Low noise HTS gradiometers and magnetometers constructed from YBa/sub 2/Cu/sub 3/O/sub 7-x//PrBa/sub 2/Cu/sub 3/O/sub 7-y/ thin films

D.C. SQUID based gradiometers and magnetometers have been fabricated using thin films of YBa/sub 2/Cu/sub 3/O/sub 7/ and PrBa/sub 2/Cu/sub 3/O/sub 7/ on MgO substrates. Flip-chip magnetometers with 1 cm and 6.25 cm/sup 2/ substrates have been constructed. Our device on the larger substrate with a 430 mm/sup 2/ pickup loop had a sensitivity of (42/spl plusmn/3) fT//spl radic/Hz above 3 Hz and (75/spl plusmn/7) fT//spl radic/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 /spl 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/spl plusmn/3) pT/m//spl radic/Hz at 300 Hz and (103/spl plusmn/7) pT/m//spl radic/Hz at 1 Hz. >