Exceptionally low magnetic damping in Co$_{0.25}$Fe$_{0.75}$ epitaxial films*

Ferromagnetic alloy Co$_{x}$Fe$_{1-x}$ has a wide range of applications in magnetic devices and spintronics due to its strong magnetization and relatively low damping. It was recently shown that polycrystalline Co$_{x}$Fe$_{1-x}$ films of various Co concentrations grown on a Cu seed layer on Si exhibit minimal magnetic damping at x$=$0.25 [1]. We grow both polycrystalline and epitaxial Co$_{0.25}$Fe$_{0.75}$ films using off-axis sputtering. The polycrystalline films show FMR linewidths of approximately 23 G at 10 GHz, comparable to the values reported in ref. 1. Remarkably, the epitaxial Co$_{0.25}$Fe$_{0.75}$ films grown on MgO (100) have much smaller gilbert damping, $\alpha =$8x10$^{-4}$, and much narrower FMR linewidths, less than 10 G at 10 GHz, which are both comparable to those of high quality Y$_{3}$Fe$_{5}$O$_{12}$ films. The metallic nature of this material combined with its very low damping offers the opportunity to explore low-loss, charge-based, dynamic spin transport that cannot be achieved with an insulating ferrimagnet such as Y$_{3}$Fe$_{5}$O$_{12}$. 1. M. A. W. Schoen et. al., Nature Phys. 12, 839 (2016).