Vertically Graded Single-Layer Fe-Ni Films with Low Damping and Unconventional Spin-Orbit Torques

Energy-efficient spintronic devices require a large spin-orbit torque (SOT) and low damping to excite magnetic precession. While ultrathin-ferromagnet bilayers attain a larger SOT at the expense of higher damping, single-layer ferromagnets with bulk inversion asymmetry [1-3] may enable both low damping and sizable SOTs. Here, we examine the impact of intentional asymmetry on damping and SOTs in 10-nm-thick symmetric and vertically graded films consisting of two ferromagnetic elements: Fe with low intrinsic damping [4] and Ni with sizable spin-orbit coupling [5]. Out-of-plane ferromagnetic resonance confirms intrinsic damping parameters of ≈0.003 for both symmetric and graded films, indicating that low damping is maintained even with steep compositional gradients. Further, dc-biased spin-torque ferromagnetic resonance reveals a damping-like SOT in each Fe-Ni system, with an effective Spin-Hall angle exceeding ~0.01. Remarkably, we find that the magnitude of the SOT does not correlate with the vertical gradient, pointing to a nuanced origin of the SOT not captured by conventional mechanisms. Our results are a step toward understanding and engineering low-damping single-layer ferromagnetic films for SOT-driven applications.