Spin Splitting in Altermagnetic RuO$_2$ Enables Field-free Spin-Orbit Torque Switching via Dominant Out-of-Plane Spin Polarization

Researchers have recently identified a novel class of magnetism, termed "altermagnetism", which exhibits characteristics of both ferromagnetism and antiferromagnetism. Here, we report a groundbreaking discovery of efficient field-free spin-orbit torque (SOT) switching in a RuO$_2$ (101)/Co/Pt/Co/Pt/Ta structure. Our results demonstrate that the spin current flows along the [100] axis, induced by the in-plane charge current, with the spin polarization direction aligned parallel to the N\'eel vector. These z-polarized spins generate an out-of-plane anti-damping torque, enabling deterministic switching of the Co/Pt layer without the necessity of an external magnetic field. The altermagnetic spin splitting effect (ASSE) in RuO$_2$ promotes the generation of spin currents with pronounced anisotropic behavior, maximized when the charge current flows along the [010] direction. This unique capability yields the highest field-free switching ratio, maintaining stable SOT switching within an external field range of approximately 400 Oe. Notably, ASSE dominates the spin current, especially when the current is aligned with the [010] direction ({\theta} = 90{\deg}). Here, the spin polarization component creates a substantial field-like effective field, surpassing the damping-like field from . This highlights the crucial role of in enhancing spin-torque efficiency and elucidating spin flow modulation mechanics in this crystalline context. Our study highlights the potential of RuO$_2$ as a powerful spin current generator, paving the way for practical applications in spin-torque switching technologies and other cutting-edge spintronic devices.