Role of self-torques in transition metal dichalcogenide/ferromagnet bilayers

In recent years, transition metal dichalcogenides (TMDs) have been extensively studied for their efficient spin-orbit torque generation in TMD/ferromagnetic bilayers, owing to their large spin-orbit coupling, large variety of crystal symmetries, and pristine interfaces. Although the TMD layer was considered essential for the generation of the observed spin-orbit torques (SOTs), recent reports show the presence of a self-torque in single-layer ferromagnetic devices with magnitudes comparable to TMD/ferromagnetic devices. Here, we perform second-harmonic Hall SOT measurements on metal-organic chemical vapor deposition (MOCVD) grown MoS2/permalloy/Al2O3 devices and compare them to a single-layer permalloy/Al2O3 device to accurately disentangle the role of self-torques, arising from the ferromagnetic layer, from contributions from the TMD layer in these bilayers. We report a fieldlike spin-torque conductivity of σFL=(-2.8±0.3)×103ℏ2e(ωm)-1 in a single-layer permalloy/Al2O3 device, which is comparable to our MoS2/permalloy/Al2O3 devices and previous reports on similar TMD/ferromagnetic bilayers, indicating only a minor role of the MoS2 layer. In addition, we observe a comparatively weak dampinglike torque in our devices, with a strong device-to-device variation. Finally, we find a linear dependence of the SOT conductivity on the Hall bar arm/channel width ratio of our devices, indicating that the Hall bar dimensions are of significant importance for the reported SOT strength. Our results accentuate the importance of delicate details, like device asymmetry, Hall bar dimensions, and self-torque generation, for the correct disentanglement of the microscopic origins underlying the SOTs, essential for future energy-efficient spintronic applications.