Spatial coexistence of multiple modes in a nanogap spin Hall nano-oscillator with extended Pt/Ni/Fe trilayers

We experimentally study the microwave generation characteristics of a spin Hall nano-oscillator driven by a local spin current injected into a nanoscale region of extended Pt/Ni/Fe trilayers due to the spin Hall effect in the Pt layer and the interfacial Rashba effect at the Pt/Ni interface. The dependence of the generated microwave spectra on the exciting current and on the magnitude and angle of the magnetic field indicates that multiple spin-wave modes with either higher frequencies than the ferromagnetic resonance frequency ${f}_{\mathrm{FMR}}$ at low out-of-plane tilting angles and small fields or lower frequencies than ${f}_{\mathrm{FMR}}$ at high tilting angles and large fields are excited by local spin currents. Furthermore, the temperature dependence of the generated spectra in the extended Pt/Ni/Fe trilayers demonstrates that the thermally activated mode transition between these distinct dynamical modes is significantly suppressed. These results suggest that the observed multiple modes are physically separated in different local potential wells created by the spatial inhomogeneity of the internal field in the asymmetric ferromagnetic bilayer with strong interfacial exchange coupling and anisotropy fields. Finally, the very weak dependence of the minimum linewidth on temperature further confirms the lack of thermal-fluctuation-induced mode coupling and mode transition between these individual dynamical modes.