Ultra-low magnetic damping of a metallic ferromagnet

Abstract

Materials with low magnetic damping are important for a range of applications but are typically insulating, which limits their use. Thanks to a unique feature of the band structure, similar levels of damping can now be achieved in a metallic alloy. 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 materials1,2,3. This presents a challenge for a broad range of applications in spintronics4 and spin-orbitronics that depend on materials and structures with ultra-low damping5,6. 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 insulators7,8. 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.