Magneto-optical studies of current distributions in high-T_csuperconductors

Abstract

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-T c superconductors (HTS). This has been related to the significant progress in quantitative high-resolution magnetooptical 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 j c (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.

Sheet-current distribution in a dc SQUID washer probed by vortices

“…So far there has been only one method of imaging the current distribution in superconductors: The magnetic field distribution on top of a superconducting thin film is measured, e.g. by magneto-optics, from which the current distribution can then be calculated by inverting the Biot-Savart law [1].In this paper we present a novel method to directly image the sheet-current distribution in a YBa 2 Cu 3 O 7 thin film. We use low-temperature scanning electron microscopy (LTSEM) [2,3,4,5] to image vortices in dc SQUID washers [6,7].…”

“…So far there has been only one method of imaging the current distribution in superconductors: The magnetic field distribution on top of a superconducting thin film is measured, e.g. by magneto-optics, from which the current distribution can then be calculated by inverting the Biot-Savart law [1].…”

Advances in magneto‐optical imaging applied to rock magnetism and paleomagnetism

“…The first is a shape effect. Above an uniformly magnetized infinite plane, the external field is always zero due to the mutual cancellation of individual source elements, but will result in a rimmed MO pattern for grains of finite width (e.g., MOI of magnetized ∼200 nm thick and 1 mm 2 films in the work by Jooss et al [2002]). This holds if the plane dimensions are at least ∼30 times larger than the distance at which the field is measured.…”

“…These merits suggest that the MOI technique can be very helpful to identify minerals carrying a magnetic moment in geological samples. The technique is, in fact, already widely used in material science for the observation of magnetic domain structures in magnetic materials [Polyanski et al, 1999;Welp et al, 2003] or flux penetration in superconductors [Dorosinskii et al, 1992;Jooss et al, 2002]. This technique has also been recently applied to geological samples [Bobyl et al, 2007].…”

Measurement of fluctuation-induced diamagnetism in BSCCO-2212 single crystals using magneto-optics

“…Compared to traditional magnetization measurements, MO has certain advantages ( see our earlier works [7,17] and some review articles [18] ). Among those, adjustable measurement time scales may be the most important one for our purpose.…”