Influence of Structural Disorder on Magnetic Relaxation in Fe Thin Films

How magnetic relaxation is impacted by the structural properties of thin films is an important open question, especially for practical applications (e.g. spin-torque memories). In this work, we examine magnetic relaxation in polycrystalline and amorphous Fe films grown on different seed layers. Out-of-plane ferromagnetic resonance (FMR) measurements reveal Gilbert damping parameters of ~0.0025 for both polycrystalline and amorphous Fe films thicker than 6 nm. This damping parameter range is in quantitative agreement with that for epitaxial Fe [1], demonstrating that the intrinsic Gilbert damping of Fe is remarkably insensitive to the film structure. The in-plane FMR linewidths of both polycrystalline and amorphous Fe films exhibit distinct nonlinear frequency dependences, which are quantitatively reproduced by a grain-to-grain two-magnon scattering model [2]. However, the effective damping parameters derived from this model differ by up to
a factor of ~3 from the intrinsic damping parameters derived from out-of-plane FMR. This finding points to the need to consider additional possible contributions to magnetic relaxation in disordered ferromagnetic metal thin films.
[1] Khodadadi et al., Phys. Rev. Lett. 124, 957201 (2020)
[2] Kalarickal et al., Phys. Rev. B 77, 054427 (2008)