Thickness dependence of spin-orbit torques generated by WTe2

TL;DR

This study investigates how the thickness of WTe2 affects the spin-orbit torques generated in WTe2/permalloy bilayers, revealing a robust out-of-plane antidamping torque even in monolayer samples and clarifying the origin of the field-like torque.

Contribution

It provides the first detailed analysis of thickness-dependent spin-orbit torques in WTe2, including the sign reversal of the antidamping torque at a monolayer step and the origin of the field-like torque.

Findings

Out-of-plane antidamping torque exists and reverses sign at monolayer step.

Even monolayer WTe2 generates a strong, comparable torque to thicker samples.

Field-like torque mainly originates from the Oersted field.

Abstract

We study current-induced torques in WTe2/permalloy bilayers as a function of WTe2 thickness. We measure the torques using both second-harmonic Hall and spin-torque ferromagnetic resonance measurements for samples with WTe2 thicknesses that span from 16 nm down to a single monolayer. We confirm the existence of an out-of-plane antidamping torque, and show directly that the sign of this torque component is reversed across a monolayer step in the WTe2. The magnitude of the out-of-plane antidamping torque depends only weakly on WTe2 thickness, such that even a single-monolayer WTe2 device provides a strong torque that is comparable to much thicker samples. In contrast, the out-of-plane field-like torque has a significant dependence on the WTe2 thickness. We demonstrate that this field-like component originates predominantly from the Oersted field, thereby correcting a previous inference drawn by our group based on a more limited set of samples.