A four-junction switch for controlling the opening and closing of a pickup coil in high-T_c superconducting quantum interference device magnetometer

We study a method to prevent thermally activated flux entry through a flux dam in a high-T_c superconducting quantum interference device (SQUID) magnetometer. For this purpose, the circulating current flowing in a pickup loop of the magnetometer must be much below the critical current of the flux dam. In order to forcibly reduce the circulating current due to a residual magnetic field, we propose a switch consisting of four bicrystal junctions, instead of the conventional flux dam. Using the switch, we can electrically control the opening and closing of the pickup loop, and therefore, can reduce the circulating current to zero. The usefulness of the switch is demonstrated.