Pinning-to-barrier crossover in YBa₂Cu₃O_7-δ single crystals

Abstract

The behaviour of magnetic flux in high purity untwinned YBa2Cu3O7-δ (YBCO) single crystals is visualized by means of the magneto-optical technique. It is observed that after zero field-cooling at high temperatures near Tc, flux penetrates directly to the sample center over a flux free edge area, in contrast to the usual Beanlike flux penetration from the edges. This fact clearly shows that volume pinning becomes negligible compared with the edge barrier. The role of the recently rediscovered geometrical barrier in the crystal magnetization is discussed.