We numerically investigate the effect of an edge indentation on the threshold field of thermomagnetic instabilities in superconducting films subjected to a ramping magnetic field, applied perpendicular to the plane of the film. In particuar, we are able to address the question on whether edge indentations promote magnetic flux avalanches. For magnetic field-independent critical current densities model, the triggering of the first magnetic flux avalanche systematically occurs at the edge indentation. In contrast to that, for the more realistic field-dependent critical current density model the first flux avalanche can take place either at or away from the indentation. This selective triggering of magnetic flux avalanches is shown to result from the variation of the threshold magnetic field for the first flux avalanche triggered at the indentation and the reduction of the critical current density by large local magnetic fields at the tip of the indentation which translates in a lower power density dissipated near the tip. We demonstrate that this interplay can be tuned by varying the indentation size, ramp rate of applied fieldḢa, and working temperature T0. We build up a phase diagram in the µ0Ḣa − T0 plane with well-defined boundaries separating three distinct regimes of thermomagnetic instability.

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Purpose Finite element (FE) models are considered for the penetration of magnetic flux in type-II superconductor films. A shell transformation allows boundary conditions to be applied at infinity with no truncation approximation. This paper aims to determine the accuracy and efficiency of shell transformation techniques in such non-linear eddy current problems. Design/methodology/approach A three-dimensional H – ϕ formulation is considered, where the reaction field is calculated in the presence of a uniform applied field. The shell transformation is used in the far-field region, and the uniform applied field is introduced through surface terms, so as to avoid infinite energy terms. The resulting field distributions are compared against known solutions for different geometries (thin disks and thin strips in the critical state, square thin films). The influence of the shape, size and mesh quality of the far-field regions are discussed. Findings The formulation is shown to provide accurate results for a number of film geometries and shell transformation shapes. The size of the far-field region has to be chosen in such a way to properly capture the asymptotic decay of the fields, and a practical procedure to determine this size is provided. Originality/value The importance of the size of the far-field region in a shell transformation and its proximity to the conducting domains are both highlighted. This paper also provides a numerical way to apply a constant magnetic field in a given region, while the source, on which only the far-field behaviour of the applied field depends, is excluded from the model.

Local polarization of magnetic materials has become a well-known and widely used method for storing binary information. Numerous applications in our daily life such as credit cards, computer hard drives, and the popular magnetic drawing board toy, rely on this principle. In this work, we review the recent advances on the magnetic recording of inhomogeneous magnetic landscapes produced by superconducting films. We summarize the current compelling experimental evidence showing that magnetic recording can be applied for imprinting in a soft magnetic layer the flux trajectory taking place in a superconducting layer at cryogenic temperatures. This approach enables the ex situ observation at room temperature of the imprinted magnetic flux landscape obtained below the critical temperature of the superconducting state. The undeniable appeal of the proposed technique lies in its simplicity and the potential to improve the spatial resolution, possibly down to the scale of a few vortices.

This paper examines the magnetic field and supercurrent distributions in three-dimensional structures made of superposed superconducting films. A recent experimental study of such structures (Tamegai 2017) has shown unusual patterns of the trapped magnetic flux, with an unexpected discontinuity line in the region where the films overlap. In this work, a finite element model is used to reproduce and understand these patterns. It is shown that the newly observed discontinuity line originates from the magnetic field dependence of the critical current density. Moreover, in contrast to common observations made in planar superconducting structures, the network of critical lines is found to evolve with the applied field when the three-dimensional structure is magnetized after zero-field cooling. These findings underline the importance of the magnetic coupling between the components of a multi-layered superconducting structure in building its critical state.