Spin-orbit torque for field-free switching in C_{3v} crystals

TL;DR

This paper investigates unconventional spin-orbit torques in C_{3v} crystals, revealing new torque components caused by Fermi surface warping that enable field-free magnetic switching.

Contribution

It provides a symmetry-based analysis and computational evidence of novel torque components in C_{3v} crystals, advancing understanding of field-free switching mechanisms.

Findings

New torque components identified from symmetry analysis.

Fermi surface warping correlates with large unconventional torques.

Unconventional torques can match damping-like torque magnitude.

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 C_{3v} point group display spin-orbit torques that can promote field-free switching [Liu et al. Nature Nanotechnology 16, 277 (2021)]. In the present work, we analyze the general form of the torques expected in C3v crystals using the Invariant Theory. We uncover several new components that arise from the coexistence of the three-fold 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.