Vertically Graded Fe-Ni Alloys with Low Damping and a Sizeable Spin-Orbit Torque

TL;DR

This study demonstrates that a 10 nm thick Fe-Ni alloy with a vertical compositional gradient exhibits both low damping and sizable spin-orbit torque, offering a promising approach for energy-efficient spintronic devices.

Contribution

It introduces a novel single-layer ferromagnet with a compositional gradient that achieves low damping and strong SOT without relying on ultrathin layers or interfaces.

Findings

Low effective damping parameter of < 5×10⁻³ in FeNi alloys.

Sizable anti-damping SOT efficiency of ~0.05.

Lattice strain gradient identified as key to symmetry breaking.

Abstract

Energy-efficient spintronic devices require a large spin-orbit torque (SOT) and low damping to excite magnetic precession. In conventional devices with heavy-metal/ferromagnet bilayers, reducing the ferromagnet thickness to $\sim$1 nm enhances the SOT but dramatically increases damping. Here, we investigate an alternative approach based on a 10 nm thick single-layer ferromagnet to attain both low damping and a sizable SOT. Instead of relying on a single interface, we continuously break the bulk inversion symmetry with a vertical compositional gradient of two ferromagnetic elements: Fe with low intrinsic damping and Ni with sizable spin-orbit coupling. We find low effective damping parameters of $\alpha_\mathrm{eff} < 5\times10^{-3}$ in the FeNi alloy films, despite the steep compositional gradients. Moreover, we reveal a sizable anti-damping SOT efficiency of $|\theta_\mathrm{DL}| \approx 0.05$, even without an intentional compositional gradient. Through depth-resolved x-ray diffraction, we identify a lattice strain gradient as crucial symmetry breaking that underpins the SOT. Our findings provide fresh insights into damping and SOTs in single-layer ferromagnets for power-efficient spintronic devices.