LOW-NOISE HTS GRADIOMETERS AND MAGNETOMETERS CONSTRUCTED FROM YBI2CU3O7-X/PRBA2CU3O7-Y THIN-FILMS

D.C. SQUID based gradiometers and magnetometers have been fabricated using thin films of YBa2Cu3O7 and PrBa2Cu3O7 on MgO substrates. Flip-chip magnetometers with 1 cm(2) and 6.25 cm(2) substrates have been constructed. Our device on the larger substrate with a 430 mm(2) pickup loop had a sensitivity of (42+/-3) fT/root Hz above 3 Hz and (75+/-7) fT/root Hz at 1 Hz. Numerical simulations of d.c. SQUIDs at 77 K have been conducted over an inductance range from 2 to 1000 pH and a critical current range from 3.2 to 322 mu A. The results emphasise the need to reduce SQUID inductances in order to obtain lower flux noise. Three layer gradiometric SQUIDs with c-axis microbridge junctions and low inductance have been fabricated. First order gradiometric flux transformers were also made with twin 20 turn spiral SQUID input coils and a pair of larger area balanced sensing coils. These were mounted with the gradiometric SQUIDs in a flip chip configuration to complete the highly symmetrical gradiometers. Measurements at 79 K on one gradiometer gave a gradient sensitivity of (49+/-3) pT/m/root Hz at 300 Hz and (103+/-7) pT/m/root Hz at 1 Hz.