  The multilayer thin film structure of the superconductor has been proposed by
A. Gurevich to enhance the maximum gradient of SRF cavities. The lower critical
field Hc1 at which the vortex starts penetrating the superconducting material
will be improved by coating Nb with thin film superconductor such as NbN. It is
expected that the enhancement of Hc1 depends on the thickness of each layer. In
order to determine the optimum thickness of each layer and to compare the
measurement results with the theoretical prediction proposed by T. Kubo, we
developed the Hc1 measurement system using the third harmonic response of the
applied AC magnetic field at KEK. For the Hc1 measurement without the influence
of the edge or the shape effects, the AC magnetic field can be applied locally
by the solenoid coil of 5mm diameter in our measurement system. ULVAC made the
NbN-SiO2 multilayer thin film samples of various NbN thicknesses. In this
report, the measurement result of the bulk Nb sample and NbN-SiO2 multilayer
thin film samples of different thickness of NbN layer will be discussed.
