Current-induced spin-orbit torque on the surface of a transition metal dichalcogenide connected to a two-dimensional ferromagnet CrI$_3$: Effects of twisting and gating
Motivated by recent progress in employing two key classes of two-dimensional materials-topological insulators and transition-metal dichalcogenides (TMDCs)-as spin sources for generating spin-orbit torque (SOT), we investigate current-induced spin polarization and the resulting SOT in bilayers composed of a TMDC (WSe$_2$ or MoSe$_2$) and ferromagnetic chromium iodide (CrI$_3$), beyond the linear response regime. Using the steady-state Boltzmann equation, we find that intra-band transitions yield a strong field-like torque on the CrI$_3$ layer, while inter-band transitions give rise to a comparatively weaker damping-like torque in the WSe$_2$/CrI$_3$ system. Remarkably, the damping-like component is enhanced by up to three orders of magnitude in n-doped MoSe$_2$, reaching a strength comparable to the field-like torque, which itself is an order of magnitude larger than that in the WSe$_2$-based bilayer. Both torque components exhibit strong asymmetry between n-type and p-type doping in WSe$_2$ and MoSe$_2$ systems. Furthermore, we demonstrate that the twist angle plays a crucial role: depending on the TMDC and chemical potential, twisting can reverse the sign of the SOT and significantly modulate its magnitude. Finally, we show that a transverse gate electric field enables substantial tunability of the SOT, by nearly one order of magnitude, and induces a sign reversal at a twist angle of $10.16^{\circ}$.