  Although the flux density map of a bulk superconductor provides in principle
sufficient information for calculating the magnitude and the direction of the
supercurrent flow, the inversion of the Biot-Savart law is ill conditioned for
thick samples, thus rendering this method unsuitable for state of the art bulk
superconductors. If a thin (< 1 mm) slab is cut from the bulk, the inversion is
reasonably well conditioned and the variation of the critical current density
in the sample can be calculated with adequate spatial resolution. Therefore a
novel procedure is employed, which exploits the symmetry of the problem and
solves the equations non-iteratively, assuming a planar z-independent current
density. The calculated current density at a certain position is found to
depend on the magnetic induction. In this way the average field dependence of
the critical current density Jc(B) is obtained also at low fields, which is not
accessible to magnetisation measurements due to the self-field of the sample.
It is further shown that an evaluation of magnetisation loops, taking the
self-field into account, results in a similar dependence in the field range
accessible to this experiment.
