  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
$k\Omega$ is realized, among the largest of all SOT systems. The SOT switching
current density can be reduced to $2.8\times10^5 A/cm^2$. 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
$1.56\pm 0.12 T/ (10^6 A/cm^2)$ and the interfacial charge-to-spin conversion
efficiency to $3.9\pm 0.3 nm^{-1}$ (nominal spin Hall angle to $23.2\pm 1.8$).
The findings establish VBST as an extraordinary candidate for energy-efficient
magnetic memory devices.

Mesoscale and Nanoscale Physics;Materials Science