Room-temperature intrinsic and extrinsic damping in polycrystalline Fe thin films
Shuang Wu, D. A. Smith, P. Nakarmi, Anish Rai, Michael Clavel, Mantu K. Hudait, Jing Zhao, F. Marc Michel, Claudia Mewes, Tim Mewes, Satoru Emori
Abstract
We examine room-temperature magnetic relaxation in polycrystalline Fe films. Out-of-plane ferromagnetic resonance measurements reveal Gilbert damping parameters of $\ensuremath{\approx}0.0024$ for Fe films with thicknesses of 4--25 nm, regardless of their microstructural properties. This observation runs counter to the intuition that various film defects heavily influence Gilbert damping. The remarkable invariance with film microstructure suggests that room-temperature intrinsic Gilbert damping in the Fe films is mostly fixed by the bcc crystal structure within the bulk of nanoscale 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 dependencies, 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.