Mu-metal

Mu-metal is a nickel–iron soft ferromagnetic alloy known for its high magnetic permeability, making it ideal for shielding sensitive electronic devices from static or low-frequency magnetic fields. Its composition has evolved over time; early formulations included 77% nickel, 16% iron, 5% copper, and 2% chromium or molybdenum, while more recent versions, such as ASTM A753 Alloy 4, consist of approximately 80% nickel, 5% molybdenum, small amounts of other elements, and 12–15% iron. The name originates from the Greek letter mu (μ), representing permeability. Mu-metal's properties include a relative permeability of 80,000–100,000, low magnetic anisotropy, magnetostriction, and coercivity, allowing it to saturate at low magnetic fields. This results in low hysteresis losses in AC magnetic circuits. Annealing in a hydrogen atmosphere enhances its permeability by aligning grains and reducing impurities. However, bending or mechanical shock can disrupt grain alignment, which may be restored through re-annealing. Common applications include shielding electric power transformers, hard disks, cathode-ray tubes, MRI equipment, and magnetometers. Mu-metal is also used in proximity sensors, fluxgate magnetometers, and superconducting coils. Similar materials like Co-Netic, supermalloy, and ceramic ferrites offer comparable or specialized properties. Developed by British scientists Willoughby S. Smith and Henry J. Garnett in 1923 for submarine telegraph cables, Mu-metal was used to reduce signal distortion by confining magnetic fields. Its development competed with permalloy, another high-permeability alloy. During World War II, it found widespread use in electronics and military applications. The trademark "MUMETAL" was last held by Magnetic Shield Corporation. Mu-metal's versatility, combining high permeability with ductility, ...