Local threshold field for dendritic instability in superconductingMgB2films

Abstract

Using magneto-optical imaging the phenomenon of dendritic flux penetration in superconducting films was studied. Flux dendrites were abruptly formed in a 300 nm thick film of MgB2 by applying a perpendicular magnetic field. Detailed measurements of flux density distributions show that there exists a local threshold field controlling the nucleation and termination of the dendritic growth. At 4 K the local threshold field is close to 12 mT in this sample, where the critical current density is 10 7 A/cm 2 . The dendritic instability in thin films is believed to be of thermo-magnetic origin, but the existence of a local threshold field, and its small value are features that distinctly contrast the thermo-magnetic instability (flux jumps) in bulk superconductors.

From the second magnetization peak to peak effect. A study of superconducting properties in Nb films and MgB2bulk samples

“…We clearly see that the flux jumps are still present. Very recently it was discovered that the compound MgB 2 is a superconductor with relatively high critical temperature T c = 39 K. As a result of the renewed interest, many groups have done reliable studies on TMI in MgB 2 samples, either in thin film or in bulk form, by performing thorough magnetic measurements, or by using more direct techniques such as magneto-optical imaging [33,34,35,36]. The observed behavior in MgB 2 films is very similar to the behavior that we observed in our Nb films.…”

Flux dendrites of opposite polarity in superconductingMgB2rings observed with magneto-optical imaging

“…• they occur above a minimum applied magnetic field B th [104,100] • the stability depends on sample size [102] The references in this list include reports on different materials, indicating that these stability thresholds are generic properties of the avalanches. This claim is further supported by reported magnetisation measurements if noise in the M − H curves are interpreted as dendritic avalanches, for example in plain Pb films [101].…”

Dynamics and stability of vortex-antivortex fronts in type-II superconductors

“…An example is the nucleation of dendritelike patterns in Nb and MgB 2 films [3][4][5]. These complex structures consist of alternating low and high vortex density regions and are found in a certain temperature window.…”

“…This results from a thermomagnetic coupling [4,5,14,15] where a higher temperature leads to a higher mobility, enhanced flux flow, and hence a larger heat generation. However, the cause of the instability at the boundary between fluxes of opposite sign is still being debated.…”