Theory of damping in magnetization dynamics, dispelling a myth and pointing a way forward

TL;DR

This paper refutes the myth that the Gilbert damping parameter is infinite at zero temperature in pure metals, clarifies calculation methods, and proposes a general torque formula applicable to inhomogeneous magnetic materials.

Contribution

It identifies and corrects a common misconception about damping in pure metals and introduces a unified approach and a new torque formula for complex magnetic systems.

Findings

The damping parameter is finite at T=0 in pure metals.

A unified view of damping calculation methods is provided.

A general torque formula for inhomogeneous materials is proposed.

Abstract

There is a widely-held belief amongst theoreticians that the Gilbert damping parameter {\alpha} in magnetization dynamics is infinite for a pure metal at T=0. The basic error leading to this belief is pointed out explicitly and the various methods of calculation used are viewed in a unified way based on the Lorentzian lineshape of ferromagnetic resonance spectra. A general torque formula for {\alpha} is proposed as a good starting-point for treating inhomogeneous materials such as alloys, compounds and layered structures. Local spin density functional theory provides a simple physical picture, in terms of a non-uniform precessional cone angle in ferromagnetic resonance, of how such inhomogeneity contributes to the damping. In a complementary many-body theory this contribution is given by a vertex correction to the torque-torque response function.