Vortex avalanche in gold-coated and finite-sized MgB$_{2}$ thin films.

The vortex avalanches in carbon-free MgB$_{2}$ thin films were studied using magneto-optical imaging. The main focus of this study was the effect of a gold coating and the sample size on the occurrence of the dendritic flux avalanches. For this purpose, we prepared two different sets of MgB$_{2}$ films. In the first, the films had gold coatings of various thicknesses, and in the second set bare MgB$_{2}$ films were patterned into long strips of different widths. The dendritic flux avalanches essentially disappeared for gold coatings thicker than 2.5 $\mu $m. Also for strip widths less than 0.3¥ª3 mm$^{2}$ the formation of flux dendrites are suppressed. The results are consistent with a thermomagnetic origin of the dendritic instability.