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

## TL;DR

This study explores how the thickness of Td-WTe2 affects its spin–orbit torque properties, showing potential for use in spintronic devices.

## Contribution

The paper reports thickness-modulated out-of-plane spin–orbit torque in polycrystalline few-layer Td-WTe2 films.

## Key findings

- Angle-resolved spin-torque ferromagnetic resonance showed high spin Hall conductivities in Td-WTe2/Ni80Fe20 heterostructures.
- Out-of-plane magnetoresistance increased with decreasing Td-WTe2 thickness due to anisotropic charge carrier scattering.
- Spin pumping measurements revealed a thickness-dependent out-of-plane spin diffusion length of approximately 14 nm.

## Abstract

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.