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The following is taken from page 9-26 of Handbook of Engineering Fundamentals by Eshbach second edition.

Ferromagnetic substances including iron, steel, nickel, cobalt, and magnetic alloys, have relative permeabilities greatly in excess of unity. Under certain conditions the relative permeability of steel may exceed 2000.

All the other elements air included are close to a permeability of 1. Thus, at frequency of zero the comparison of the magnetic conductivity is over 2000 times that of other elements. What this means is the lines of magnetic force will try and stay within the ferromagnetic substance and not go back into the air unless it can help it. Thus, the easiest magnetic path from one side of box to the other is through the outer shell. This reduces the internal field strength thus the shielding effect. This does not happen with non-ferromagnetic substances like copper, brass, aluminum, lead and etc.

As the frequency rises eddy currents start gradually to take effect and the electrical conductivity properties begins to take over. By using a ferromagnetic substance that is a good conductor you have the best of both worlds some shielding at both high and low frequencies ELF.

Offered by Mike.