Enhanced torque efficiency in ferromagnetic multilayers by introducing naturally oxidized Cu

TL;DR

This study demonstrates that introducing a naturally oxidized Cu layer in ferromagnetic multilayers significantly enhances spin-orbit torque efficiency and spin-Hall angle, leveraging both spin-orbit and orbital torques for improved spintronic device performance.

Contribution

It reveals that naturally oxidized Cu layers can substantially boost torque efficiency via combined spin-orbit and orbital torques, a novel approach in multilayer design.

Findings

Torque efficiency increased by a factor of ten.

Spin-Hall angle was significantly enhanced.

Tuning Cu-CuO_x thickness controls torque performance.

Abstract

Spin-orbit torque (SOT) in the heavy elements with a large spin-orbit coupling (SOC) has been frequently used to manipulate the magnetic states in spintronic devices. Recent theoretical works have predicted that the surface oxidized light elements with a negligible SOC can yield a sizable orbital torque (OT), which plays an important role in switching the magnetization. Here, we report anomalous-Hall-resistance and harmonic-Hall-voltage measurements on perpendicularly magnetized Ta/Cu/[Ni/Co]$_5$/Cu-CuO$_x$ multilayers. Both torque efficiency and spin-Hall angle of these multilayers are largely enhanced by introducing a naturally oxidized Cu-CuO$_x$ layer, where the SOC is negligible. Such an enhancement is mainly due to the collaborative driven of the SOT from the Ta layer and the OT from the Cu/CuO$_x$ interface, and can be tuned by controlling the thickness of Cu-CuO$_x$ layer. Compared to the Cu-CuO$_x$-free multilayers, the maximum torque efficiency and spin-Hall angle were enhanced by a factor of ten, larger than most of the reported values in the other heterostructures.