AC Defluxing of SQUIDs

Magnetic flux trapping is a very serious problem in SQUID-based instrumentation and superconducting electronics. A critical consideration for avoiding flux trapping is a prevention of SQUID sensors or circuits from being exposed to magnetic fields. Unfortunately, this measure cannot be applied, if sensors must be exposed to a strong magnetic field or transients. For example, unavoidable exposures take place in ultralow field magnetic resonance imaging and superparamagnetic relaxation measurements using unshielded thin-film planar SQUID sensors. SQUID sensors stop working after being exposed to magnetic fields of only a few mT in strength. Earlier, we reported successful results of defluxing of planar LTS SQUID-gradiometers using strong decaying ac magnetic pulses. In this paper, we present further experimental results and discuss a possible mechanism to explain the observed defluxing effects. The new ac defluxing technique is much faster and dissipates much less energy than a conventional thermal cycling.