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 dc 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 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 dc SQUIDs with junctions of width. The best field sensitivity we obtained using a pick-up loop of 8 mm in diameter which was coupled directly to a dc SQUID having an inductance of 150 pH. In magnetic shielding, the field resolution of a SQUID with a single junction resistance of was at 77 K and 1 kHz.