Control of spin-orbit torques through crystal symmetry

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. WTe₂, a semi-metallic transitional metal dichalcogenide, is one such low symmetry material. Consistent with the symmetries of the WTe₂ crystal structure, 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 [1]. We show directly that the sign of this out-of-plane antidamping torque reverses across a monolayer step in the WTe₂, and that the magnitude of this torque depends only weakly on the thickness of WTe₂ from 16 nm down to the single monolayer limit [2]. Finally, we compare our observations of spin-orbit torques generated in WTe₂ to that another low symmetry crystal, TaTe₂ – a monoclinic metallic transition metal dichalcogenide.
[1] D. MacNeill, G. M. Stiehl et al., Nature Physics 13, 300–305 (2017).
[2] D. MacNeill, G. M. Stiehl et al., Phys. Rev. B 96, 054450 (2017).