77 K SQUIDs operating in the Earth's magnetic field

Weak magnetic fields like the Earth's field (/spl sim/50 /spl mu/T) can adversely affect the operation of 77 K thin-film high-T/sub c/ SQUIDs through flux penetration into either the junction or the body of the SQUID. Josephson vortex penetration into the junction causes suppression of the critical current, increased white noise and possible cessation of normal SQUID operation. We have used various techniques to overcome this problem, including development of narrow (/spl sim/1 /spl mu/m wide) step-edge junctions with high critical current density. The adverse effects of Abrikosov vortex penetration into the SQUID include magnetic hysteresis and increased low-frequency noise. Film quality (J/sub c/) is found to be important In minimising these effects.