Room-Temperature Intrinsic and Extrinsic Damping in Polycrystalline Fe Thin Films

TL;DR

This study investigates magnetic relaxation in polycrystalline Fe films at room temperature, revealing that intrinsic damping is mainly determined by nanoscale grains, while extrinsic damping involves complex microstructural effects.

Contribution

It demonstrates that intrinsic Gilbert damping in polycrystalline Fe films is a local property of nanoscale grains, and highlights the complexity of modeling extrinsic damping due to microstructural disorder.

Findings

Intrinsic damping is approximately 0.0024 regardless of microstructure.

In-plane FMR linewidths show nonlinear frequency dependence, indicating extrinsic damping.

Two-magnon scattering models do not fully explain the extrinsic damping observed.

Abstract

We examine room-temperature magnetic relaxation in polycrystalline Fe films. Out-of-plane ferromagnetic resonance (FMR) measurements reveal Gilbert damping parameters of $\approx$ 0.0024 for Fe films with thicknesses of 4-25 nm, regardless of their microstructural properties. The remarkable invariance with film microstructure strongly suggests that intrinsic Gilbert damping in polycrystalline metals at room temperature is a local property of nanoscale crystal grains, with limited impact from grain boundaries and film roughness. By contrast, the in-plane FMR linewidths of the Fe films exhibit distinct nonlinear frequency dependences, indicating the presence of strong extrinsic damping. To fit our in-plane FMR data, we have used a grain-to-grain two-magnon scattering model with two types of correlation functions aimed at describing the spatial distribution of inhomogeneities in the film. However, neither of the two correlation functions is able to reproduce the experimental data quantitatively with physically reasonable parameters. Our findings advance the fundamental understanding of intrinsic Gilbert damping in structurally disordered films, while demonstrating the need for a deeper examination of how microstructural disorder governs extrinsic damping.