Mu-metal is a nickel-iron alloy (approximately 75% nickel, 15% iron, plus copper and molybdenum) that has very high magnetic permeability. The high permeability makes mu-metal very effective at screening static or low-frequency magnetic fields, which cannot be attenuated by other methods. The name came from the Greek letter mu (μ) which represents permeability. A number of different proprietary formulations of the alloy are sold under trade names such as Mumetal, MuMetal, and MuShield; this article will cover their common features.
Mu-metal can have relative permeabilities of 80,000–100,000 compared to several thousand for ordinary steel. In addition it has low coercivity and magnetostriction resulting in low hysteresis loss. Other high permeability alloys such as permalloy have similar magnetic properties; mu-metal's advantage is that it is more ductile and workable.
Mu-metal objects require heat treatment after they are in final form — annealing in a magnetic field 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, which obstruct the free motion of the magnetic domain boundaries. Bending or mechanical shock after annealing may disrupt the material's grain alignment, leading to a drop in the permeability of the affected areas, which can be restored by repeating the hydrogen annealing step.
The high permeability of mu-metal provides a low reluctance path for magnetic flux, leading to its major use, in magnetic shields against static or slowly varying magnetic fields. Magnetic shielding made with high permeability alloys like mu-metal works not by blocking magnetic fields but by shunting them—providing a path for the magnetic field lines around the shielded area. So the best shape for shields is a closed container surrounding the shielded space. The effectiveness of mu-metal shielding decreases with the alloy's permeability, which drops off at both low field strengths and, due to saturation, at high field strengths. So mu-metal shields are often made of several enclosures one inside the other, each of which successively reduces the field inside it. RF magnetic fields above about 100 kHz can be shielded by Faraday shields, ordinary conductive metal sheets or screens which are used to shield against electric fields.
Full article ▸