Ultra-low magnetic damping of a metallic ferromagnet

Magnetic damping is of critical importance for devices that seek to exploit the electronic spin degree of freedom, as 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^1-3. This presents a challenge for a broad range of applications in spintronics⁴ and spin-orbitronics that depend on materials and structures with ultra-low damping^5,6. It is believed that achieving ultra-low damping in metallic ferromagnets is limited by the scattering of magnons by the conduction electrons. However, we report on a binary alloy of cobalt and iron that overcomes this obstacle and exhibits a damping parameter approaching 10^-4, which is comparable to values reported only for ferrimagnetic insulators^7,8. We explain this phenomenon by a unique feature of the band structure 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 and a test of the theoretical predictions proposed by Mankovsky and colleagues³.