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 Tc 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∼1 μT. Devices with flux dams can show no deterioration of the noise characteristics for B as large as 34 μT. The flux dams are part of the pickup loop and limit the circulating current Icirc. If Icirc is kept sufficiently low no vortices are forced into the SQUID loop and the noise performance of the magnetometer remains good. © 1997 American Institute of Physics.