Evaluating the entry of vortices into a slotted high-Tc SQUID magnetometerwith a double-pickup-coil configuration

A direct-coupling-type high-Tc SQUID magnetometer with two pickup loops per SQUID on a substrate has been fabricated. To suppress the flux trapping an d vortex entry, the superconducting film of the whole device except a 5-mum -wide transmission-line-shaped SQUID is composed of narrow (5 mum) lines wi th slots and holes. The threshold field for the flux trapping during field cooling was evaluated to be around 130 muT, a much larger field than the ge omagnetic field. Moreover, applying a field after cooling reduced the criti cal current of the SQUID to zero at 6 muT. And vortex entry into the pickup loop occurred at a field exceeding about 10 muT.