Noise performance of high-T/sub c/ DC SQUID magnetometers

High-T/sub c/ DC SQUID magnetometers operating at 77 K have been fabricated from epitaxial YBCO and SrTiO/sub 3/ films and characterized. The DC SQUID washers incorporated either bicrystal or step-edge microbridge junctions of widths down to about 3 /spl mu/m. Three types of DC SQUID magnetometer are described-(a) a single layer device with a direct-coupled loop; (b) a DC SQUID plus flip-chip coupled flux transformer; (c) a fractional turn device with a direct-coupled multi-loop. The 1/f noise in the spectra of uncoupled DC SQUIDs and some magnetometers is significantly reduced by using AC bias modulation. Low frequency energy sensitivities of uncoupled SQUIDs at 77 K have been improved by use of AC bias to 7/spl times/10/sup -30/ J/Hz at 1 Hz (45 pH inductance). For flip-chip coupled magnetometers (15-turn input coil, pick-up coil area 13 mm/sup 2/), excess low frequency noise was observed, only partially reduced by AC biasing, with field sensitivities of 1 pT rms//spl radic/(Hz) and 130 fT rms/spl radic/(Hz) above 200 Hz. >