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Flux noise and flux creep in YBCO thin films

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7 Author(s)
Ferrari, M.J. ; Dept. of Phys., California Univ., Berkeley, CA, USA ; Johnson, Mark ; Wellstood, F.C. ; Clarke, John
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The authors used a DC SQUID (superconducting quantum interference device) to measure the low-frequency magnetic flux noise produced by thin-film rings of YBa2Cu3O7-δ (YBCO) with various microstructures. Below the transition temperature T c of the YBCO, the spectral density of the noise scales as 1/f (f is the frequency) from 1 Hz to 1 kHz. This noise generally increases with temperature and vanishes abruptly at Tc . Improvements in crystalline microstructure greatly reduce the magnitude of the noise, which was lowest for a highly orientated sample with its c-axis perpendicular to the substrate. Making a radial cut to interrupt current paths around the sample ring does not significantly affect the magnitude of the noise, demonstrating that the noise arises from a local mechanism such as the thermally activated hopping of flux bundles. Flux creep was observed in one sample cooled in a magnetic field of 1 mT, and the creep rate exhibited a sharp maximum near 80 K. It is concluded that SQUIDs and flux transformers of YBCO must be fabricated from highly orientated films to obtain low noise at low frequencies

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Magnetics, IEEE Transactions on  (Volume:25 ,  Issue: 2 )