^19F NMR Study of the Bilayer Iron-Based Superconductor KCa_2Fe_4As_4F_2

We report a ^19\!F nuclear magnetic resonance (NMR) study on single-crystal KCa_2Fe_4As_4F_2 (T_\rm c \sim 33.3 K). The ^19F NMR spectral shape of KCa_2Fe_4As_4F_2 is weakly dependent on temperature and the Knight shift is small, which implies weak coupling between the CaF layer and the FeAs layer. The temperature dependence of 1/^19\!T_1 shows a hump below T_\rm c, however the 1/^75\!T_1 decreases just below T_\rm c, which implies that there are strong in-plane magnetic fluctuations in the CaF layers than in the FeAs layers. This may be caused by the motion of vortices. The absence of the coherence peak suggests unconventional superconductivity in KCa_2Fe_4As_4F_2.