized bullet modes at certain currents and magnetic fields. Additionally, the observed linear temperature dependence of the minimum line width, which resembles the thermal broadening of single-mode oscillation, further confirms that two localized bullet modes are independent, spatially separated, and lack thermally activated mode-transition behavior. Our results provide valuable information for the electronic control of coherent magnetic dynamics by combining bulk and interfacial spin-orbit coupling effects in the magnetic heterostructures.