Observation of Thickness-Modulated Out-of-Plane Spin-Orbit Torque in Polycrystalline Few-Layer Td-WTe(2) Film.

The low-symmetry Weyl semimetallic Td-phase WTe(2) 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-WTe(2) (t)/Ni(80)Fe(20)/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 σ(SH,y) = 4.93 × 10(3) (ℏ/2e) Ω(-1)m(-1) and σ(SH,z) = 0.81 × 10(3) (ℏ/2e) Ω(-1)m(-1), highlighting its potential for wafer-scale spin-orbit torque device applications. Additionally, a detailed examination of magnetotransport properties in polycrystalline few-layer Td-WTe(2) 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-WTe(2), with damping analysis yielding an out-of-plane spin diffusion length of λ(SD) ≈ 14 nm.