Magnetic mapping of defects in type-II superconductors

Abstract

Recently it was discovered that the non-uniform Meissner current flowing around the pinning sites in the type-II superconductor induces the unconventional vortex-antivortex pairs with the non-quantized magnetic flux [J.-Y. Ge, J. Gutierrez, V. N. Gladilin, J. T. Devreese, and V. V. Moshchalkov, Nat. Commun. 6, 6573 (2015)]. Here we provide the theory of this phenomenon showing that the vortex-like structures originate from the perturbation of the current streamlines by the non-superconducting defect, which results in the generation of the localized magnetic field. The position and the shape of such vortex dipoles are shown to be very sensitive to the defect form. Thus, applying the external magnetic field or current to the superconductor and using, e.g., the high-resolution scanning Hall microscope to measure the stray magnetic field one can plot the map containing the information about the position of the defects and their shape.The control of the Abrikosov vortex pinning is one of the corner-stone problems in the physics of superconducting systems. 1,2 The reduction of the vortex mobility by a lattice of the pinning centers allows to damp the energy dissipation and substantially increase the critical current, 3-7 which is extremely important for application of superconductors in electronics. During the last decades it became possible to create both low-and high-temperature superconductors with various types of natural and artificial defects (holes, 8-10 grain boundaries, 11 nanorods, 12-14 imbedded nanoparticles, 15 surface grades, 16 controllable lattice transformations, 17 etc.) which are shown to be effective barriers for the vortex motion. The corresponding enhancement of the critical current appears to be very sensitive to the particular shape and the spatial distribution of the pinning centers. This aims the efforts of both theoreticians and experimentalists at the engineering of the efficient pinning potentials and the extensive study of the magnetic flux behavior simultaneously affected by the pinning sites and the transport current.Recently the magnetic field induced by the Meissner current flowing around the pinning sites was measured with the high-resolution scanning Hall microscope. 18,19 It was found that the magnetic contrast near the defects reminds the one for the pair of vortex and antivortex. Interestingly, the magnetic flux carried by each pole of such "vortex dipole" is not quantized and depends on current. To explain this effect the authors have performed sophisticated numerical simulations based on the time-dependent Ginzburg-Landau equation accounting the non-uniform profile of the Meissner current. Here we provide the simple explanation of this phenomenon within the stationary currents theory. It is based on the fact that each defect being impervious for the Cooper pairs perturbs the streamlines of the superconducting current which gives rise to the well-localized stray magnetic field. Our analysis clearly shows that the formation of the vortex dipoles is not specific to the Meissner s...

Simulation of Real Defect Geometry and Its Detection Using Passive Magnetic Inspection (PMI) Method

“…Natural magnetic fields and other influential local magnetic sources [37], combined with internal and external stresses, can change the scattered stray magnetic field of ferromagnetic materials [43]. Internal domain walls' displacement and magnetic-moment rotation in ferromagnetic materials happen under the influence of external magnetic fields [44], and there are relationships between the micro-magnetic characteristics of these materials and their mechanical responses [45].…”