Microscopic Study of Dendritic Avalanches in MgB2 Thin Films of Various Widths

To understand the features of a dendritic avalanche, we prepared the eight sets of MgB 2 thin films with rectangular shapes of 3 mm [length L ] × w [width], where w = 0.2 to 1.6 mm; the samples were in the long-length limit of w < L . Magneto-optical images (MOIs) of the first few dendrite patterns, which were distributed sparsely, were taken at applied fields below 35 mT at a temperature of 4 K. The key features of the present measurements are as follows: (1) the number of branches and the area of the dendrites increase with increasing w ; (2) the change in the flux density (CFD) from before to after the jump at the edge of the samples increased with increasing w ; (3) the applied magnetic flux density of the first few appearances of dendrites decreased with increasing of the w ; (4) the real initial penetration flux density at the sample's edge was about 20 mT, regardless of w ; (5) the critical current densities obtained from the MOI were almost the same for all samples, which implies that Bean's criti...