Efficient field-free magnetization switching through orbital-current-enhanced magnon torque

Efficient control of magnetization is essential for the development of spintronics devices. Various mechanisms, including spin currents, orbital currents, and magnon currents, have been explored to achieve magnetization switching. Among them, magnon currents facilitate low-dissipation transport of spin angular momentum, potentially mitigating energy dissipation associated with Ohmic loss. So far, the magnon currents excited by spin currents have been well-established, while the role of orbital currents in magnon excitation remains much less explored. Here, we report efficient magnon torques in a RuO₂/NiO/ferromagnet system, where spin and orbital currents generated by RuO₂ jointly excite magnon currents in NiO. Notably, the magnon currents exhibit both in-plane and out-of-plane polarizations, manifesting a high in-plane spin Hall conductivity σ_s,y = 143.8 × 10³ ℏ /2e Ω^-1 m^-1 and out-of-plane spin Hall conductivity σ_s,z = 8.3 × 10³ ℏ /2e Ω^-1 m^-1, which enables field-free switching of perpendicular magnetization with low-power consumption. Our findings reveal that spin and orbital currents in RuO₂ can jointly excite magnon currents in NiO, enabling efficient field-free switching of perpendicular magnetization and paving the way to low-power spintronic devices.