Flux Noise and Flux Creep in YBCO Thin Films

摘要

We have used a dc SQUID to measure the low-frequency magnetic flux noise produced by thin-film rings of YBa sub 2 Cu sub 3 O/sub 7/minus/delta/ (YBCO) with various microstructures. Below the transition temperature T/sub 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 T/sub c/. Improvements in crystalline microstructure dramatically reduce the magnitude of the noise, which was lowest for a highly oriented 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. We conclude that SQUIDs and flux transformers of YBCO must be fabricated from highly oriented films to obtain low noise at low frequencies. 13 refs., 6 figs., 1 tab. (ERA citation 14:018970)