Flux creep and irreversibility line in YBCO ceramics

The intergranular losses in ac susceptibility of two-dimensional high-Tc YBa2Cu3Ox ceramic samples are studied experimentally as a function of the magnetic field (H0 = 0–40 Oe, h = 0.005–1 Oe), the external magnetic field being H = H0+h·cos ωt) and frequency (f = 0.275–20 kHz) The sample thickness D varied from 1.4 to 0.42 mm. It is shown that the experimental results can be described by the “giant” flux creep model in the limit of a sufficiently strong pinning. For this purpose, it is necessary to take into account the critical line on which the measured critical density is equal to zero. This line has a dynamic character and is probably the irreversibility line.

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