The Role of Self-Torques in Transition Metal Dichalcogenide/Ferromagnet Bilayers

Abstract

Recently, 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, variety in crystal symmetries, and pristine interfaces. Although the TMD layer was considered essential for the generation of the observed 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 MoS${}_2$/permalloy/Al${}_2$O${}_3$ devices and compare them to a single-layer permalloy/Al${}_2$O${}_3$ device to accurately disentangle the role of self-torques from contributions from the TMD layer. We report a damping-like self-torque conductivity of opposite sign in our single-layer permalloy/Al${}_2$O${}_3$ device compared to one MoS${}_2$/permalloy/Al${}_2$O${}_3$ device, and find no significant one for all other MoS${}_2$/permalloy/Al${}_2$O${}_3$ devices. This indicates a competition between the self-torque and the torque arising from the TMD layer, which would reduce the observed torque in these bilayers. In addition, we find a field-like spin-torque conductivity of comparable magnitude to control MoS${}_2$/permalloy/Al${}_2$O${}_3$ devices, indicating only a minor role of the MoS${}_2$ layer. Finally, we find a linear dependence of the SOT conductivity on the Hall bar leg/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.

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