what is mu metal?

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

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.
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