The magnetic flux-penetration into single- and polycrystals has been studied by means of the magneto-optical Faraday effect. The flux density profile in YBaCuO single- and polycrystals reveals a hyperbolic behaviour in contrast to the parabolic decrease as observed in Nb. The volume pinning forces fp in grains of sintered specimens determined according to Friedel et al.1, show a reciprocal dependence on the grain size of sintered samples. This points to surface pinning effects.
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