Noise performance of high-Tc DC SQUID magnetometers

摘要

High-Tc DC SQUID magnetometers operating at 77 K have been fabricated from epitaxial YBCO and SrTiO3 films and characterized. The DC SQUID washers incorporated either bicrystal or step-edge microbridge junctions of widths down to about 3 μm. 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 7×10-30 J/Hz at 1 Hz (45 pH inductance). For flip-chip coupled magnetometers (15-turn input coil, pick-up coil area 13 mm2), excess low frequency noise was observed, only partially reduced by AC biasing, with field sensitivities of 1 pT rms/√(Hz) and 130 fT rms√(Hz) above 200 Hz.