Damping in Ferromagnets: Landau-Lifshitz versus Gilbert

We first note a number of qualitative and quantitative arguments favoring Landau-Lifshitz over Gilbert damping in ferromagnets. We then explicitly demonstrate a classical Fokker-Planck-like derivation of the macroscopic damping rate in terms of thermodynamic fluctuations (fluctuation-dissipation). Because out-of-equilibrium fluctuations are driven toward equilibrium by their excess thermodynamic energy, the damping term is proportional to the transverse component of the effective field, thus yielding Landau-Lifshitz damping with an explicit expression for the damping coefficient. This damping is unaffected to lowest order in systems with spin transfer torque (STT). Recent experiments on current-driven domain wall motion have a natural interpretation in terms of the so-called adiabatic STT.