Magnetic flux penetration and trapped flux patterns formed in a MgB2 film during increases and subsequent decreases of the applied external magnetic field were studied using magneto-optical imaging. The film was grown by electron-beam evaporation and exhibited a granular structure. A non-homogeneous fan-like shaped penetration, already pointed out in the literature, was observed. In order to investigate the origin of this kind of penetration, a quantitative approach was used. The induction magnetic field map and the corresponding contour map of a framed zone exhibited many isolated loops originating from the granular nature of the sample. Within such loops, an estimation of the local current density was made by inverting the Biot–Savart law. All of the results pointed towards the conclusion that the fan-like shaped patterns mirrored the percolation of dissipation paths. Along these paths, some interfaces - distributed in an hierarchical order - played the role of pinning barriers or easy-flow channels.

Quantitative Magneto-Optical Analysis of Macroscopic Supercurrent Flow in MgB2. L.Gozzelino, F.Laviano, D.Botta, A.Chiodoni, R.Gerbaldo, G.Ghigo, E.Monticone, C.Portesi, E.Mezzetti: Superconductor Science and Technology, 2003, 16, 199-204