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RESEARCH PRODUCT

Vortex creep crossover in YBCO/PrBCO superlattices during standard magnetization relaxation measurements

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We investigated the relaxation of the irreversible magnetization in a series of 200?nm thick YBa2Cu3O7/PrBa2Cu3O7 [(YBCO)n/(PrBCO)m] superlattices, where the thickness m of the nonsuperconducting PrBCO layer (measured in unit cells) was kept to m = 4 (sufficient to decouple the superconducting YBCO layers), whereas the thickness n of the YBCO layer was varied between 2 and 20 unit cells. The analysis of standard zero-field-cooling dc magnetization relaxation data obtained in the low temperature T region with the applied magnetic field H oriented along the c axis reveals the occurrence of a crossover elastic (collective) vortex creep at low T?plastic vortex creep at high T, generated by the T dependent macroscopic currents induced in the sample during measurements. For thin superlattices (n < 20) the creep crossover temperature , and Tcr decreases linearly with increasing ln(H) for a fixed n. This crossover represents an alternative to the elastic vortex glass behavior reported for superlattices, as well as to 'quantum vortex creep' at unexpectedly high T inferred for thin films. We also discuss the absence of an increase of the magnetically determined critical current density with decreasing YBCO thickness in our superlattices, which apparently contradicts the collective pinning theories.