Newton's second law in spin-orbit torque

Spin-orbit torque (SOT) refers to the excitation of magnetization dynamics via spin-orbit coupling (SOC) under the application of a charged current. In this work, we introduce a simple and intuitive description of the SOT in terms of spin force. In Rashba SOC system, the damping-like SOT can be expressed as Tso = Rc × Fso, in analogy to the classical torque-force relation, where Rc is the effective radius characterizing the Rashba splitting in the momentum space. As a consequence, the magnetic energy is transferred to the conduction electrons, which dissipates through Joule heating at a rate of (je · Fso), with je being the applied current. Finally, we propose an experimental verification of our findings via measurement of the anisotropic magnetoresistance effect.