Properties of a flux dam inserted in the pickup coil of a high-T_c superconducting quantum interference device magnetometer

The properties of a flux dam inserted in a pickup coil of a high-T_c superconducting quantum interference device (SQUID) are studied. First, the effect of thermal noise at T = 77 K on the magnetic flux entry into in the pickup coil through the flux dam is discussed. It is shown that the thermal noise causes the flux entry even when the circulating current in the pickup coil is less than the critical current of the flux dam. The behavior of this thermally activated flux entry is analyzed using the so-called potential-barrier model. It is shown that the flux entry into the coil proceeds logarithmically with time, indicating the necessity of a long waiting time in the use of the flux dam. It is also shown that the critical current of the flux dam must be large enough to obtain a quasistable state, i.e., a state where no thermally activated flux entry occurs during the measurement time. The critical current should be larger than 50 μA in the magnetic field under operation. Next, in order to satisfy this requirement, the magnetic field dependence of the critical current of the flux dam is studied when the flux dam is made of a wide bicrystal junction. It is shown that the magnetic field dependence of the wide junction can be explained reasonably well if we take into account the so-called overlap-type-junction model and the flux-focusing effect of the bicrystal junction. The obtained results will be useful in the design of the flux dam.