Spin-orbit torque for field-free switching in C3v crystals

Abstract

Spin-orbit torques in noncentrosymmetric polycrystalline magnetic heterostructures are usually described in terms of field-like and damping-like torques. However, materials with a lower symmetry point group can exhibit torques whose behavior substantially deviates from the conventional ones. In particular, based on symmetry arguments it was recently proposed that systems belonging to the ${\text{C}}_{3\mathrm{v}}$ point group display spin-orbit torques that can promote field-free switching [Liu et al., Nat. Nanotechnol. 16, 277 (2021)]. In the present work, we analyze the general form of the torques expected in ${\text{C}}_{3\mathrm{v}}$ crystals using the invariant theory. We uncover several new components that arise from the coexistence of the threefold rotation and mirror symmetries. Using both tight binding model and first principles simulations, we show that these unconventional torque components arise from the onset of trigonal warping of the Fermi surface and can be as large as the damping-like torque. In other words, the Fermi surface warping is a key indicator to the onset of field-free switching in low symmetry crystals.