Aspects of magnetic-flux dynamics in different types of samples of the high-temperature superconductor YBa2Cu3Ox have been investigated in magnetic fields below 1 Oe and at 77 K. The experiments were carried out in an arrangement including a field coil, a flat sample perpendicular to the field, and a radio frequency-superconducting quantum interference device (RF-SQUID) along a common axis. For epitaxial thin films the Meissner effect was established. For a ceramic sample it was found that field changes of order 10(-2) Oe affect the flux distribution inside the sample. The observations can explain why RF-SQUID' s made from this ceramic operate in the nonhysteretic mode. The central object was an epitaxial film with a large density of defects. In this film the dynamics of the mixed state show features expected in ''spin-glass'' models.
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