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 transition temperature dc superconducting quantum interference devices (SQUIDs) with narrow linewidths was independent of B-0, the magnetic field 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 with B-0. Estimates indicate that the 1/f flux noise produced by the pickup loop of a directly coupled magnetometer is negligible. (C) 1996 American Institute of Physics.