Rare-earth-based permanent magnets have the unique ability to create Tesla-strength magnetic fields without the need for power supplies. Moreover, Halbach showed that the fields of assemblies of magnets can be calculated analytically, because the permeability of permanent magnets is very close to unity. But that only worked in the absence of materials with a non-unity permeability, which implies that numerical tools must be employed to account, for example, for materials used to shield stray fields. Here we employ the method of images to derive analytic expressions for the magnetic field of Halbach multipoles that are enclosed in infinite-permeability shielding.
Volker Ziemann (Mon,) studied this question.