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). SmBa(2)Cu(3)O(y) (SmBCO) and Er(0.95)La(0.1)Ba(1.95)Cu(3)O(y) (L1ErBCO) having T(c)'s around 90 K were used for base- and counter-electrodes, respectively. SrSnO(3) (SSO) was used for insulating layers. A black-colored Pr(1.4)Ba(1.6)Cu(2.6)Ga(0.4)O(y) (P4G4) layer was first grown on MgO substrate via a thin BaZrO(3) 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 x 1.0, 1.0 x 1.0 and 3.0 x 3.0 mm(2). 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 similar to 4.5 mu Phi(0)/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.