Ferromagnetic resonance linewidth in ultrathin films with perpendicular magnetic anisotropy

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 a material in which the PMA could be systematically reduced by irradiation with Helium ions: $\text{Ni}\ensuremath{\mid}\text{Co}$ multilayers. 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, leading to a large reduction in 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 ($\ensuremath{\alpha}\ensuremath{\approx}0.04$, compared to $\ensuremath{\alpha}\ensuremath{\approx}0.002$ for pure Fe).