Magnetic imaging of YBa2Cu3Oy sampler circuits with moats by a scanning SQUID microscope

We have investigated the flux state in YBa 2 Cu 3 O y (YBCO) sampler circuits having superconducting ground planes with 5 or 7-μm-wide moats by a scanning SQUID microscope (SSM) system. It was found that magnetic flux was trapped into continuous moats with the length larger than 20 μm, and magnetic flux was excluded from the moat-surrounded area which includes ramp-edge Josephson junctions and ground plane contacts. We ensured that flux trapping into ground plane contacts is suppressed by reducing its size to typically 5 × 5 μm 2 . The threshold value for complete flux exclusion is about 0.8 μT slightly depending on chips, which can be readily achieved with dual μ-metal magnetic shielding. In addition, we have observed the magnetic flux due to electric noise which comes from outside the SSM chamber.