Anomalous flux propagation through interface separating regions of different critical currents in Bi₂Sr₂CaCu ₂O_8+δ crystals

Magneto-optical imaging was employed to study magnetic flux propagation through an interface separating between heavy-ion irradiated and non-irradiated parts of the same Bi₂Sr₂CaCu₂O _8+δ crystal. As the flux front hits the interface the local induction increases abruptly at a rate considerably larger than that of the external field. This induction jump is accompanied by the appearance of a finger-like flux pattern at the interface. At low temperatures or high field ramp-rates, the induction at the interface increases gradually, finger patterns do not form and the flux front crosses the interface smoothly. We explain the flux behavior at the interface on the basis of the magnetic diffusion equation, and propose that the induction jump triggers the formation of finger patterns.