HTS SQUID MAGNETOMETERS WITH INTERMEDIATE FLUX TRANSFORMERS

We have modeled and fabricated a novel HTS magnetometer with a d.c. SQ UID which couples to an on-chip flux transformer via an intermediate f lux transformer. This configuration allows high efficiency coupling be tween a 65 mm(2), 28 nH pickup loop and a low inductance (<15 pH) SQUI D. The small SQUID inductance is required for low noise and high parti al derivative V/partial derivative 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 u se of CAM junction technology, double thickness (approximate to 0.7 mu m) YBa2Cu3O7 layers, and a groundplaned SQUID slot. The measured resp onsivity at 77 K was 0.57 mm(2) (3.6 nT/Phi(0)) with partial derivativ e V/partial derivative Phi = 98 mu V/Phi(0) which is close to that pre dicted by our model. No resonances, which degrade the performance of s imilarly configured LTS devices, were observed. We attribute this to t he use of PrBa2Cu3O7 as the isolation barrier. Excess white noise, pro bably due to Johnson noise from the PrBa2Cu3O7, limited the sensitivit y in our first device to 33 fT/root Hz at 77 K.