Magnetic field-induced noise in directly coupled high Tc 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 $\text{B∼1}$ 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 $I_{\mathrm{circ}}.$ If $I_{\mathrm{circ}}$ is kept sufficiently low no vortices are forced into the SQUID loop and the noise performance of the magnetometer remains good.