agnetometers with Intermediate lux Transformers

We have modeled and fabricated a novel HTS 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’, 28 nH pickup nd a low inductance (45 pH) SQUID. The small S inductance is required for low noise and high av/a(r, 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 (.I 0.7 pm) YBazCusO7 layers, and a groundplaned SQUID slot. The measured responsivity at 77 K was 0.57 mm2 (3.6 nT/@o) with aV/i3D = 98 pV/Qo 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 PrBazCu& as the isolation barrier. Excess white noise, probably due to Johnson noise from the PrBazCu30.1 , limited the sensitivity in our first device to 33 ff/& at 77 K.