  The emerging field of superconductor (SC) spintronics has attracted intensive
attentions recently. Many fantastic spin dependent properties in SC have been
discovered, including the observation of large magnetoresistance, long spin
lifetimes and the giant spin Hall effect in SC, as well as spin supercurrent in
Josephson junctions, etc. Regarding the spin dynamic in SC films, few studies
has been reported yet. Here, we report the investigation of the spin dynamics
in an s-wave superconducting NbN film via spin pumping from an adjacent
insulating ferromagnet GdN layer. A profound coherence peak of the Gilbert
damping is observed slightly below the superconducting critical temperature of
the NbN layer, which is consistent with recent theoretical studies. Our results
further indicate that spin pumping could be a powerful tool for investigating
the spin dynamics in 2D crystalline superconductors.
