Few-layer dependence of Spin-Orbit torques in 1T’-MoTe₂/ ferromagnet heterostructures.

Single-crystal materials with sufficiently low crystal symmetry can be used to generate novel forms of spin-orbit torques on adjacent ferromagnets. For example, we have previously shown that when WTe₂ is used as a spin generation layer, an out-of-plane antidamping torque is generated in heterostructures of WTe₂/Py that is consistent with the WTe₂ crystal symmetries. Here, we present measurements of spin-orbit torques produced by another low-symmetry material, 1T’-MoTe₂, which manifests different crystal symmetries as compared to WTe₂. We perform systematic studies of the spin-orbit torques using spin-torque ferromagnetic resonance and second harmonic Hall techniques down to the monolayer limit. We report the presence of an out-of-plane antidamping torque in MoTe₂/Py heterostructures when the MoTe₂ is a monolayer or trilayer thick, but surprisingly find that the out-of-plane antidamping torque goes to zero in bilayer MoTe₂. Finally, we compare these results to those found in the few-layer limit of WTe₂.