77-K SQUIDS OPERATING IN THE EARTH MAGNETIC-FIELD

Weak magnetic fields like the Earth's field (similar to 50 mu T) can a dversely affect the operation of 77 K thin-film high-T-c SQUIDs throug h flux penetration into either the junction or the body of the SQUID. Josephson vortex penetration into the junction causes suppression of t he critical current, increased white noise and possible cessation of n ormal SQUID operation. We have used various techniques to overcome thi s problem, including development of narrow (similar to 1 mu m wide) st ep-edge junctions with high critical current density. The adverse effe cts of Abrikosov vortex penetration into the SQUID include magnetic hy steresis and increased low-frequency noise. Film quality (J(c)) is fou nd to be important in minimising these effects.