Absence of Correlation between Damping and Crystal Quality in Epitaxial Fe

Most studies on ferromagnetic relaxation in metallic thin films are performed on polycrystalline structures, which makes it difficult to compare the experimental findings with theoretical calculations. Also it is unclear how microstructure – in addition to electronic properties – would affect damping in such polycrystalline metallic thin films. Our experimental study investigates the interplay between structure and damping in epitaxial thin films of pure Fe, grown on single-crystal MgO and MgAl₂O₄ substrates by magnetron sputtering. Our structural characterization by X-ray diffraction indicates significantly higher crystalline quality for Fe/MgAl₂O₄ as evidenced by pronounced Laue oscillations and an order of magnitude narrower rocking curve, compared to Fe/MgO. However, we find no difference in the value of Gilbert damping parameter α from broadband ferromagnetic resonance measurements up to 50 GHz, i.e., both Fe/MgAl₂O₄ and Fe/MgO consistently exhibit α ≈ 0.003. Our experimental observations are contrary to the expectations that structural disorder should impact ferromagnetic relaxation, and may provide insight into the fundamental mechanism of damping in metallic ferromagnets.