We report a detailed comparison of experimental data and theoretical predictions for the dendritic flux instability, believed to be a generic behavior of type-II superconducting films. It is shown that a thermomagnetic model published very recently [Phys. Rev. B 73, 014512 (2006)10.1103/PhysRevB.73.014512] gives an excellent quantitative description of key features like the stability onset (first dendrite appearance) magnetic field, and how the onset field depends on both temperature and sample size. The measurements were made using magneto-optical imaging on a series of different strip-shaped samples of MgB2. Excellent agreement is also obtained by reanalyzing data previously published for Nb.

The dendritic patterns of magnetic flux motion formed during field penetration into an MgB 2 film were observed using magneto-optic imaging. To investigate the origin of the dendrites, experiments were performed where the sample was partially covered with an thermally conducting foil serving as an efficient heat sink. We observed that the dendrites are formed only in areas lacking the thermal conductor. When dendrites develop in the uncovered part they never invade into the covered region. The results strongly suggest that the dendritic instability is thermal in origin. Ó 2001 Published by Elsevier Science B.V.

Magneto-optical imaging reveals that in superconducting films of MgB2 a pulse of transport current creates avalanche-like flux dynamics where highly branching dendritic patterns are formed. The instability is triggered when the current exceeds a threshold value, and the superconductor, shaped as a long strip, is initially in the critical state. The instability exists up to 19 K, which is a much wider temperature range than in previous experiments, where dendrites were formed by a slowly varying magnetic field. The instability is believed to be of thermomagnetic origin indicating that thermal stabilization may become crucial in applications of MgB2.

The dynamics of magnetic flux distributions across a YBa 2 Cu 3 O 7−δ strip carrying transport current is measured using magneto-optical imaging at 20 K. The current is applied in pulses of 40-5000 ms duration and of magnitude close to the critical one, 5.5 A. During the pulse some extra flux usually penetrates the strip, so the local field increases in magnitude. When the strip is initially penetrated by flux, the local field either increases or decreases depending on both the spatial coordinate and the current magnitude. Meanwhile, the current density always tends to redistribute more uniformly. Despite the relaxation, all distributions remain qualitatively similar to the Bean-model predictions.