Production of HTS-SQUID Magnetometer With Ramp-Edge Junctions Exhibiting Lowered Noise in AC Biasing Mode

We fabricated HTS-SQUID magnetometers with ramp-edge-type Josephson junctions and measured their noise properties using dc-ac biasing mode. Some exhibited lowered low-frequency noise by ac biasing, and others did not. We compared various parameters to find correlative ones with effectiveness of ac biasing. We found that the junction width and the critical current density of the counter electrode layer were weakly correlated with the low-frequency noise. The narrower width and the higher density seemed to result in lowered noise by applying ac bias current. It is likely that disorder in the counter electrode and/or disorder in the underlying barrier region give rise to fluctuation of inhomogeneous current flow in junctions or flux trapping, leading to low-frequency noise which cannot be reduced by ac biasing. Generation probability of such inhomogeneous portion seems to be reduced by making narrow junctions. The high quality electrode seems to contain lesser disorder causing flux trapping.