Ultrasonic studies at 85 kHz on polycrystalline YBa2Cu 3O7-δ (Y123) below Tc, with an applied magnetic field, have yielded details of the interaction of the magnetic field with the superconductor. When the magnetic field is applied to the sample there is an immediate increase in acoustic damping, which then decreases logarithmically over an extended period of time to a trapped flux value. When the field is turned off a similar process is observed. The results are interpreted in terms of the Anderson-Kim model for thermally assisted flux creep, and they allow the determination of the apparent magnetic flux pinning energy. The results indicate two distinct pinning energies, one for the normal long term flux creep observed over minutes, and another lower value, which predominates during the first 5 s after a magnetic field is applied.
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