Investigation of thermo-magnetic instability in superconducting NbN thin-films by automated real-time magneto-optical imaging

Abstract

If a magnetic field above a certain critical field (Bc1) is imposed on a type II superconductor, it will start to penetrate in the form of magnetic flux quanta, or vortices, resulting in a meta-stable non-equilibrium critical state. The critical state can collapse due to thermo-magnetic instability in the superconductor, resulting in a sudden large scale redistribution of vortices. This might cause the superconductor to heat up to above the critical temperature, at least locally, where sudden resistivity can have catastrophic consequences.

In this thesis we have studied thermo-magnetic instability in superconductor thin-films, in a wide range of fields and temperatures, by automating the experimental setup and developing algorithms for graphical recognition of flux avalanches, and determination of threshold fields for the formation of these avalanches.

When constructing a phase diagram of the threshold fields as a function of temperature, we found a hysteretic behaviour of the upper thresholds for increasing and decreasing magnetic fields. An explanation is given based on a theoretical model of the correlation between the threshold field and the critical current density. We also found that sufficiently large primary avalanches have the ability to move independently of the shielding current inside of the flux front. At decreasing fields we observed secondary avalanches starting at the center d-line. Heat from a primary avalanche reduce the pinning force, which in some locations will let the Lorentz force overcome the pinning and move vortices. This create a local instability that can sometimes trigger a secondary avalanche.