Bending of magnetic avalanches in MgB2 thin films

Abstract

The penetration of magnetic flux into a superconductor above the lower critical field does not necessarily happen in a regular way. It is found that under particular conditions, a chaotic penetration in form of magnetic avalanches occurs. These avalanches are closely related to the so-called thermomagnetic instability, which identifies local heating due to flux line movement as the origin of the effect. In case of MgB2 thin films, these avalanches are found only below T=10K and are suppressed by a covering metallic layer with high thermal conductivity and sufficient thickness. These avalanches are observed in the case of partly gold covered MgB2 films by the magneto-optical Faraday effect. The investigation of avalanches propagating into a gold-covered region revealed a change of the propagation direction depending on the incident angle of these avalanches.

Dynamics and morphology of dendritic flux avalanches in superconducting films

“…The trees are seen to have a morphology that strongly resembles the flux structures observed experimentally in many superconducting films. [3][4][5][6][7][8][9] The simulations also reproduce the experimental finding that, once a flux tree is formed, the entire dendritic structure remains unchanged as H a continues to increase. The Supplementary Material 23 includes a video clip of the dynamical process and shows a striking resemblance to the magneto-optical observations of the phenomenon.…”

“…When repeating identical experiments, one finds that the patterns are never the same, although qualitative features of the morphology, such as the degree of branching and overall size of the structure, show systematic dependences on, e.g., temperature. Using magneto-optical imaging, flux avalanches with these characteristics have been observed in films of Nb, 3 YBa 2 Cu 3 O 7−x , 4 MgB 2 , 5,6 Nb 3 Sn, 7 YNi 2 B 2 C, 8 and NbN. 9 Investigations of onset conditions for the avalanche activity have identified material-dependent threshold values in temperature, 5 applied magnetic field, 9,10 and transport current, 11 as well as in sample size.…”

Flux penetration in a superconducting film partially capped with a conducting layer

“…It has been shown recently that magnetic flux avalanches triggered in a superconducting film are diverted from their initial trajectory when they encounter a conductive layer deposited on top of the superconductor, but electrically insulated from it [1][2][3][4]. This phenomenon arises from the electromagnetic braking of the flux propagation, caused by the eddy currents induced in the conductive layer [5][6][7][8].…”

Ray optics behavior of flux avalanche propagation in superconducting films

“…It is well known that a metal layer causes inductive braking of the avalanche propagation [22,24], and hence such boundaries should display refraction of dendrites provided they have a traveling wave nature. Indeed, previous work by Albrecht et al [25] showed that the propagation of flux dendrites crossing borders between regions of different material properties depends on the incidence angle of the avalanche. The present Rapid Communication gives direct experimental evidence that the electromagnetic modes excited in the dendritic avalanches in NbN cause systematic refraction at boundaries between different media.…”