Mu-metal

Mu-metal is a range of nickel-iron alloys that are notable for their high magnetic permeability, composed of approximately 77% nickel, 16% iron, 5% copper and 2% chromium or molybdenum. The high permeability makes mu-metal useful for shielding against static or low-frequency magnetic fields. The name came from the Greek letter mu (μ) which represents permeability in physics and engineering formulae. A number of different proprietary formulations of the alloy are sold under trade names such as MuMETAL, Mumetall, and Mumetal2.