Effect of a static magnetic field on a slotted high-T/sub c/ SQUID magnetometer without a flux dam

Abstract

A direct-coupling-type high-critical-temperature (high-T/sub c/) SQUID magnetometer with two pickup loops per SQUID on a bicrystal substrate has been fabricated. To avoid flux trapping, the superconducting films of the pickup loops have slits with a separation of 5 /spl mu/m. To suppress flux entry into the pickup loops, there are no weak links on the pickup loops, i.e., no flux dams. In field cooling, noise did not substantially increase in the low frequency region unless the field reached around 100 /spl mu/T. Regardless of whether the magnetometer was previously zero-field cooled or field-cooled, the flux did not enter the pickup loop until the changing field exceeded about 10 /spl mu/T. The magnetometer cooled under geomagnetic- and environmental-field noise could work in an unshielded environment. A gradiometer was made by connecting the feedback coils of two magnetometers in series. The environmental field noise was reduced by the gradiometer by about 35 dB.