Nearly isotropic spin-pumping related Gilbert damping in Pt/Ni$_{81}$Fe$_{19}$/Pt

TL;DR

This study investigates the anisotropy of Gilbert damping in Pt/NiFe/Pt films, finding minimal and opposite-sign anisotropy compared to theoretical predictions, indicating a different underlying mechanism for spin-current absorption.

Contribution

The paper provides experimental measurements of damping anisotropy in Pt/NiFe/Pt, challenging existing theories based on Rashba spin-orbit coupling and suggesting alternative mechanisms.

Findings

Minimal damping anisotropy observed

Sign of anisotropy opposite to Rashba effect predictions

Indicates a different mechanism for spin-current absorption

Abstract

A recent theory by Chen and Zhang [Phys. Rev. Lett. 114, 126602 (2015)] predicts strongly anisotropic damping due to interfacial spin-orbit coupling in ultrathin magnetic films. Interfacial Gilbert-type relaxation, due to the spin pumping effect, is predicted to be significantly larger for magnetization oriented parallel to compared with perpendicular to the film plane. Here, we have measured the anisotropy in the Pt/Ni$_{81}$Fe$_{19}$/Pt system via variable-frequency, swept-field ferromagnetic resonance (FMR). We find a very small anisotropy of enhanced Gilbert damping with sign opposite to the prediction from the Rashba effect at the FM/Pt interface. The results are contrary to the predicted anisotropy and suggest that a mechanism separate from Rashba spin-orbit coupling causes the rapid onset of spin-current absorption in Pt.