Mechanisms of noise increase in direct-coupled high T-c superconducting quantum interference device magnetometers exposed to magnetic fields

We have investigated the behavior of high-critical-temperature (high T-c) d irect-coupled superconducting quantum interference device (SQUID) magnetome ters in static and fluctuating magnetic fields. The magnetometers consist o f narrow-linewidth superconducting films to prevent flux trap during field cooling. Moreover, they have no superconducting films crossing the bicrysta l lines of the substrates (except at the Josephson junctions); i.e., they h ave no flux dams. When one of these magnetometers was cooled in a static ma gnetic field B-cool, the low-frequency noise when B-cool< 100 <mu>T was as low as that under zero-field cooling, but above 100 muT the noise increased substantially. On the other hand, when a field B-ext of less than 4 muT wa s applied after zero-field cooling, the low-frequency noise increased in pr oportion to B-ext. It returned to its original value reversibly when B-ext was turned off. However, when B-ext was greater than or equal to 4 muT, the output of the flux-locked-loop started to drift with time and the low-freq uency noise increased further. This additional noise increase remained afte r turning off B-ext. These results suggested that there are two modes of in crease for the low-frequency noise induced by flux penetration due to the s hielding current: a "reversible" mode and an "irreversible" mode. We found that the low-frequency noises of the two modes were additive with respect t o their power, suggesting that the two noises derived from independent sour ces at different sites on the magnetometer. We also found that the reversib le-mode noise could be reduced by improving the profile of the film edge. ( C) 2001 American Institute of Physics.