Observation of magnetic-field penetration via dendritic growth in superconducting niobium films.

Abstract

Using a high-resolution magneto-optical technique, we have studied the penetration and expulsion of magnetic flux in superconducting niobium thin films. We have found that over a wide region of the phase diagram, the picture of uniform flux fronts as described by a critical-state model breaks down and that the penetration of the field takes place through the growth of magnetic dendrites. Magnetic decoration studies with single vortex resolution show that the dendrites have a complex structure and consist of walls of both high and low magnetic field and are filled with a state of matter which structurally appears to be a vortex fluid. Some of the dendrites are seen to grow by nucleating regions with hundreds of antivortices in the film.