Observation of Thickness-Modulated Out-of-Plane Spin-Orbit Torque in Polycrystalline Few-Layer Td-WTe2 Film.

The low-symmetry Weyl semimetallic Td-phase WTe2 exhibits both a distinct out-of-plane damping torque (τDL) and exceptional charge-spin interconversion efficiency enabled by strong spin-orbit coupling, positioning it as a prime candidate for spin-orbit torque (SOT) applications in two-dimensional transition metal dichalcogenides. Herein, we report on thickness-dependent unconventional out-of-plane τDL in chemically vapor-deposited (CVD) polycrystalline Td-WTe2 (t)/Ni80Fe20/MgO/Ti (Td-WTN-t) heterostructures. Angle-resolved spin-torque ferromagnetic resonance measurements on the Td-WTN-12 structure showed significant spin Hall conductivities of σSH,y = 4.93 × 103 (ℏ/2e) Ω-1m-1 and σSH,z = 0.81 × 103 (ℏ/2e) Ω-1m-1, highlighting its potential for wafer-scale spin-orbit torque device applications. Additionally, a detailed examination of magnetotransport properties in polycrystalline few-layer Td-WTe2 films as a function of thickness revealed a marked amplification of the out-of-plane magnetoresistance, which can be ascribed to the anisotropic nature of charge carrier scattering mechanisms within the material. Spin pumping measurements in Td-WTN-t heterostructures further revealed thickness-dependent spin transport properties of Td-WTe2, with damping analysis yielding an out-of-plane spin diffusion length of λSD ≈ 14 nm.