S. Leonhardt

Magneto-optical studies of current distributions in high-Tc superconductors

In the past few years magneto-optical flux imaging (MOI) has come to take an increasing role in the investigation and understanding of critical current densities in high-Tc superconductors (HTS). This has been related to the significant progress in quantitative high-resolution magneto-optical imaging of flux distributions together with the model-independent determination of the corresponding current distributions. We review in this article the magneto-optical imaging technique and experiments on thin films, single crystals, polycrystalline bulk ceramics, tapes and melt-textured HTS materials and analyse systematically the properties determining the spatial distribution and the magnitude of the supercurrents. First of all, the current distribution is determined by the sample geometry. Due to the boundary conditions at the sample borders, the current distribution in samples of arbitrary shape splits up into domains of nearly uniform parallel current flow which are separated by current domain boundaries, where the current streamlines are sharply bent. Qualitatively, the current pattern is described by the Bean model; however, changes due to a spatially dependent electric field distribution which is induced by flux creep or flux flow have to be taken into account. For small magnetic fields, the Meissner phase coexists with pinned vortex phases and the geometry-dependent Meissner screening currents contribute to the observed current patterns. The influence of additional factors on the current domain patterns are systematically analysed: local magnetic field dependence of jc(B), current anisotropy, inhomogeneities and local transport properties of grain boundaries. We then continue to an overview of the current distribution and current-limiting factors of materials, relevant to technical applications like melt-textured samples, coated conductors and tapes. Finally, a selection of magneto-optical experiments which give direct insight into vortex pinning and depinning mechanisms are reviewed.

Influence of vortex-vortex interaction on critical currents across low-angle grain boundaries in YBa2Cu3O7-δ thin films

Low-angle grain boundaries with misorientation angles u,5° in optimally doped thin films of YBa2Cu3O72d are investigated by magneto-optical imaging. By using a numerical inversion scheme of BiotSavart’s law, the critical current density across the grain boundary can be determined with a spatial resolution of about 5mm. Detailed investigation of the spatially resolved flux density and current density data shows that the current density across the boundary varies with varying local flux density. Combining the corresponding flux and current pattern, it is found that there exists a universal dependency of the grain boundary current on the local flux density. Considering the magnetic vortex-vortex interaction in and in the vicinity of the grain boundary, a model is developed that is able to describe the experimental data. DOI: 10.1103/PhysRevB.63.014507

Anisotropic flux pinning in thin YBCO-films by substrate modifications

Abstract The magnetooptical Faraday effect is an excellent tool to investigate the spatially resolved flux density distribution and, by a numerical inversion of the Biot–Savart law, the local current density in a superconducting film with a spatial resolution better than 10 μm [Ch. Jooss, R. Warthmann, A. Forkl, H. Kronmuller, Physica C 299 (1998) 215]. Irradiating the surface of SrTiO 3 (001) substrates with a focused ion beam in a line pattern leads to an anisotropy of the flux and current density distribution in a well-defined area. While we measure an enhancement of j c parallel to the lines, a reduction of the value of the current perpendicular to the lines is found. This is in qualitative agreement with experiments on YBCO films grown on vicinal substrate surfaces [T. Haage, J. Zegenhagen, J.Q. Li, H.-U. Habermeier, M. Cardona, Ch. Jooss, R. Warthmann, A. Forkl, H. Kronmuller, Phys. Rev. B 56 (1997) 8404] and based on the data gained on these systems, a pinning model for the films on the irradiated substrates is suggested.

Influence of substrate irradiation on critical current density and microstructure in YBCO thin films

Abstract Strontiumtitanate substrates have been irradiated in defined areas with a focused ion beam (FIB) of gallium ions. This leads to a charge of the surface structure of the substrates. Afterwards YBCO was grown epitactically on these substrates by pulsed laser deposition. The effect of the modification on the flux density distribution in the YBCO thin films has been observed magnetooptically and the local current distribution has been determined. The correlation between the change in jc and the microstructure of the YBCO samples, investigated by transmission electron microscopy (TEM), is discussed.

Current densities in low-angle grain boundaries of YBCO

Abstract Magnetooptical imaging has been used to investigate low-angle grain boundaries (LAGBs) in YBCO thin films at T = 5K. This technique allows us to visualize the flux density distribution with a spatial resolution of a few micrometers. By a numerical analysis of the experimental data the local current distribution in a LAGB has deen determined. It is shown that the current density acros the grain boundary depends strongly on the local flux density which leads to a crossover in the pinning scenario at the LAGB.

Enhancement of the Critical Current Density of YBA2CU3O7–δ–Films by Substrate Irradiation

One of the most striking properties of YBa2Cu3O7_8 (YBCO) thin films is their high critical current density. This is due to strong pinning centers which are generated during film growth. As YBCO grows epitactically, the film growth mechanism depends strongly on the roughness of the substrate surface. With a Focused Gallium Ion Beam (FIB) the structure of the substrate surface is modified. Magneto-optical measurements show the local flux densities in the superconductor and the local current density distribution can be calculated by a numerical inversion of Biot-Savart’s law [1]. It is shown that the critical current density in the superconductor is enhanced by specific substrate modification.