Fabrication of Integrated Two-Axis High- $T_{\rm c}$ Planar Gradiometer

We have fabricated a two-axis high-T_c planar gradiometer. Two gradiometers with planar gradiometric pickup coils lie diagonally on a square substrate. Therefore, they can measure field gradients, dBz/dx and dBz/dy at the same site. The two-axis gradiometer was fabricated on a 15 × 15 mm MgO (100) substrate using an HTS multilayer process including crossovers, superconductive contacts and ramp-edge junctions. The width of the pickup coil and the baseline length are 6 mm and 8.5 mm, respectively. Voltage modulations in V-Φ characteristics more than 20 μV at 77 K were obtained for both the gradiometers. Parasitic effective areas measured by applying a uniform magnetic field were about 0.0001 mm^-2. The gradiometer balance was estimated to be about 10^-2. The parasitic effective area was not due to the imbalance of the gradiometric pickup coil, but due to a sensitivity of SQUID itself. Flux noise level as low as 10 μΦ₀/Hz^1/2 at frequencies down to 3 Hz and a cross-talk ratio less than 1/230 were obtained.