K. Guth

Self-enhanced flux penetration into small angle grain boundaries in YBCO thin films

Grain boundaries in high-temperature superconductors are well known to strongly decrease the critical current. However, most information about their superconducting properties is based on transport measurements which always give non-local, averaged data over the whole sample size. In this paper, we report on the spatially resolved magneto-optical measurements of the magnetic flux and current density distribution in bicrystalline Y Ba2Cu3O7−x thin films with small angle grain boundaries. The current distributions are obtained by a model-independent inversion of Biot–Savarts law. In contrast to Bean-like models of the current pattern at the grain boundary, we find an inhomogeneous current density across the grain boundary, which is strongly influenced by the magnetic stray fields of the currents in the grains. The self-field effects are also responsible for a self-enhanced flux penetration where the penetration depth is significantly increased compared to a Bean-like model.

The visualization of current-limiting defects in YBa2Cu3O7 films on ion-beam assisted deposition buffer layers of yttrium-stabilized ZrO2 and Gd2Zr2O7

For the production of high-current-carrying, long-length superconducting wires or tapes, it is necessary to use biaxially textured metallic substrates or buffer layers. Though being highly textured, the deposited superconducting film exhibits a complex defect structure which (locally) suppresses the critical current and alternates characteristically the magnetic flux distribution seen in magneto-optical imaging. In this paper, we report on pulsed laser deposited YBaCuO films on biaxially textured yttrium-stabilized ZrO2?(YSZ) and Gd2Zr2O7?(GZO) buffers which were grown by ion beam assisted deposition?(IBAD) on polycrystalline substrates. The current-limiting defect structure turns out to resemble closely a combination of a dense distribution of pinhole-like induced growth distortions and a fine grain boundary network. The current suppression is caused on the one hand by the dense packing of pinhole-like defects. On the other hand, we observe a substantial current anisotropy being related to the surface morphology of the buffer layers and the direction of the IBAD-beam.

Quantitative Magneto-Optics: Flux, Current and Electric Field Imaging

Magneto-optical imaging (MOI) is an excellent tool to visualize magnetic flux patterns in superconductors with high spatial as well as temporal resolution. Furthermore, full quantitative MOI allows the determination of main properties of the (metastable) critical state, such as Meissner and critical current density distributions or electric field distributions related to thermally activated or current driven relaxation. A key step is the accurate calibration of the magneto-optically measured light intensities into a local magnetic flux density distribution, where properties of the MO layer as well as of the imaging system have to be taken into account. In this article, we give a short introduction into flux imaging, calibration techniques, current imaging and a completely new account on electric field imaging of relaxing magnetized states of high-temperature superconducting thin films. For all methods presented, we give examples using single crystalline and bicrystalline YBa2Cu3O7 thin films.

Magneto-optical study of grain boundaries, interfaces and grain boundary networks in YBaCuO

Using quantitative magneto-optics and an inversion scheme of Biot-Savarts law the local current carrying capability of various types of grain boundaries (GBs) and interfaces in YBaCuO thin films and bulk material was investigated. In all GBs and interfaces a spatial variation of the local critical current density jc was observed which, however, has different reasons, such as microstructural inhomogeneities, magnetic field dependence of jc and size effects in extended GBs and networks of low angle grain boundaries (LAGBs).

Modifying the Current Distribution of Grain Boundaries in YBCO Films

The current distribution in thin superconducting films can be influenced by magnetic surroundings. A soft magnet put parallel to a thin film edge can reduce or prevent flux entry and therefore stabilise Meissner screening currents in the film. This is particularly interesting for the investigation of currents across grain boundaries in high temperature superconductors, where the critical current density strongly depends on the flux which penetrates into the grain boundary. Furthermore, using special magnetic arrangements, asymmetric flux and current distributions can be obtained. An increase of the grain boundary critical current density is obtained in the Meissner state, compared to flux penetrated states. We show, that flux penetration into grain boundaries can be suppressed up to a certain external field H*, which depends on the temperature, allowing larger intergranular current densities. All investigations are done by magneto-optical imaging and the inversion of Biot and Savart.

Grain Boundary Critical Currents in High‐Tc Superconductors: A Magneto‐Optical Study

Using magneto‐optical imaging the magnetic field and current distribution in decreasing external fields in bi‐crystalline YBa2Cu3O7−δ thin films containing small angle grain boundaries have been studied with high spatial resolution. In this work, we focus on the influence of an external magnetic field on the remanent state current distribution in these films. It is found, that the external field has strong influence on the inter‐ to intra‐grain current ratio. For a symmetric 4° tilt grain boundary the inter‐ to intra‐grain current ratio was increased from 0.45 to 0.63 by the application of an external field of 80 mT at high temperatures. This shift is based on two effects: Firstly, we find a suppression of the intra‐granular currents and, secondly, an increase of the currents crossing the grain boundary.