Sensitive high-Tc SQUID magnetometers for unshielded operation

We have developed sensitive and highly reliable high-T-c SQUID magnetometers using a reproducible SNS junction fabrication process. In order to enable unshielded operation in the earth's field, we have incorporated flux dams Inco 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(2) has been achieved on 24 mm x 24 mm substrates using 100 mu m wide pickup coils. We have demonstrated a magnetic field noise sensitivity of 80 fT/root Hz at 10 Hz and 77 K in an externally looked field of 60 mu T. This magnetic field sensitivity unchanged from the zero-field measurement.