Dramatic Role of Critical Current Anisotropy on Flux Avalanches inMgB2Films

Abstract

Anisotropic penetration of magnetic flux in MgB(2) films grown on vicinal sapphire substrates is investigated using magneto-optical imaging. Regular penetration above 10 K proceeds more easily along the substrate surface steps, the anisotropy of the critical current being 6%. At lower temperatures the penetration occurs via abrupt dendritic avalanches that preferentially propagate perpendicular to the surface steps. This inverse anisotropy in the penetration pattern becomes dramatic very close to 10 K where all flux avalanches propagate in the strongest pinning direction. The observed behavior is fully explained using a thermomagnetic model of the dendritic instability.

Oscillatory regimes of the thermomagnetic instability in superconducting films

“…1. These thresholds have been explained on the basis of linear stability analysis of the nonlinear and non-local equations governing the electrodynamics of such films [15][16][17][18][19][20]. The theoretical works show that in order to trigger avalanches an electrical field in the range E = 30-100 mV/m is required.…”

Lightning in superconductors

“…The most unstable mode has a finite wave-number, which means that the instability will nucleate by fingering2728293031. As the instability develops, numerical analysis of the non-linear and non-local dynamics of magnetic flux and temperature have revealed that the instabilities develop into complex branching structures2832.…”

Finger patterns of magnetic flux in bulkBi2Sr2CaCu2O8+δBarness1, Sinvani2, Shaulov3 et al. 2008Phys. Rev. B10060View full textAdd to dashboardCite

“…1- 23 Such patterns have been directly observed in a large number of superconducting films employing magneto-optical imaging techniques. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] The existing experimental data, and the recently developed theoretical models, [14][15][16][17][18][19][20][21] suggest that the origin of these patterns is thermomagnetic instability of the vortex matter in the superconducting films. [21][22][23] The instability arises from local temperature increase due to flux motion, which, in turn, decreases flux pinning and hence facilitates further flux motion.…”