Onset of flux-bundle migration into superconducting niobium strips

Penetration of magnetic flux into type-II superconducting Nb strips is obse rved by a flux jump readout technique. To investigate the effect of the geo metry, strips of different width are subjected to an increasing perpendicul ar magnetic field. We demonstrate that conventional geometrical barrier mod els cannot account for the onset of the flux migration into the strip volum e. The migration starts for a local magnetic held at the strip equator high er than H-c1, allowing the irreversible penetration of the mixed state on t he edges. The flux entry consists of flux bundles, each of them containing several hundreds of flux quanta. [S0163-1829(99)02134-7].