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High-T, DC SQUID magnetometers operating 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 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 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 7~1 0 -~~J / H z at 1 Hz (45 pH inductance). For flip-chip coupled magnetometers (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. INTRODUCITONHigh-Tc SQUID magnetometers have been under development 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 single 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 develop a high-Tc SQUID technology which is hoped to be eventually used in biomagnetic applications. These include magnetocardiography (MCG) and also magnetoencephalography (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 gradiometers are desired. The MEG requirement is the most stringent, with near field sensitivities of < 10 ff rmsh!Hz at = 1 Hz necessary for acceptable signal to noise.In this paper, we report on noise measurements carried out on three types of high-Tc DC SQUID magnetometers fabricated from YBCO and STO films and operating at or near 77 K: (a) DC SQUID washers plus 3-layer transformers coupled in flip chip (YBCO/STO/YBCO, where YBCO = YBa2Cu307-6, STO = SrTiO3); (b) single layer magnetometers with direct-coupled flux transformer loops; (c) integrated fractional-turn magnetometers with direct coupled loops. For the latter, preliminary data only will be presented. II. DC SQUID FABRICATION AND PROPERTIES A. DC SQUID FabricationDC SQUIDs have been fabricated with square washer geometries similar to the Ketchen-Jaycox design [4] with junctions on the inside near the washer m t r e (type 1, Fig. 1) or on the outside end of the slot (type 2, Fig. 1).The epitaxial YBCO and STO fiis for ...

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