spinorbittorque

Spin-orbit torque refers to a class of torques exerted on the magnetization of a ferromagnetic layer by spin currents generated through spin-orbit coupling. In typical heavy-metal/ferromagnet bilayers or trilayers, an in-plane charge current in a nonmagnetic layer with strong spin-orbit coupling is converted into a transverse spin current or nonequilibrium spin accumulation. This spin polarization diffuses into the adjacent ferromagnet and applies torque to its magnetization, enabling switching or manipulation without requiring spin-polarized current through the magnetic layer itself.

The torque can be decomposed into a damping-like component, proportional to m × (m × σ), and a field-like component, proportional to m × σ, where m is the magnetization and σ is the spin polarization direction. The damping-like torque is commonly associated with the spin Hall effect in the heavy metal, while the field-like torque may arise from interfacial Rashba-Edelstein effects or from spin Hall current reflection at the interface.

Common material systems include Pt/Co, Ta/CoFeB/MgO, and W/CoFeB. The efficiency is often described by the spin Hall angle θ_SH or a damping-like torque efficiency ξ_DL; larger values enable magnetization switching at lower current densities. Materials such as β-phase W and topological insulators have been explored to boost efficiency, as have engineered interfaces.

Applications include all-electrical switching of perpendicular-magnetic-anisotropy ferromagnets for spin-orbit torque MRAM, and current-driven domain-wall or skyrmion motion in nanowires. SOT offers potential advantages in scalability and energy efficiency over traditional spin-transfer torque devices.

Measurement techniques include spin-torque ferromagnetic resonance, harmonic Hall voltage, and current-induced switching experiments. Ongoing research addresses interfacial quality, Joule heating, and reliability, as well as novel materials such as topological insulators and two-dimensional materials to enhance efficiency and enable field-free switching.