Low-frequency flux noise in YBCO dc SQUIDs cooled in static magnetic fields

The low-frequency flux noise in bicrystal and step-edge YBa2Cu3Ox de SQUIDs has been investigated. The width, w, of the superconducting strips forming the SQUID frame was varied from 4 to 42 mu m. The SQUIDs were cooled in static magnetic fields up to 150 mu T. Two types of low-frequency noise dominated, namely 1/f-like noise and random telegraph noise giving a Lorentzian frequency spectrum. The 1/f noise performance of the w = 4, 6 and 7 mu m SQUIDs was almost identical, while the SQUIDs with w = 22 and 42 mu m showed an order of magnitude higher noise level. Our analysis of the data suggests an exponential increase of the 1/f noise versus the cooling held, exhibiting a characteristic magnetic field around 40 mu T.