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Mu-metal

Mu-metal is a nickel-iron alloy (75% nickel, 15% iron, plus copper and molybdenum) that has very high magnetic permeability. Permeability is represented by μ.

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The high permeability makes mu-metal very effective at screening static or low-frequency magnetic fields, which cannot be attenuated by other methods.^[1]

Mu metal requires special heat treatment — annealing in hydrogen atmosphere, which reportedly increases the magnetic permeability about 40 times. The annealing alters the material's crystal structure, aligning the grains and removing some impurities, especially carbon. Mechanical treatment may disrupt the material's grain alignment, leading to drop of permeability in the affected areas, which can be restored by repeating of the hydrogen annealing step.

Uses and properties

Mu-metal is used to shield equipment from magnetic fields. For example:

Vacuum chambers for experiments with low-energy electrons, for example photoelectron spectroscopy
Magnetic resonance imaging equipment
The magnetometers used in magnetoencephalography and magnetocardiography
Photomultipliers
Cathode-ray tubes used in analog oscilloscopes
Superconducting circuits and esp. Josephson junction circuits
Electric power transformers, which are built with mu-metal shells to prevent them from affecting nearby circuitry
Magnetic cartridges, which have a mu-metal case to reduce interference when LPs are played back
Hard Drives, which have mu-metal backings to the magnets found in the drive

Other materials with similar magnetic properties are supermalloy, supermumetal, nilomag, sanbold, Molybdenum Permalloy, Ultraperm, M-1040, etc.