Vortex creep crossover in YBCO/PrBCO superlattices during standard magnetizationrelaxation measurements

Abstract

We investigated the relaxation of the irreversible magnetization in a series of 200 nm thickYBa2Cu3O7/PrBa2Cu3O7 [(YBCO)n/(PrBCO)m] superlattices, wherethe thickness m of the nonsuperconducting PrBCO layer (measured in unit cells) was kept tom = 4 (sufficient to decouple the superconducting YBCO layers), whereas the thicknessn of the YBCO layer was varied between 2 and 20 unit cells. The analysis of standardzero-field-cooling dc magnetization relaxation data obtained in the low temperatureT region with the appliedmagnetic field H oriented along the c axis reveals the occurrence of a crossover elastic (collective) vortex creep at lowT–plastic vortexcreep 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 decreaseslinearly with increasing ln(H) for a fixed n. This crossover represents an alternative to the elastic vortex glass behaviorreported for superlattices, as well as to ‘quantum vortex creep’ at unexpectedly highT inferred for thin films. We also discuss the absence of an increase of the magneticallydetermined critical current density with decreasing YBCO thickness in our superlattices,which apparently contradicts the collective pinning theories.