The formation and propagation of flux avalanches in tailored MgB2 films

The applications of superconducting magnesium diboride are substantially limited by the presence of magnetic flux avalanches at low temperatures. Here, quickly moving magnetic vortices create large amounts of heat and magnetic noise. Such avalanches can be suppressed by evaporating metal layers to the surface of the superconductor, which acts both as a heat sink and as an electromagnetic drag by induced eddy currents. We show that it is necessary to distinguish between the mechanisms that are responsible for the formation and the propagation of avalanches. A high critical current favors avalanche formation but avalanche propagation is suppressed. The diverse consequences for creation and propagation explain the preference of avalanches for inhomogeneous superconductors. After the discovery of the metallic superconductor MgB2 in 2001 [1], it was soon realized that at temperatures below T = 10K a chaotic behavior of flux penetration can occur that had already been found in conventional superconductors [2] and YBa2Cu3O7−δ [3]. The magnetic flux penetrates into the superconductor by forming vortex avalanches on a wide range of lengthscales [4]. Since vortex velocities in avalanches are extremely high [5], this dissipative flux motion leads to a high level of disadvantageous magnetic noise. Magnesium diboride films are, in principle, attractive for application in making magnetic sensor devices. The reasons for this are the high superconducting transition temperature and the advantages of processing a metallic material. Additionally, the coherence length is large when compared to the size of grain 3 Author to whom any correspondence should be addressed. New Journal of Physics 12 (2010) 093043 1367-2630/10/093043+07$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft