SpinOrbitTorques

Spin-orbit torques are torques exerted on the magnetization of a ferromagnet that arise from spin–orbit coupling in an adjacent nonmagnetic layer or at an interface. An electric current in the nonmagnetic material can generate a nonequilibrium spin accumulation or spin current via the spin Hall effect or the Rashba–Edelstein effect, which then transfers angular momentum to the ferromagnet, altering its orientation.

Commonly two torque components are distinguished: damping-like torque and field-like torque. The damping-like torque tends to align or anti-align the magnetization with the spin polarization, while the field-like torque acts perpendicular to it. In many devices, the damping-like component provides the dominant switching influence, while the field-like component can modify switching thresholds and dynamics.

Spin currents in spin-orbit torque devices can originate from the bulk spin Hall effect in heavy metals (for example platinum, tantalum, tungsten) or from interfacial Rashba effects in oxide or topological-insulator based structures. The efficiency of these torques is often characterized by a damping-like torque efficiency or spin-torque angle, which depends on material properties and layer interfaces.

Materials and structures include ferromagnet/heavy-metal bilayers, topological insulator/ferromagnet stacks, and certain two-dimensional materials. Pt/CoFeB, Ta/CoFeB, and W/CoFeB are common examples; interface quality and layer order strongly influence the torque efficiency and switching behavior.

Experimental methods to quantify spin-orbit torques include spin-torque ferromagnetic resonance, harmonic Hall measurements, and time-resolved magneto-optical Kerr effect experiments.

Applications target fast, scalable magnetization switching for spintronic memory (SOT-MRAM); SOT devices can operate with lower write currents and decoupled read/write paths compared with conventional spin-transfer torque devices, though materials optimization remains a challenge.

The reciprocal effect, inverse spin-orbit torque, describes generation of charge currents from magnetization dynamics via the inverse spin Hall effect, enabling electrical detection of magnetization dynamics. History and outlook trace the concept to the spin Hall and Rashba effects, with rapid experimental progress in the following decades and ongoing materials and device development.