Effect of Meissner Screening and Trapped Magnetic Flux on Magnetization Dynamics in Thick Nb/Ni₈₀Fe₂₀/Nb Trilayers

We investigate the influence of Meissner screening and trapped magnetic flux on magnetization dynamics for a Ni₈₀Fe₂₀ film sandwiched between two thick Nb layers (100 nm) using broadband (5–20 GHz) ferromagnetic resonance (FMR) spectroscopy. Below the superconducting transition Tc of Nb, significant zero-frequency line broadening (5–6 mT) and dc-resonance field shift (50 mT) to a low field are both observed if the Nb thickness is comparable to the London penetration depth of Nb films (≥100 nm). We attribute the observed peculiar behaviors to the increased incoherent precession near the Ni₈₀Fe₂₀/Nb interfaces and the effectively focused magnetic flux in the middle Ni₈₀Fe₂₀ caused by strong Meissner screening and (defect-)trapped flux of the thick adjacent Nb layers. This explanation is supported by static magnetic properties of the samples and comparison with FMR data on thick Nb/Ni₈₀Fe₂₀ bilayers. Great care should, therefore, be taken in the analysis of FMR response in ferromagnetic Josephson structures with thick superconductors, a fundamental property for high-frequency device applications of spin-polarized supercurrents.