Influence of microscopic defects in type-II superconducting thin films on the magnetic flux penetration

Abstract

The magnetic flux penetration into thin type-II superconducting films with circular defects is investigated. The artificial circular defects ͑diameter ϭ40 m) in an YBa 2 Cu 3 O 7Ϫ␦ thin film ͑thickness Ϸ300 nm) were prepared by pulse-laser irradiation. The flux penetration into the zero-field-cooled superconducting film was visualized by means of the magneto-optic method. A stepwise increase of the external magnetic field allowed a detailed investigation of the influence of local defects on the flux penetration. For a magnetic field parallel to a long sample ͑longitudinal geometry͒ with a long cylindrical defect a single parabolic discontinuity line appears. Also in the case of a thin superconducting film exposed to a transverse magnetic field ͑transverse geometry͒, a single parabolic discontinuity line has been supposed in the vicinity of a local defect. On the contrary, our investigations show that the flux and current distribution around a single defect in a superconducting thin film can be determined not by a single, but by two discontinuity parabolas. In thin superconducting films in transverse geometry screening currents in the Meissner region ( jϽ j c ) are present in contrast to extended infinitely long samples in the longitudinal geometry. We explain our experimental results by the influence of these Meissner screening currents on the temporal formation of the shape of an approaching flux front.

Stability of degenerate vortex states and multi-quanta confinement effects in a nanostructured superconductor with Kagome lattice of elongated antidots

“…One should note that the deformed magnetic field profiles are the combined effects of vortex current around antidots and external current crowding at antidots. The similar phenomena were also observed at topological defects due to the Meissner current or external current [62][63][64][65][66]. By comparing the evolutions of magnetic field of S A and S B , one can see that the equivalence of the unit cells of the observed ice-like states are broken by the external current (symmetry break) although the different vortexoccupations in unit cells contribute the same free energy to the vortex system in the case of no applied current.…”

Anisotropic vortex channeling inYBa2Cu3O7–δthin films with

“…These are related to the singularities in magnetic induction found at the edges of current-carrying 2D conductors, and were observed in the magneto-optical experiments of Ref. 12. In our case one would not expect to resolve this fine structure experimentally due to the small actual hole size combined with the relatively large distance between sample and Hall probe.…”

“…This indicates a strong interaction between supercurrents circulating the holes and those due to bulk induction gradients, similar to that revealed in magneto-optical images of flux penetration in a YBCO film with several larger ͑40-m͒ circular holes. 12 While we believe that the vortices flow out of the sample upon field removal, we cannot rule out the possibility that antivortices are channeling in the opposite direction, and the process is one of systematic vortex-antivortex annihilation. The latter would be the expected mechanism of magnetization reversal in homogeneous thin films.…”

Mechanism for flux guidance by micrometric antidot arrays in superconducting films

“…The motion of magnetic flux in type-II superconducting films can to a large extent be controlled by introduction of artificial micro-and nanostructures, such as antidots (holes), [1][2][3] magnetic dots, 4,5 thickness modulations, 6,7 grain boundaries, 8,9 slits, 10 or magnetic domain walls in superconductor/ferromagnet hybrids. 11,12 Such structures are key building blocks for successful realization of fluxonics devices like vortex ratchets, pumps and lenses, etc.…”