Abstract
Magnetic flux penetration and trapped flux patterns formed into a MgB2 film during the increase and the subsequent decrease of the applied external magnetic field were studied using magneto-optical imaging. The film was grown by electron-beam evaporation and exhibits a granular structure. The non-homogeneous fan-like shaped penetration, already pointed out in the literature, was observed. To investigate the origin of this kind of penetration, a quantitative approach was chosen. The induction magnetic field map and the corresponding contour map of a framed zone show many isolated loops originating from the granular nature of the sample. Inside such loops, an estimation of the local current density was made through the inversion of the Biot–Savart law. All the results point towards the conclusions that the fan-like shaped patterns do mirror the percolation of dissipation paths. Along these paths, some interfaces, distributed in a hierarchical order, play the role of either pinning barriers or easy-flow channels.
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