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 (<i>t</i>)/Ni₈₀Fe₂₀/MgO/Ti (Td-WTN-<i>t</i>) heterostructures. Angle-resolved spin-torque ferromagnetic resonance measurements on the Td-WTN-12 structure showed significant spin Hall conductivities of <i>σ</i>_SH,y = 4.93 × 10³ (ℏ/2e) Ω^-1m^-1 and <i>σ</i>_SH,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-<i>t</i> heterostructures further revealed thickness-dependent spin transport properties of Td-WTe₂, with damping analysis yielding an out-of-plane spin diffusion length of <i>λ</i>_SD ≈ 14 nm.