Low-frequency noise and linearity of a YBa2Cu3O7 dc superconducting quantum interference device magnetometer in static magnetic fields

We have investigated the low-frequency noise and effective flux sensing area of a field cooled dc superconducting quantum interference device magnetometer from YBa2Cu3O7 in magnetic fields up to 200 mu T, both before and after it was patterned with holes to reduce the maximum structural width of the pickup loop. We find that even in low fields the noise of the unpatterned magnetometer steadily increases with the applied field. However, after the patterning for the holes, the noise remains at the zero field level up to a threshold field of 35 mu T and is always less than in the unpatterned case. This threshold field is also found in field dependent measurements of the total harmonic distortion. The effective area of the magnetometer depends on the cooling field, and the dependence is different in the unpatterned and the patterned case. (C) 1999 American Institute of Physics. [S0021-8979(99)05523-1].