Abstract
We performed magnetization relaxation experiments for optimally dopedYBa2Cu3O7−δ (YBCO) films with the external magnetic fieldH = 20–30 kOe oriented along thec axis and at relativelylow temperatures T = 10–32 K, where a crossover elastic (collective) vortex creep–plastic creepis expected. The creep crossover is generated in this case by theT dependent macroscopic currents induced in the specimen, and manifestsitself through the occurrence of a maximum in the current densityJ (orT) variation of the normalized vortex-creep activation energyU*, at which the creep exponent changes sign. With the same magnetizationrelaxation data the widely used Maley technique to construct theJ dependence of the vortex-creep activation energyU suppliesan U(J) variation close to the logarithmic one. It appears that the Maley technique maylead to unreliable results, due to the creep crossover characteristic to standardzero-field-cooling magnetization measurements and the reduced overall relaxation (in aconvenient relaxation time window) for highly disordered specimens. It is suggestedthat the results obtained with this scheme should be checked by determining theU*(T,J) variation.
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