Effect of a static magnetic field on a slotted high-T_c SQUID magnetometer without a flux dam

A direct-coupling-type high-critical-temperature (high-T_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 μm. 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 low frequency region unless the field reached around 100 μT. 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 μT. The magnetometer cooled under the 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.