Intrinsic and Extrinsic Damping in Polycrystalline Fe Thin Films

Advancing the understanding of magnetic relaxation is essential for energy-efficient spintronic memories, oscillators, etc. Here, we examine the intrinsic and extrinsic damping in two series of polycrystalline Fe thin films with distinct structural properties: (1) strongly textured, smooth Fe films grown on Cu and (2) weakly textured, rough Fe films grown on Ag.

Out-of-plane FMR measurements reveal that intrinsic Gilbert damping parameters for all Fe films thicker than 4 nm are in quantitative agreement with that for epitaxial Fe [1]. The remarkable invariance with microstructure strongly suggests that intrinsic Gilbert damping is a local property of nanoscale crystal grains with limited impact from film microstructure. By contrast, the in-plane FMR linewidths of the Fe films exhibit pronounced nonlinear frequency dependencies indicating the presence of strong two-magnon scattering. These in-plane FMR results cannot be reproduced quantitatively by a standard grain-to-grain two-magnon scattering model [2], even with different types of correlation functions. Our finding points to the need to further examine the fundamental impact of film microstructure on extrinsic damping.

[1] Khodadadi et al., Phys. Rev. Lett. 124, 957201 (2020)

[2] Kalarickal et al., Phys. Rev. B 77, 054427 (2008)