  The phenomenology of magnetic damping is of critical importance for devices
that seek to exploit the electronic spin degree of freedom since damping
strongly affects the energy required and speed at which a device can operate.
However, theory has struggled to quantitatively predict the damping, even in
common ferromagnetic materials. This presents a challenge for a broad range of
applications in spintronics and spin-orbitronics that depend on materials and
structures with ultra-low damping. Such systems enable many experimental
investigations that further our theoretical understanding of numerous magnetic
phenomena such as damping and spin-transport mediated by chirality and the
Rashba effect. Despite this requirement, it is believed that achieving
ultra-low damping in metallic ferromagnets is limited due to the scattering of
magnons by the conduction electrons. However, we report on a binary alloy of Co
and Fe that overcomes this obstacle and exhibits a damping parameter
approaching 0.0001, which is comparable to values reported only for
ferrimagnetic insulators. We explain this phenomenon by a unique feature of the
bandstructure in this system: The density of states exhibits a sharp minimum at
the Fermi level at the same alloy concentration at which the minimum in the
magnetic damping is found. This discovery provides both a significant
fundamental understanding of damping mechanisms as well as a test of
theoretical predictions.
