Ultralow Gilbert damping in CrO2 epitaxial films

In this study, we report the observation of ultralow Gilbert damping in epitaxial ${\mathrm{CrO}}₂$ films. The dynamic properties of (100)- and (110)-oriented ${\mathrm{CrO}}₂$ epitaxial films grown on ${\mathrm{TiO}}₂$ substrates were studied using ferromagnetic resonance measurements based on a resonant cavity and a coplanar waveguide in a large frequency range. The Lande $g$ factor was found to be 1.98, and it was independent of film orientation and thickness. The effective damping constant rapidly increased with film thickness when the film thickness was smaller than 50 nm, which might be attributed to magnon scattering. Extremely low damping was observed in the (110)-oriented ${\mathrm{CrO}}₂$ film with a thickness of 364 nm, and the damping constant was obtained as $(6.2\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. This value is about half an order of magnitude lower than that of ultralow-damping CoFe systems $[(1.3\text{--}2.0)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}]$ and is comparable with the lowest value observed recently in some Heusler alloy systems. The extremely low damping behavior in the ${\mathrm{CrO}}₂$ system is strongly correlated with its half-metallic nature.