How does a magnetic shield work?
There is no known material that can block magnetic fields without itself being attracted to the magnetic force. A magnetic shield acts as a kind of sponge redirecting the magnetic field around the shield instead of passing through the sensitive instrument which is being shielded. To be a good magnetic shielding material it must have a high permeability which means that the magnetic field lines are strongly attracted to the shielding material. Mumetal, Supra 50, and Supra 36 are the most common alloys and are chosen based on the intensity of the magnetic field. If the magnetic field is too high for the material chosen it will saturate and become ineffective. In this case you can use a multi layer shield with a combination of the above alloys. Alloys should also have a very low remanence to prevent them becoming permanently magnetised.
