Gilbert Damping in Magnetic Multilayers

TL;DR

This paper develops a theoretical framework to understand how adjacent normal metal layers enhance ferromagnetic relaxation in multilayers, highlighting the role of s-d exchange interaction and electron-electron interactions.

Contribution

The paper derives a quantitative expression for Gilbert damping enhancement due to normal metal layers, incorporating electron-electron interactions and comparing with experimental data.

Findings

Enhancement of damping is proportional to the square of the s-d exchange constant.

Electron-electron interactions significantly increase relaxation rates.

The theory aligns with recent spin-pumping experimental results.

Abstract

We study the enhancement of the ferromagnetic relaxation rate in thin films due to the adjacent normal metal layers. Using linear response theory, we derive the dissipative torque produced by the s-d exchange interaction at the ferromagnet-normal metal interface. For a slow precession, the enhancement of Gilbert damping constant is proportional to the square of the s-d exchange constant times the zero-frequency limit of the frequency derivative of the local dynamic spin susceptibility of the normal metal at the interface. Electron-electron interactions increase the relaxation rate by the Stoner factor squared. We attribute the large anisotropic enhancements of the relaxation rate observed recently in multilayers containing palladium to this mechanism. For free electrons, the present theory compares favorably with recent spin-pumping result of Tserkovnyak et al. [Phys. Rev. Lett. \textbf{88},117601 (2002)].