Suppression of thermally activated flux entry through a flux dam in high T _c superconducting quantum interference device magnetometer

Thermally activated magnetic-flux entry into a pickup coil through a flux dam in high-T _c superconducting quantum interference device (SQUID) magnetometer is studied. Since the thermally activated flux entry strongly depends on the circulating current in the pickup coil, the behavior of the circulating current is studied with numerical simulation when an external field is applied. It is shown that the circulating current becomes near the critical current of the flux dam after the large external field is applied. In this case, thermally activated flux entry becomes large, and degrades the SQUID performance. In order to suppress the thermal activation, the circulating current must be much below the critical current. For this purpose, two methods are proposed. One is to use a compensation field in the case of the flux dam, and the other is to use a switch instead of the flux dam. Numerical simulation shows that we can rapidly decrease the circulating current much below the critical current, and hence, can suppress the thermally activated flux entry by using these methods. The usefulness of these methods is confirmed experimentally.