Fabrication of SQUID chip for large planer gradiometer using HTS coated conductor

We have fabricated a large high-Tc superconducting (HTS) SQUID gradiometer with a baseline length of 130 mm for geophysical application. It consists of an HTS-SQUID gradiometer chip with an integrated input coil and a large pickup coil made of an HTS coated conductor (CC). In this work, fabrication of the chip containing ramp-edge Josephson junctions and multilayer structures is reported. An MgO(1 0 0) substrate with a size of 15 × 15 mm2 was used. The half area of the substrate was occupied by gold pads for connection with an HTS-CC. A directly-coupled thin-film gradiometer with a baseline length of 7 mm was prepared in the remained half area. SmBCO and La-doped ErBCO having Tc’s around 90 K were used for base- and counter-electrodes, respectively. A 5.5-turn input coil made of La-doped ErBCO was formed on the thin-film pickup coil made of SmBCO. The fabricated gradiometer chip was operated by supplying electrical current through the input coil, but modulation corresponding to only three flux quanta was observed in the V–Φ curve. We found that the critical current density of the thin-film pickup coil was insufficient. We speculated that the rather wide pickup coil was not fully oxygenated. After longer annealing treatment, the number of modulation peaks in the V–Φ curve was significantly increased.

► We have fabricated a large HTS-SQUID gradiometer with a baseline length of 130 mm.
► The gradiometer had an integrated input coil and a coated-conductor pickup one.
► The oxygenation for the lower SmBCO layer is very important.