MuMETAL is an alloy of nickel and iron used for magnetic shielding. You can find it inside many electronic devices, from hard drives to CRT televisions.
MuMETAL is made of mostly nickel. It contains about 75-80% nickel, with iron and other elements making up the balance. In comparison, regular steel is an alloy made of mostly iron. Steel is often more than 99% iron.