Giant Hall Switching by Surface-State-Mediated Spin-Orbit Torque in a Hard Ferromagnetic Topological Insulator*

Topological insulators (TI) and magnetic topological insulators (MTI) can apply highly efficient spin-orbit torque (SOT) and manipulate the magnetization with their unique topological surface states with ultra-high efficiency. Here, we demonstrate efficient SOT switching of a hard MTI, V-doped (Bi,Sb)₂Te₃ (VBST) with a large coercive field that can prevent the influence of an external magnetic field. A giant switched anomalous Hall resistance of 9.2 kΩ is realized, among the largest of all SOT systems. The SOT switching current density can be reduced to 2.8×10⁵ A/cm². Moreover, as the Fermi level is moved away from the Dirac point by both gate and composition tuning, VBST exhibits a transition from edge-state-mediated to surface-state-mediated transport, thus enhancing the SOT effective field to 1.56±0.12 T/ (10⁶ A/cm²) and the spin Hall angle to 23.2±1.8 at 5 K. The findings establish VBST as an extraordinary candidate for energy-efficient magnetic memory devices.