Local threshold field for dendritic instability in superconducting MgB₂ films

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 10⁷ A/cm² . The dendritic instability in thin films is believed to be of thermomagnetic origin, but the existence of a local threshold field and its small value are features that distinctly contrast with the thermomagnetic instability (flux jumps) in bulk superconductors.