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

• Received 27 May 2019
• Revised 6 September 2019

DOI:https://doi.org/10.1103/PhysRevB.100.184402