Abstract
Abstract Magnetic relaxations have been measured for up to 20 h for a flux-grown YBa 2 Cu 3 O 7- δ single crystal, with the applied field (up to 7 T) parallel to the c -axis. A ln( t ) relaxation of the magnetisation, found from the Anderson-Kim flux creep model, only describes our data for low temperatures and low fields. However, we have found that a power-law relaxation ( M )( t )∝ t -α ), which points to a logarithmic current-dependence of the effective activation energy [ U ( J )= U 0 ln ( J 0 / J )], fits the data well for a wide range of temperatures and applied fields. Values for U 0 ( B, T ) have been obtained from the power-law relation, and compared with the values derived from the logarithmic relaxation rates. Discrepancies between them are discussed. Possible connections between our experimental data and Feigel'man's collective flux creep theory are also discussed.
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