Noise Performance of High-Tc DC SQUID Magnetometers

High-T, DC SQUID magnetometers op- erating at 77 K have been fabricated from epitaxial YBCO and SrTiOg films and characterized. The DC SQUID washers incorporated either bicrystal or step-edge microbridge junctions of widths down to about 3 pm. Three types of DC SQUID magne- tometer 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 l/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-~~J/Hz at 1 Hz (45 pH inductance). For flip-chip coupled mag- netometers (15-turn input coil, pick-up coil area 13 mm2), excess low fre-quency noise was observed, only partially reduced by ac biasing, with field sensitivities of 1 pT rms/dHz at 1 Hz and 130 fT rmsdHz above 200 Hz. I. INTRODUCITON High-Tc SQUID magnetometers have been under develop- ment by a number of research groups world wide and rapid progress has been reported during the past two years, with improvements to the 77 K operation of high-Tc films, Josephson junctions, DC SQUIDs, flux transformers and coupled devices. For example in (l), there was reported a sin- gle layer DC SQUID magnetometer coupled to a single layer transformer (loop diameter = 50 mm) with a field sensitivity at 77 K of 31 ff rms/dHz above 5 Hz. A large area rf SQUID washer, coupled in a similar manner, was also reported to reach 24 ff rms/dHz for fkquencies down to 1 Hz (21. One of the goals of the work reported here has been to de- velop a high-Tc SQUID technology which is hoped to be eventually used in biomagnetic applications. These include magnetocardiography (MCG) and also magnetoencephalogra- phy (MEG), where the high-Tc SQUIDs would eventually replace their low-Tc counterparts in SQUID systems with a large number of channels (3). For such applications, compact arrays of high sensitivity SQUID magnetometers and grad- iometers are desired. The MEG requirement is the most