V. V. Yurchenko

Very strong intrinsic flux pinning and vortex avalanches in (Ba,K)Fe2As2 superconducting single crystals

We report that the (Ba,K)Fe2As2 crystal with Tc =3 2 K shows a pinning potential, U0, as high as 104 K, with U0 showing very little field dependence. The (Ba,K)Fe2As2 single crystals become isotropic at low temperatures and high magnetic fields, resulting in a very rigid vortex lattice, even in fields very close to Hc2. The isotropic rigid vortices observed in the two-dimensional (2D) (Ba,K)Fe2As2 distinguish this compound from 2D high-Tc cuprate superconductors with 2D vortices. The vortex avalanches were also observed at low temperatures in the (Ba,K)Fe2As2 crystal. It is proposed that it is the K substitution that induces both almost isotropic superconductivity and the very strong intrinsic pinning in the (Ba,K)Fe2As2 crystal.

Mapping flux avalanches in MgB2 films—equivalence between magneto-optical imaging and magnetic measurements

Vortex avalanches are known to occur in MgB2 films within a certain range of temperatures and magnetic fields. These events, resulting from a thermomagnetic instability, were first revealed by real-time magneto-optical imaging, which exposed dendritic paths of abrupt flux propagation. This very powerful technique has, however, a practical limitation, since sensors that are currently available cannot be used at high magnetic fields. This letter shows that results obtained using dc magnetometry are in good correspondence with those furnished by magneto-optical imaging, demonstrating that the two techniques can be efficiently used as complementary tools to map vortex avalanches in superconducting films.

Magneto-optical imaging of magnetic flux patterns in superconducting films with antidots

We present the results of experiments on visualization of magnetic flux distribution and its dynamics in high-temperature superconductors with artificial defects. High-Tc superconductor thin films were equipped with a special arrangement of antidots in order to separate the streams of magnetic flux moving in (or out of) the sample. A possibility to alter the direction and depth of flux penetration is clearly demonstrated by means of magneto-optical imaging. The resolution was sufficient for observation of flux in particular antidots, which allows more detailed dynamic analysis of such systems. 2006 Elsevier B.V. All rights reserved.

Suppression of dendritic flux jumps in MgB2 films coated with a gold rim

An effective method for suppressing dendritic flux avalanches in MgB2 films is demonstrated by coating the films with a metal rim along the edges, which is where the avalanches nucleate. The effect of the partial coating has been investigated by means of magneto-optical imaging on a series of samples with golden rims of different width. Measurements of the onset field of instability reveal that such rims can substantially improve the thermo-magnetic stability of the superconducting films.

Dendritic flux avalanches in superconducting films

Thermomagnetic instability in general, and dendritic flux avalanches in particular, have attracted considerable attention of both scientists and engineers working on superconductor applications. Though being harmful for the performance of many superconducting devices, the avalanches provide a fruitful playground for experimental and theoretical studies of complex dynamics of the vortex matter. In this paper we report on the progress in understanding the mechanisms responsible for the development of the giant magnetic avalanches. We review recent results on magneto-optical imaging of the fingering instability in superconducting films and analyze them on the basis of recent theoretical model that establishes criteria for onset of the dendritic avalanches.

Mechanism for flux guidance by micrometric antidot arrays in superconducting films

A study of magnetic flux penetration in a superconducting film patterned with arrays of micron sized antidots (microholes) is reported. Magneto-optical imaging (MOI) of a YBCO film shaped as a long strip with perpendicular antidot arrays revealed both strong guidance of flux, and at the same time large perturbations of the overall flux penetration and flow of current. These results are compared with a numerical flux creep simulation of a thin superconductor with the same antidot pattern. To perform calculations on such a complex geometry, an efficient numerical scheme for handling the boundary conditions of the antidots and the nonlocal electrodynamics was developed. The simulations reproduce essentially all features of the MOI results. In addition, the numerical results give insight into all other key quantities, e.g., the electrical field, which becomes extremely large in the narrow channels connecting the antidots.

Width-dependent upper threshold field for flux noise in MgB2 strips

The authors measured magnetization hysteresis curves and used magneto-optical imaging to visualize the flux distributions in superconducting MgB2 films in order to study dendritic flux avalanches. The flux avalanches are found to disappear above some upper threshold field that is typically ∼1kOe, but strongly depends on the film width. If the film is made wider, this threshold field first increases and then tends to saturate. This behavior is quantitatively explained using a thermomagnetic model for the dendritic avalanches and taking into account the field dependence of Jc. The results demonstrate that patterning superconducting films into narrow strips substantially increases the range of magnetic fields for which they can be used for applications.

Magnetic field penetration in MgB2 single crystals: Pinning and Meissner holes

The evolution of flux distribution in MgB2 single crystals during their remagnetization was imaged with magneto-optical technique. Meissner holes, formed as the areas where the annihilation of vortices and antivortices takes place, were found at the boundary between oppositely magnetized parts of the crystal. Gradient of magnetic induction in the vicinity of Meissner holes was found to be enhanced. Finger-like structures of convex shape, formed during the penetration of magnetic flux inside the crystal, were observed and explained as an effect of inhomogeneous pinning and demagnetizing field redistribution in the sample.

Magneto-optical investigations of Ag-sheathed Bi-2223 tapes with ferromagnetic shielding

An increase in the critical current and suppression of AC losses in superconducting wires and tapes with soft magnetic sheath have been predicted theoretically and confirmed experimentally. In this work we present the results of magneto-optical investigations on a series of Ag-sheathed Bi-2223 tapes with Ni coating. We visualize distributions of magnetic field at increasing external field and different temperatures, demonstrating a difference between the flux propagation in the superconductor with Ni rims and a reference sample without Ni coating.

UPPER THRESHOLD FIELDS OF DENDRITIC FLUX JUMPS IN GOLD-COATED MgB2 THIN FILMS

We measured the magneto-optical images (MOIs) and the magnetic hysteresis (M – H) curves of c-axis-oriented MgB2 thin films to investigate the flux penetration in the form of dendritic avalanches. In order to understand the role of the thermal effects, we prepared Au-coated MgB2 thin films with different thicknesses of gold. While the MOI provides a spatially resolved flux pattern, the M – H curve presents global and average information about the flux noise associated with avalanches. These two types of measurements complement each other. The upper threshold field, above which the flux noise disappears, was determined from the M-H curves while the lower threshold field was determined from both the M – H curves and the MO images. The field range where the flux penetrates via avalanches is found to be smaller for thicker gold layers. These results are important for many superconducting applications.