High-performance high-Tc SQUID sensors for multichannel systems in magnetically disturbed environment

We have fabricated and characterized high-T-c SQUID sensors that are suited for operation in multichannel systems in magnetically disturbed environment. Utilizing the superior properties of our 30 degrees SrTiO3 bicrystal junctions, we are able to fabricate high-performance single layer direct-coupled SQUID magnetometers. System noise levels down to 24 ft Hz(-1/2) and typical 1/f corners of 4 Hz were obtained for devices having a 9 mm x 9 mm pickup loop and a nominal SQUID inductance of 100 pH. To protect the SQUIDs from moisture, they are hermetically encapsulated in a ceramic housing. A thick film resistor integrated on the chip carrier allows one to heat the SQUID device above T-c in order to release trapped magnetic flux. For the operation of our low-noise SQUIDs, a compact direct-coupled read-out electronics with a preamplifier voltage noise of 0.4 nV Hz(-1/2) and 100 kHz bias reversal was developed. Sensor dynamics and linearity were investigated with regard to the operation in magnetically disturbed environment. The maximum system bandwidth and slew rate of our bias reversal electronics were 1 MHz and about 5 x 10(5) Phi(0)/s, respectively. For signal frequencies less than or equal to 50 Hz, a minimum total harmonic distortion of -118 dB was measured, dominated by nonlinearities in the read-out electronics. A new magnetometer design with improved noise performance in magnetically disturbed environment is presented.