The avalanche process in gold covered MgB2 films

Abstract

Below a certain threshold temperature, the critical state of superconducting thin films can become unstable, which results in large magnetic flux jumps, called flux avalanches. The phenomenon of magnetic flux avalanches is strongly connected to the thermal and electrical conductivity in the material. Enhancing both by metallic cover layers on the superconducting film can suppress the magnetic avalanches. It was found that it is feasible to subdivide the avalanche process into a formation and a propagation step. In this work we want to address the question regarding the stage at which the metallic cover layer influences the avalanche mechanism. The investigations are carried out on thin MgB2 films with an inhomogeneous current density distribution, where both the formation and propagation of magnetic flux avalanches are highly supported. Evaporating a gold cover layer on top of the inhomogeneous MgB2 films leads to a suppression of the instabilities in this case. Magnetization measurements and magneto-optical imaging are used to observe the instabilities in pure MgB2 films and MgB2 films covered with gold layers. We investigate how the two steps of the avalanche process, nucleation and propagation, are influenced by the additional gold layer. We find a particular influence on the initial phase of the avalanche formation.