Low-frequency noise in field-cooled, directly coupled magnetometers

We have studied the low-frequency noise properties of high-T-c directly coupled magnetometers involving de Superconducting Quantum Interference Devices (SQUIDs) connected to four different types of-pickup loop. Two of the pickup loops have a solid-washer design, while the other two comprise structures with narrow (4 mu m) linewidths. The devices were fabricated from YBCO thin films laser deposited on bicrystal substrates. The noise properties of the magnetometers cooled and maintained in static magnetic fields were measured at 77 K. The pickup loop contributes to the 1/f noise of held-cooled devices solely through vortex motion at tapered outer edges; this noise was eliminated by repatterning the edges to make them sharp. We demonstrate that solid-him washers can be replaced with narrow-linewidth structures without any loss in effective area.