Gradiometric DC superconducting quantum interference devices (SQUIDs)
have been constructed from thin-film YBa2Cu3O7-x on MgO substrates. Th
e SQUIDs were fabricated as first-order gradiometers with PrBa2Cu3O7-x
insulator providing electrical isolation at the crossovers. Highly sy
mmetrical first-order gradiometer flux transformers have also been fab
ricated. These couple to the SQUIDs and enhance the sensitivity to mag
netic-field gradients. Each flux transformer has two 20 turn flat spir
al SQUID input coils and a pair of larger-area balanced pickup coils.
Flux transformers with both parallel and series configured pickup loop
s have been fabricated. When coupled to a SQUID as a flip chip, one gr
adiometer was operated at 77 K with a gradient sensitivity of (0.79 +/
- 0.03) nT/m/root Hz at 10 Hz and (2.0 +/- 0.1) nT/m/root Hz at 1 Hz.