Meissner Holes In Superconductors

Features of the magnetic induction patterns at the boundary between oppositely magnetized parts in YBCO crystals are studied using an advanced magneto-optical technique. Vortices of opposite polarities are revealed to concentrate at the remagnetization front, equivalent to a local increase of the current along it. A model is suggested considering the presence at the front of a flux-free cylinder formed by closed vortex loops. Inside the cylinder the loops collapse because their line tension exceeds the pinning force. Currents along the cylinder surface and in its nearest environment can be large and determine observed flux patterns. A role of the Meissner holes in the appearance of macroturbulent flux instabilities is discussed.