Magnetic field-induced noise in directly coupled high T-c superconducting quantum interference device magnetometers

We have measured the noise of several directly coupled high T-c superconducting quantum interference device (SQUID) magnetometers as we change a static magnetic field B while the devices are superconducting. Devices without ''flux dams'' show an increase in noise at relatively low magnetic fields B similar to 1 mu T. Devices with flux dams can show no deterioration of the noise characteristics for B as large as 34 mu T. The flux darns are part of the pickup loop and limit the circulating current I-circ. If I-circ is kept sufficiently low no vortices are forced into the SQUID loop and the noise performance of the magnetometer remains good. (C) 1997 American Institute of Physics.