XMCD characterization of rare-earth dopants in Ni$_{81}$Fe$_{19}$(50nm): microscopic basis of engineered damping

TL;DR

This study uses XMCD to link local orbital moments of rare-earth dopants Gd and Tb in NiFe thin films to changes in magnetization damping, revealing an atomistic control mechanism based on spin-orbit coupling.

Contribution

It provides direct experimental evidence connecting rare-earth orbital moments to damping enhancement in ferromagnetic thin films, highlighting a scalable atomistic control method.

Findings

Tb doping increases damping and has a significant orbital moment

Gd doping does not affect damping and has negligible orbital moment

Orbital moments correlate with damping enhancement in RE-doped NiFe

Abstract

We present direct evidence for the contribution of local orbital moments to the damping of magnetization precession in magnetic thin films. Using x-ray magnetic circular dichroism (XMCD) characterization of rare-earth (RE) M$_{4,5}$ edges in Ni$_{81}$Fe$_{19}$ doped with $<$ 2% Gd and Tb, we show that the enhancement of GHz precessional relaxation is accompanied by a significant orbital moment fraction on the RE site. Tb impurities, which enhance the Landau-Lifshitz(-Gilbert) LL(-G) damping $\lambda(\alpha)$, show a spin to orbital number ratio of 1.5$\pm$0.3; Gd impurities, which have no effect on damping, show a spin to orbital number ratio of zero within experimental error. The results indicate that the dopant-based control of magnetization damping in RE-doped ferromagnets is an atomistic effect, arising from spin-lattice coupling, and thus scalable to nanometer dimensions.