Suppression of geometrical barrier in Bi₂Sr₂CaCu ₂O_8+δ crystals by Josephson vortex stacks

Differential magneto-optics are used to study the effect of a dc in-plane magnetic field on hysteretic behavior due to geometrical barriers in Bi ₂Sr₂CaCu₂O_8+δ crystals. In the absence of an in-plane field a vortex dome is visualized in the sample center surrounded by barrier-dominated flux-free regions. With an in-plane field, stacks of Josephson vortices form vortex chains, which are surprisingly found to protrude out of the dome and into the vortex-free regions. The chains are imaged to extend up to the sample edges, thus providing easy channels for vortex entry and for drain of the dome through geometrical barrier, suppressing the magnetic hysteresis. Reduction of the vortex energy due to crossing with Josephson vortices is evaluated to be about two orders of magnitude too small to account for the formation of the protruding chains. We present a model and numerical calculations that qualitatively describe the observed phenomena by taking into account the demagnetization effects in which flux expulsion from the pristine regions results in vortex focusing and in the chain protrusion. Comparative measurements on a sample with narrow etched grooves provide further support to the proposed model.