A magnetic shield or screen protects a delicate instrument from external magnetic fields. The shield should be made of a material with high magnetic permeability to be effective, which means it can conduct magnetic fields effectively and redirect them away from the protected instrument.

The most common materials used for magnetic shielding are ferromagnetic materials, such as mu-metal or permalloy. These materials have high magnetic permeability and can redirect magnetic fields around the shielded area, reducing the magnetic field strength within the shielded region.

Mu-metal is a nickel-iron alloy often used for magnetic shielding due to its high permeability and ability to reduce magnetic interference. Permalloy is another nickel-iron alloy that is commonly used for shielding applications.

Other materials that can be used for magnetic shielding include superconductors, which can completely exclude magnetic fields, and non-magnetic materials, such as aluminum or copper, which can reflect or absorb magnetic fields but are less effective than ferromagnetic materials.