Mu-metal

Not to be confused with nu metal.
Assortment of mu-metal shapes used in electronics, 1951
Five-layer mu-metal box. Each layer is about 5 mm thick. It reduces the effect of the Earth's magnetic field inside by a factor of 1500.

Mu-metal is a nickel–iron soft ferromagnetic alloy with very high permeability, which is used for shielding sensitive electronic equipment against static or low-frequency magnetic fields.