Magnetic instability in YBa2Cu3O7−δ films

Abstract

. Thcfilms had a thckness of 300 nm and a critlcal current plied. Magnetic flux penetrated into the superconducting film firs1 from the edges and from defects which were in conracr with the edges until a localequilibrium of the flux distribution due to tbe pinning lorce and the ~~iagneric force was reached. This induces a current distribution in the superconducting film. In order to disturb the equilibrium of tlus current distribution and to iuitiare a magnetic instability, a pulse of a frequency doubled Nd : YAG laser = 532 nm, halfwidrh T = 7 ns) was foc~ised onto the filn1 from the substrate side. The energy deas~ty i o the laser spot (diameter 30pm) was up to 30 mJ/m2. The san~ple temperature iu the locus could not be measured directly, but we estimate that the temperarure is well above the cnrical temperature.If the perfurbation is sufficiently strong, e.g. Cor energy densities of rhe laser pulse above 7 rnJ/cm2, this trigzers a maguetic instability, in which a ~nagnetic flux avalanche penetrates into the sin. Fig. 1 shows a mapnetooptical image of the resulting flux distribution aller the laser pulse. Bright redons correspond to high magnetic flux density. In contrast to tlie niore or less llomogeneous flux fronts which proplgate towards the sample center w h n thc external field is gradually increased, this instability develops in the form oC a dendritic pattern as already observed earlier [I]. The total area covered by the flux branches is found to increase linearly with the ex~ernal magnetic field B,,, above a threshold (B,,, = 7.5 mT), whereas the widrh o l the branches (0.1 mm) remains constant witbin our accuracy.

The diversity of flux avalanche patterns in superconducting films

“…One sees that large l also gives large avalanches and many branches. This results is in agreement with previous experiment by Bolz et al 4 The images seen in Fig. 6 show also a striking resemblance with previous experimental and simulation results at different substrate temperatures T 0 .…”

“…Evidently, within this part of parameter space one finds patterns which show striking similarities with those observed in experiments. [1][2][3][4][5][6][7][8][9] In addition, we find some type of patterns not previously reported.…”

Origin of dendritic flux patterns in MgB2 films

“…Spatially resolved studies using magneto-optical (MO) imaging have shown that in films these jumps are associated with an abrupt invasion of dendritic flux structures [6,7]. Similar dendritic structures have been observed earlier in films of Nb [8][9][10] and YBa 2 Cu 3 O 7 [11,12]. So far, there is no general agreement about the mechanism underlying this dramatic instability.…”

Finger patterns produced by thermomagnetic instability in superconductors

“…Numerous magneto-optical studies have revealed that the thermomagnetic instability in thin superconducting samples results in dendritic flux patterns. [7][8][9][10][11][12][13][14][15][16][17][18] In course of the dendritic instability the flux forms narrow "fingers" of typical width 20-50 µm and length up to the size of the sample. Such a behavior clearly contradicts to the conventional theoretical concepts and needs elucidation.…”