Abstract. -We have studied the dynamics of magnetic-flux avalanches in superconducting YBa2Cu3O 7−δ films by means of a fast magneto-optic pump-probe technique. Two regimes of propagation are found: in regime I, directly after the nucleation of the avalanches by a femtosecond laser pulse, the velocity v of the flux dendrites depends strongly on the magnetic field, and values of v up to 180 km/s are observed. Within some ten nanoseconds after nucleation regime II is reached, where the propagation velocity has dropped by one order of magnitude and is nearly independent of sample temperature and magnetic field. Comparison with previous experiments on flux jumps in superconductors shows good qualitative agreement.

Dendritic flux patterns in superconducting YBCO films are studied on a nanosecond time-scale. It is found that dendrites only develop for certain values of the external field and temperature. It is well-known that superconductors carrying high critical current densities tend to develop instabilities in their magnetic flux patterns which manifest themselves as macroscopic flux jumps [1]. A combination of magneto-optics and pulsed laser irradiation can be used to investigate the flux propagation with high spatial and temporal resolution in the micrometer and nanosecond range. Using this technique it was found that for thin superconducting films the flux jump instability can give rise to dendritic magnetic flux avalanches propagating with tip velocities as high as 50 km/s [2,3].In this work we report on a systematic magneto-optic study of various types of dynamic flux patterns in thin films. Depending on temperature T and magnetic field B we observe two completely different regimes of flux penetration: either in the form of a smooth flux front, or as a dendritic flux avalanche.In our experiments we used YBCO films with thicknesses 100, 200, 330, and 690 nm, grown on a SrTiO 3 substrate by thermal reactive co-evaporation (the c-axis is perpendicular to the substrate) [3]. The superconducting transition temperatures are between 86 and 90 K and critical current densities are between 2 Â 10 6 and 4 Â 10 6 A/m 2 at 77 K. To visualize the magnetic c-component of the field a Bi-doped yttrium iron garnet (YIG) film on gadolinium gallium garnet with in-plane anisotropy is used as magneto-optic layer [4], which is mounted on top of the superconductor [5]. In order to distinguish between positive and negative direction of the field the polarizer and analyzer in our polarization microscope were not crossed completely and fields between )50 and +180 mT could be observed. The YIG film allows to obtain a time resolution of about 200 ps (see [6]).Each experiment is done as follows. The sample is zero field cooled below T c . After reaching a stable temperature a magnetic field B a perpendicular to the sample surface is applied, and then the laser pulse is triggered. The thin film geometry enhances the field at the edges and the magnetic flux penetrates into the superconducting film from there. The first part of the laser pulse is now used to change the local magnetic field. This is done by focusing the laser pulse with a cylindrical lens onto the film from the substrate side of the sample. A stripe of 50 lm width is heated above T c , dividing the quadratic sample into two regions of equal size. Magnetic flux fills the non-superconducting stripe and the T/s). Now the magnetic flux starts to penetrate perpendicular to the stripe into the superconductor. Fig. 1 shows the flux profile at 30 K after a time delay of 67.8 ns (top) and the final state (bottom). We found that this homogeneous flux penetration is restricted to a region in the T =T c versus B a =j c diagram as shown in Fig. 2. Above a certain magnetic field which de...

We suggest a new theoretical approach describing the velocity of magnetic flux dendrite penetration into thin superconducting films. The key assumptions for this approach are based upon experimental observations. We treat a dendrite tip motion as a propagating flux jump instability. Two different regimes of dendrite propagation are found: A fast initial stage is followed by a slow stage, which sets in as soon as a dendrite enters into the vortex-free region. We find that the dendrite velocity is inversely proportional to the sample thickness. The theoretical results and experimental data obtained by a magneto-optic pump-probe technique are compared and excellent agreement between the calculations and measurements is found.

We report the discovery of a new mechanism of spontaneous generation of a magnetic flux in a superconductor cooled through T c . Values of the spontaneous flux appear random from one cooldown to the next, following a Gaussian distribution. The width of the distribution increases with the size of the temperature gradient in the sample. Our observations appear inconsistent with the well-known mechanisms of flux generation. The dependence on the temperature gradient suggests that the flux may be generated through an instability of the thermoelectric superconducting-normal quasiparticle counterflow.

Wc measured the velocity of Ihc flux front of an artificially nucleated dendritic instability in YNi2B2C. The required time resolution In the nanosecond regime was achieved by our magneto-optic pumpprobe technique, utilizing a femtosecond laser system. The penetration vclocity of the flux front is on the order of 360 km s-'.