REDUCTION OF 1 F NOISE IN HIGH-T-C DC SUPERCONDUCTING QUANTUM INTERFERENCE DEVICES COOLED IN AN AMBIENT MAGNETIC-FIELD/

The spectral density S-Phi(f) of the low-frequency 1/f noise of high t ransition temperature dc superconducting quantum interference devices (SQUIDs) with narrow linewidths was independent of B-0, the magnetic f ield in which they were cooled, up to a threshold value, about 33 mu T in the best case. Above this threshold, which is associated with the entry of flux vortices into the film, the noise increased rapidly. By contrast, for large square washer SQUIDs, S-Phi(f) scaled linearly wit h B-0. Estimates indicate that the 1/f flux noise produced by the pick up loop of a directly coupled magnetometer is negligible. (C) 1996 Ame rican Institute of Physics.