Control of spin-orbit torques through crystal symmetry in WTe2/ferromagnet bilayers

TL;DR

This paper demonstrates that by using WTe2 with low crystal symmetry, it is possible to generate out-of-plane antidamping spin-orbit torques, enabling more versatile control of magnetic devices beyond previous symmetry limitations.

Contribution

The study shows that crystal symmetry in WTe2/ferromagnet bilayers can be exploited to control the direction of spin-orbit torques, enabling out-of-plane antidamping torques not possible in higher-symmetry systems.

Findings

Out-of-plane antidamping torque generated along low-symmetry axes.

Symmetry-dependent control of spin-orbit torques demonstrated.

Potential for optimizing magnetic device control via crystal symmetry.

Abstract

Recent discoveries regarding current-induced spin-orbit torques produced by heavy-metal/ferromagnet and topological-insulator/ferromagnet bilayers provide the potential for dramatically-improved efficiency in the manipulation of magnetic devices. However, in experiments performed to date, spin-orbit torques have an important limitation -- the component of torque that can compensate magnetic damping is required by symmetry to lie within the device plane. This means that spin-orbit torques can drive the most current-efficient type of magnetic reversal (antidamping switching) only for magnetic devices with in-plane anisotropy, not the devices with perpendicular magnetic anisotropy that are needed for high-density applications. Here we show experimentally that this state of affairs is not fundamental, but rather one can change the allowed symmetries of spin-orbit torques in spin-source/ferromagnet bilayer devices by using a spin source material with low crystalline symmetry. We use WTe2, a transition-metal dichalcogenide whose surface crystal structure has only one mirror plane and no two-fold rotational invariance. Consistent with these symmetries, we generate an out-of-plane antidamping torque when current is applied along a low-symmetry axis of WTe2/Permalloy bilayers, but not when current is applied along a high-symmetry axis. Controlling S-O torques by crystal symmetries in multilayer samples provides a new strategy for optimizing future magnetic technologies.