Layer-dependent spin-orbit torques generated by the centrosymmetric transition metal dichalcogenide β-MoTe2

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 beta-MoTe2, which, unlike WTe2, retains bulk inversion symmetry. We measure spin-orbit torques on beta-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 beta-MoTe2-the out-of-plane antidamping torque is present in MoTe2/Permalloy heterostructures when the beta-MoTe2 is a monolayer or trilayer thick, but goes to zero for devices with bilayer beta-MoTe2.