Anomalously low magnetic damping of a metallic ferromagnet
Justin M. Shaw, Martin Schoen, Danny Thonig, Michael L. Schneider, Thomas J. Silva, Hans T. Nembach, Olle Eriksson, Olof Karis
The phenomenology of magnetic damping is of critical importance to devices which 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 spin-tronics and spin-orbitronics that depend on the development of new materials with ultra-low damping. Materials with exceptionally low values of magnetic damping are also necessary to push experimental and theoretical understanding of numerous magnetic phenomena such as effects of chiral and Rashba mediated damping and spin-transport. Despite this requirement, it is understood that achieving an ultra low damping in metallic ferromagnets is hindered due to the scattering of magnons to the conduction electrons. However, we report on a binary alloy of Co and Fe that overcomes this obstacle and exhibits a damping parameter approaching 10-4 , i.e. comparable to values only reported for ferrimagnetic insulators. We explain this phenomenon by a unique feature of the bandstructure: the density of states of this material exhibits a sharp minimum at the Fermi level at the same alloy concentration as the minimum in the magnetic damping is found. This discovery provides both a significant fundamental understanding of damping mechanisms as well as a deeper comprehension into how damping can be engineered in a wide range of materials.