The stability of dc and rf SQUIDs in static ambient fields

For real applications of SQUIDs in biomagnetism, geomagnetism and NDE, SQUID devices are required which are stable in a magnetically unshielded environment. We have developed de and rf SQUID devices with different layouts and widths of Josephson junctions, and have studied their operation in static magnetic fields and unshielded space. Additionally, the noise performance of these SQUID devices has been studied to determine whether there is an influence of the layout or the junction width on noise. Using a 0.3 mu m wide step-edge junction the transfer function of a washer-type SQUID was not depressed by external magnetic fields up to 3 G. The same stability was obtained using a direct-coupled layout for rf and de SQUIDs with junctions of 1 mu m width, The best field sensitivity we obtained using a pick-up loop of 8 mm in diameter which was coupled directly to a do SQUID having an inductance of 150 pH. In magnetic shielding, the field resolution of a SQUID with a single junction resistance of 10 Omega was 20 fT Hz(-1/2) at 77 K and 1 kHz.