Fabrication of Low-Noise HTS-SQUID Gradiometers and Magnetometers With Ramp-Edge Josephson Junctions

We have studied the fabrication process of HTS-SQUIDs with ramp-edge Josephson junctions (JJs). SmBa₂Cu₃O_y (SmBCO) and Er_0.95La_0.1Ba_1.95Cu₃O_y (L1ErBCO) having T_c´s around 90 K were used for base- and counter-electrodes, respectively. SrSnO₃ (SSO) was used for insulating layers. A black-colored Pr_1.4Ba_1.6Cu_2.6Ga_0.4O_y (P4G4) layer was first grown on MgO substrate via a thin BaZrO₃ buffer layer. It was expected to work as a temperature homogenizer over the whole substrate area during deposition of the upper layers. We fabricated planar gradiometers having pickup coils with sizes of 0.5 × 1.0, 1.0 × 1.0 and 3.0 × 3.0 mm². Their baselines are 1.0, 1.0 and 3.0 mm, respectively. The gradiometers exhibited flux noise at 77 K as low as 3.3 ~ 4.5 μΦ₀/Hz^1/2 at 1 kHz. Directly and inductively coupled magnetometers having a 13.5 mm square pickup coil were also fabricated. The latter has an integrated 20-turn input coil. They showed field noise levels of 55 and 10 fT/Hz^1/2 at 1 kHz, respectively.