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.