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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 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 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.