Mechanisms behind large Gilbert damping anisotropies

TL;DR

This paper introduces a first-principles method to calculate Gilbert damping anisotropy in magnetic alloys, revealing significant anisotropic effects influenced by interfaces and structural distortions, though not fully matching experimental ratios.

Contribution

It presents a novel real-space electronic structure approach to quantify Gilbert damping anisotropy in magnetic materials from first principles.

Findings

Damping anisotropy in FeCo alloys can reach up to 200%.

Interface and surface effects significantly influence damping anisotropy.

Microscopic mechanisms for large damping anisotropies are identified, though not matching experimental maximums.

Abstract

A method with which to calculate the Gilbert damping parameter from a real-space electronic structure method is reported here. The anisotropy of the Gilbert damping with respect to the magnetic moment direction and local chemical environment is calculated for bulk and surfaces of Fe$_{50}$Co$_{50}$ alloys from first principles electronic structure in a real space formulation. The size of the damping anisotropy for Fe$_{50}$Co$_{50}$ alloys is demonstrated to be significant. Depending on details of the simulations, it reaches a maximum-minimum damping ratio as high as 200%. Several microscopic origins of the strongly enhanced Gilbert damping anisotropy have been examined, where in particular interface/surface effects stand out, as do local distortions of the crystal structure. Although theory does not reproduce the experimentally reported high ratio of 400% [Phys. Rev. Lett. 122, 117203 (2019)], it nevertheless identifies microscopic mechanisms that can lead to huge damping anisotropies.