Sensitive high-T/sub c/ SQUID magnetometers for unshielded operation

We have developed sensitive highly reliable high-T/sub c/ SQUID magnetometers a reproducible SNS junction fabrication process. In order to enable unshielded operation in the earth's field, we have incorporated flux dams into direct-coupled SQUID magnetometers. By using up to four pickup coils in parallel, each with its own flux dam, an effective area of 0.57 mm/sup 2/ has been achieved on 24 mm/spl times/24 mm substrates using 100 /spl mu/m wide pickup coils. We have demonstrated a magnetic field noise sensitivity of 80 fT//spl radic/Hz 10 Hz and 77 K in an externally applied field 60 /spl mu/T. This magnetic field sensitivity unchanged from the zero-field measurement.