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Magneto-optical imaging was used to study a zero-field-cooled MgB2 film at 9.6K where in a slowly increasing field the flux penetrates by abrupt formation of large dendritic structures. Simultaneously, a gradual flux penetration takes place, eventually covering the dendrites, and a detailed analysis of this process is reported. We find an anomalously high gradient of the flux density across a dendrite branch, and a peak value that decreases as the applied field goes up. This unexpected behaviour is reproduced by flux creep simulations based on the non-local field-current relation in the perpendicular geometry. The simulations also provide indirect evidence that flux dendrites are formed at an elevated local temperature, consistent with a thermo-magnetic mechanism of the instability, 5 pages, 5 figures, submitted to Supercond. Sci. Technol

Magneto-optical imaging was used to study flux penetration into MgB2 films in a slowly increasing perpendicular applied field. A variety of flux jumps and avalanches have been observed at temperatures below 10 K. At small fields, jumps with typical size of 20 μm and regular shape occur at random locations along the flux front. Above some threshold field of 2–10 mT, big dendritic jumps with dimensions comparable to the sample size (mm scale) take place. The jumps are developing extremely fast, result in highly-branching irreproducible flux patterns, and effectively suppress the apparent critical current. Both types of jumps are believed to result from thermo-magnetic instability.

Using magneto-optical imaging the phenomenon of dendritic flux penetration in superconducting films was studied. Flux dendrites were abruptly formed in a 300 nm thick film of MgB2 by applying a perpendicular magnetic field. Detailed measurements of flux density distributions show that there exists a local threshold field controlling the nucleation and termination of the dendritic growth. At 4 K the local threshold field is close to 12 mT in this sample, where the critical current density is 10^7 A/cm^2. The dendritic instability in thin films is believed to be of thermo-magnetic origin, but the existence of a local threshold field, and its small value are features that distinctly contrast the thermo-magnetic instability (flux jumps) in bulk superconductors., 6 pages, 6 figures, submitted to Phys. Rev. B