  Many experiments in nuclear and neutron physics are confronted with the
problem that they use a superconducting magnetic spectrometer which potentially
affects other experiments by their stray magnetic field. The retardation
spectrometer aSPECT consists, inter alia, of a superconducting magnet system
that produces a strong longitudinal magnetic field of up to 6.2T. In order not
to disturb other experiments in the vicinity of aSPECT, we had to develop a
magnetic field return yoke for the magnet system. While the return yoke must
reduce the stray magnetic field, the internal magnetic field and its
homogeneity should not be affected. As in many cases, the magnetic shielding
for aSPECT must manage with limited space. In addition, we must ensure that the
additional magnetic forces on the magnet coils are not destructive. In order to
determine the most suitable geometry for the magnetic shielding for aSPECT, we
simulated a variety of possible geometries and combinations of shielding
materials of non-linear permeability. The results of our simulations were
checked through magnetic field measurements both with Hall and nuclear magnetic
resonance probes. The experimental data are in good agreement with the
simulated values: The mean deviation from the simulated exterior magnetic field
is (-1.7+/-4.8)%. However, in the two critical regions, the internal magnetic
field deviates by 0.2% respectively <1E-4 from the simulated values.
