Layer-dependent spin-orbit torques generated by the centrosymmetric transition metal dichalcogenide $\beta$-MoTe${}_2$

Abstract

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 WTe${}_2$/ferromagnet heterostructures. Here, we present measurements of spin-orbit torques produced by the low-symmetry material $\beta$-MoTe${}_2$, which unlike WTe${}_2$ retains bulk inversion symmetry. We measure spin-orbit torques on $\beta$-MoTe${}_2$/Permalloy heterostructures using spin-torque ferromagnetic resonance as a function of crystallographic alignment and MoTe${}_2$ 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 WTe${}_2$, 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 $\beta$-MoTe${}_2$ -- the out-of-plane antidamping torque is present in MoTe${}_2$/Permalloy heterostructures when the $\beta$-MoTe${}_2$ is a monolayer or trilayer thick, but goes to zero for devices with bilayer $\beta$-MoTe${}_2$.