Mu-metal
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Mu-metal is a nickel-iron alloy (77% nickel, 15% iron, plus copper and molybdenum) that is very efficient for screening magnetic fields. It is used to shield vacuum chambers for experiments with low-energy electrons, to shield magnetic resonance imaging equipment, and to shield the magnetometers used in magnetoencephalography and magnetocardiography.
The name of the material refers to the Greek letter μ, which is the symbol for magnetic permeability. Mu-metal has a high value of μ.
See also permalloy.
Other materials with similar properties are supermalloy, supermumetal, nilomag, sanbold, Mo-Permalloy, Ultraperm, M-1040, etc.