Julian S. Satchell

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.

Control and reproducibility of c-axis microbridges

Vertical c-axis microbridges, with dimensions comparable to the penetration depth (/spl lambda//sub c/), have been investigated experimentally and theoretically. Josephson-like effects are observed due to coherent motion of vortices across the bridge. Both the on-chip and chip to chip reproducibility has been improved from previous reports by process optimisation. Within an array it is highly dependent on the oxygenation state and the temperature of the sample. Although the spread in parameters within an array of standard junctions is almost good enough to fabricate many circuits of interest, the I/sub c/ is too high and the R/sub n/ too low for many logic applications. In an attempt to reduce the coupling strength, the growth temperature of a region of the material in the microbridge has been varied. Initial results suggest I/sub c/ can be reduced and R/sub n/ increased to a level which, with further optimisation, might be useful for logic applications, while the reproducibility was not adversely affected.