Single-crystal materials with sufficiently low crystal symmetry and strong spin-orbit interactions can be used to generate novel forms of spin-orbit torques on adjacent ferromagnets, such as the out-of-plane antidamping torque previously observed in WTe2/ferromagnet heterostructures. Here, we present measurements of spin-orbit torques produced by the low-symmetry material β-MoTe2, which unlike WTe2 retains bulk inversion symmetry. We measure spin-orbit torques on β-MoTe2/Permalloy heterostructures using spin-torque ferromagnetic resonance as a function of crystallographic alignment and MoTe2 thickness down to the monolayer limit. We observe an out-of-plane antidamping torque with a spin torque conductivity as strong as 1/3 of that of WTe2, demonstrating that the breaking of bulk inversion symmetry in the spin-generation material is not a necessary requirement for producing an out-of-plane antidamping torque. We also measure an unexpected dependence on the thickness of the β-MoTe2 -- the out-of-plane antidamping torque is present in MoTe2/Permalloy heterostructures when the β-MoTe2 is a monolayer or trilayer thick, but goes to zero for devices with bilayer β-MoTe2.
@article{arxiv.1906.01068,
title = {Layer-dependent spin-orbit torques generated by the centrosymmetric transition metal dichalcogenide $\beta$-MoTe$_2$},
author = {Gregory M. Stiehl and Ruofan Li and Vishakha Gupta and Ismail El Baggari and Shengwei Jiang and Hongchao Xie and Lena F. Kourkoutis and Kin Fai Mak and Jie Shan and Robert A. Buhrman and Daniel C. Ralph},
journal= {arXiv preprint arXiv:1906.01068},
year = {2019}
}