Origin of Nonlinear Damping Due to Mode Coupling in Auto-Oscillatory Modes Strongly Driven by Spin-Orbit Torque

We investigate the physical origin of nonlinear damping due to mode coupling between several auto-oscillatory modes driven by spin-orbit torque in constricted Pt/Py(permalloy) heterostructures by examining the dependence of the auto-oscillation on the temperature and the applied field orientation. We observe a transition in the nonlinear damping of the auto-oscillation modes extracted from the total oscillation power as a function of the drive current, which coincides with the onset of power redistri-bution among several modes and the crossover from linewidth narrowing to linewidth broadening in all individual modes. This indicates the activation of another relaxation process by nonlinear magnon-magnon scattering within the modes. We also find that both nonlinear damping and the threshold current in the mode-interaction damping regime at high drive current after transition are temperature independent, suggesting that the mode coupling occurs dominantly through a nonthermal magnon-scattering process via a dipole or exchange interaction rather than thermally excited magnon-mediated scattering. This finding presents a promising pathway toward overcoming the current limitations of efficiently controlling the inter-action between two highly nonlinear magnetic oscillators to prevent mode crosstalk or intermode energy transfer and deepens the understanding of complex nonlinear spin dynamics in multimode spin-wave systems.

Automated summary

This study investigates the physical origin of nonlinear damping in Pt/Py heterostructures driven by spin-orbit torque, specifically examining the mode coupling between multiple auto-oscillatory modes. The results show a transition in the nonlinear damping and power redistribution among the modes, indicating the activation of another relaxation process. Additionally, the study finds that the mode coupling occurs through a nonthermal magnon-scattering process, independent of temperature. These findings have significant implications for understanding the complex nonlinear spin dynamics in multimode spin-wave systems and controlling the interaction between nonlinear magnetic oscillators.