HTS SQUID magnetometers with intermediate flux transformers

We have modeled and fabricated a novel HTS magnetometer with a d.c. SQUID which couples to an on-chip flux transformer via an intermediate flux transformer. This configuration allows high efficiency coupling between a 65 mm/sup 2/, 28 nH pickup loop and a low inductance (<15 pH) SQUID. The small SQUID inductance is required for low noise and high /spl part/V//spl part//spl Phi/ so that direct readout schemes may be used. The intermediate flux transformer was flip-chipped with a sub-micron separation to the main substrate. The magnetometer makes use of CAM junction technology, double thickness (/spl ap/0.7 /spl mu/m) YBa/sub 2/Cu/sub 3/O/sub 7/ layers, and a groundplaned SQUID slot. The measured responsivity at 77 K was 0.57 mm/sup 2/ (3.6 nT//spl Phi//sub 0/) with /spl part/V//spl part//spl Phi/=98 /spl mu/V//spl Phi//sub 0/ which is close to that predicted by our model. No resonances, which degrade the performance of similarly configured LTS devices, were observed. We attribute this to the use of PrBa/sub 2/Cu/sub 3/O/sub 7/ as the isolation barrier. Excess white noise, probably due to Johnson noise from the PrBa/sub 2/Cu/sub 3/O/sub 7/, limited the sensitivity in our first device to 33 fT//spl radic/Hz at 77 K.