Spin–Orbit Torques in NbSe₂/Permalloy Bilayers

Abstract

We present measurements of current-induced spin-orbit torques generated by NbSe₂, a fully metallic transition-metal dichalcogenide material, made using the spin-torque ferromagnetic resonance (ST-FMR) technique with NbSe₂/Permalloy bilayers. In addition to the out-of-plane Oersted torque expected from current flow in the metallic NbSe₂ layer, we also observe an in-plane antidamping torque with torque conductivity σ_S ≈ 10³ (ℏ/2e)(Ωm)^-1 and indications of a weak field-like contribution to the out-of-plane torque oriented opposite to the Oersted torque. Furthermore, in some samples we also measure an in-plane field-like torque with the form m̂ × ẑ, where m̂ is the Permalloy magnetization direction and ẑ is perpendicular to the sample plane. The size of this component varies strongly between samples and is not correlated with the NbSe₂ thickness. A torque of this form is not allowed by the bulk symmetries of NbSe₂ but is consistent with symmetry breaking by a uniaxial strain that might result during device fabrication.