Fabrication of 30 mm long baseline single-layer 2nd-order high-Tc SQUID gradiometer

We have designed and fabricated a 30 mm long baseline 2nd-order SQUID gradiometer from a single-layer YBa2Cu3O7 film. The design rule was the same as that of our previous study on short baseline gradiometers. Three linearly aligned 10.2 mm x 10.2 mm pickup loops were coupled directly to a dc SQUID to measure ∂2Bz/∂x2. The device had an overall size of 70.2 mm x 10.6 mm and was patterned from a single layer of YBa2Cu3O7 film on a sapphire substrate by argon ion milling with a photoresist mask. The Josephson element of the SQUID was YBa2Cu3O7 nanobridges formed by focused ion beam writing technique. Balancing of the device was achieved by optimizing the inductance of the center loop. Estimated sensitivity of the gradiometer was ∼1.8 x 10-10 T/m2/Hz1/2, which is equivalent to a field noise of 160 fT/Hz1/2, for an intrinsic SQUID flux noise of 10-5 φ0/Hz1/2