Unusual propagation of magnetic avalanches in gold covered MgB 2

Abstract

The penetration of magnetic flux lines into a type-II superconductor above the lower critical field can happen in a form of chaotic flux avalanches. Especially, in magnesium diboride films at low temperatures this phenomenon is observed. It has been shown that a thermomagnetic instability arising from local heating due to flux line motion can be considered as the origin of the effect. Studying these avalanches is realized by means of the magnetooptical Faraday effect, which allows a direct imaging of the typically dendritic magnetic structures. We show that the propagation direction of avalanches can directly be influenced by evaporating thin metallic patterns on top of a superconducting film leading to changes of the propagation velocity, the flux density distribution and the local heat transport. As a consequence a bending of the avalanche depending on the geometric shape of the metallic pattern and the incident angle of the avalanche occurs.