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 superc onducting quantum interference device (SQUID) are studied. First, the effec t of thermal noise at T = 77 K on the magnetic flux entry into in the picku p coil through the flux dam is discussed. It is shown that the thermal nois e causes the flux entry even when the circulating current in the pickup coi l is less than the critical current of the flux dam. The behavior of this t hermally activated flux entry is analyzed using the so-called potential-bar rier model. It is shown that the flux entry into the coil proceeds logarith mically with time, indicating the necessity of a long waiting time in the u se 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 muA in the magnetic field under o peration. Next, in order to satisfy this requirement, the magnetic field de pendence of the critical current of the flux dam is studied when the flux d am is made of a wide bicrystal junction. It is shown that the magnetic fiel d dependence of the wide junction can be explained reasonably well if we ta ke into account the so-called overlap-type-junction model and the flux-focu sing effect of the bicrystal junction. The obtained results will be useful in the design of the flux dam.