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

We have studied the low-frequency noise properties of high-T-c directly cou pled magnetometers involving de Superconducting Quantum Interference Device s (SQUIDs) connected to four different types of-pickup loop. Two of the pic kup loops have a solid-washer design, while the other two comprise structur es with narrow (4 mu m) linewidths. The devices were fabricated from YBCO t hin films laser deposited on bicrystal substrates. The noise properties of the magnetometers cooled and maintained in static magnetic fields were meas ured at 77 K. The pickup loop contributes to the 1/f noise of held-cooled d evices solely through vortex motion at tapered outer edges; this noise was eliminated by repatterning the edges to make them sharp. We demonstrate tha t solid-him washers can be replaced with narrow-linewidth structures withou t any loss in effective area.