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

概要

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To understand the features of a dendritic avalanche, we prepared the eight sets of MgB2 thin films with rectangular shapes of 3 mm [length $L$] $\times$ $w$ [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 critical model works quite well.
2010-04-15