Ferromagnetic resonance linewidth in ultrathin films with perpendicular magnetic anisotropy

TL;DR

This study investigates the origins of large ferromagnetic resonance linewidths in ultrathin films with perpendicular magnetic anisotropy, revealing the role of PMA fluctuations and two magnon scattering in linewidth enhancement.

Contribution

It demonstrates that PMA fluctuations significantly contribute to linewidth broadening and shows how Helium ion irradiation reduces linewidth by decreasing PMA.

Findings

PMA fluctuations cause increased linewidth in in-plane magnetization.

Helium ion irradiation reduces PMA and linewidth.

Gilbert damping is significantly higher in PMA films.

Abstract

Transition metal ferromagnetic films with perpendicular magnetic anisotropy (PMA) have ferromagnetic resonance (FMR) linewidths that are one order of magnitude larger than soft magnetic materials, such as pure iron (Fe) and permalloy (NiFe) thin films. A broadband FMR setup has been used to investigate the origin of the enhanced linewidth in Ni$|$Co multilayer films with PMA. The FMR linewidth depends linearly on frequency for perpendicular applied fields and increases significantly when the magnetization is rotated into the film plane. Irradiation of the film with Helium ions decreases the PMA and the distribution of PMA parameters. This leads to a great reduction of the FMR linewidth for in-plane magnetization. These results suggest that fluctuations in PMA lead to a large two magnon scattering contribution to the linewidth for in-plane magnetization and establish that the Gilbert damping is enhanced in such materials ($\alpha \approx 0.04$, compared to $\alpha \approx 0.002$ for pure Fe).