Eddy current interactions in a Ferromagnet-Normal metal bilayer structure, and its impact on ferromagnetic resonance lineshapes

TL;DR

This study reveals that eddy currents in thin ferromagnet-normal metal bilayers significantly influence ferromagnetic resonance lineshapes, causing asymmetries that depend on layer thickness and sample geometry, contrary to previous assumptions.

Contribution

It demonstrates that eddy currents in NM layers thinner than the skin depth can induce measurable phase shifts and asymmetries in FMR lineshapes, which were previously neglected.

Findings

Eddy currents cause asymmetry in FMR lineshapes for thin NM layers.

Asymmetry increases as NM layer thickness decreases.

Sample geometry and layer thickness can tune the eddy-current effects.

Abstract

We investigate the effect of eddy currents on ferromagnetic resonance (FMR) in ferromagnet-normal metal (FM/NM) bilayer structures. Eddy-current effects are usually neglected for NM layer thicknesses below the microwave (MW) skin depth (approx. 800 nm for Au at 10 GHz). However, we show that in much thinner NM layers (10-100 nm of Au or Cu) they induce a phase shift in the FMR excitation when the MW driving field has a component perpendicular to the sample plane. This results in a strong asymmetry of the measured absorption lines. In contrast to typical eddy-current effects, the asymmetry is larger for thinner NM layers and is tunable through changing the sample geometry and the NM layer thickness.