Low-noise Y-Ba-Cu-O flip-chip dc SQUID magnetometers

We have prepared low-noise flip-chip SQUID magnetometers by using dc SQUIDs with large flux-to-voltage transfer functions and superconducting flux transformers. The dc SQUIDs have a square washer geometry and are prepared on a SrTiO/sub 3/ bicrystal substrate with a misorientation angle of 30/spl deg/. The maximum peak-to-peak flux modulated voltage varies typically between 20 and 60 /spl mu/V for a SQUID inductance of 80 pH. The YBa/sub 2/Cu/sub 3/O/sub 7-x/ flux transformers are prepared on a SrTiO/sub 3/ single crystal polished on both sides. The preparation process of the flux transformers involves the deposition of YBCO and SrTiO/sub 3/ layers by pulsed laser ablation and pattern definition by ion beam etching. The flip-chip magnetometers are encapsulated and robust to temperature cycling. We operate the magnetometers in magnetically shielded and in unshielded environment by using a direct-coupled read-out electronics. The field sensitivity of the magnetometers is equal to 2.2 nT//spl Phi//sub 0/. The field resolution of the magnetometers is typically <40 fT/Hz/sup 1/2/ at 1 kHz and about 100 fT/Hz/sup 1/2/ at 1 Hz inside shielding. The suitability of these devices for geophysical applications is shown by Transient Electromagnetics measurements.