FLUX PINNING AND IRREVERSIBILITY IN YBA2CU3O7 SUPERCONDUCTING CRYSTAL

Extensive ac-magnetic-permeability studies have been carried out on the irreversible behavior and flux dynamics in superconducting YBa2Cu3O7 crystals in external magnetic fields up to 7.5 T. From the analysis the irreversibility line B*(T*) is found to scale as (1-T*/Tc)n, where n 1.5 for fields higher than 1.5 T, and n 1 for lower fields. The exponents are sensitive to the choice of Tc. The frequency dependence of the irreversibility temperature is carefully studied in the low-frequency region from 10 to 105 Hz and is found to be logarithmic in the whole frequency range. The small logarithmic slope increases with increasing external field. The analysis is done within a flux-creep picture using a single-relaxation-time form for the complex permeability, and a vortex-glass model. The peak in the imaginary part of the permeability in different dc fields is successfully fitted to the predictions of the flux-creep model. A dynamical screening length is needed to account for the observed frequency dependence of the irreversibility temperature. © 1990 The American Physical Society.