Mu-metal is a nickel–iron soft ferromagnetic alloy with very high permeability, which is used for shielding sensitive electronic equipment against static or low-frequency magnetic fields. It has several compositions. One such composition is approximately 77% nickel, 16% iron, 5% copper, and 2% chromium or molybdenum. More recently, mu-metal is considered to be ASTM A753 Alloy 4 and is composed of approximately 80% nickel, 5% molybdenum, small amounts of various other elements such as silicon, and the remaining 12 to 15% iron. 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.
Mu-metal typically has relative permeability values of 80,000–100,000 compared to several thousand for ordinary steel. It is a “soft” ferromagnetic material; it has low magnetic anisotropy and magnetostriction, giving it a low coercivity so that it saturates at low magnetic fields. This gives it low hysteresis losses when used in AC magnetic circuits. Other high-permeability nickel–iron alloys such as permalloy have similar magnetic properties; mu-metal’s advantage is that it is more ductile, malleable and workable, allowing it to be easily formed into the thin sheets needed for magnetic shields.
Mu-metal objects require heat treatment after they are in final form—annealing in a magnetic field in hydrogen atmosphere, which 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.
MuMetal® is the most widely used alloy for magnetic shielding purposes. Its composition of 80% nickel, 4.5% molybdenum and balance iron gives it highly permeable properties. This tells us that the material has high magnetic susceptibility to an applied magnetic field; it readily accepts the flow of magnetic field. We can take advantage of this property to effectively redirect field lines away from areas we wish to be field-free. By building a hollow structure from MuMetal® sheet en-compassing the area of interest we create an entire region of space that is unaffected by external fields. For clarity, this effect is shown in the illustration.
Magnetic field lines are directed around the area within the shield. This diagram depicts the cross section of a cylindrical shield. The central black region indicates the field free region.
As you might expect, magnetic shielding is useful for an incredibly wide range of applications; from small components within a set of speakers to large magnetically shielded rooms, called MuRooms. This breadth of usage requires some clever design to maximise the efficiency of the shield in each case. Plainly speaking, the fewer sharp angles and joints a shield has, the better the shield. This naturally might lead to thinking that the best design for a shield would be a spherical design. However, in almost all cases, either a cylindrical shield or a MuMetal® can (MuCan) performs impressively well and is far easier and quicker to produce.
On the microscopic scale, MuMetal® can be viewed as a crystalline structure with magnetic domains. These domains are responsible for MuMetal®’s superior permeability. In order to develop the maximum number of domains within the material, a specialised heat treatment in a hydrogen furnace is performed as the final stage in the shield’s production. In this way the shield produced has the highest performance possible.
MuMetal® saturates at 0.76 T. In other words, a field stronger than this cannot be contained within the MuMetal and field will leak into the contained space. In most circumstances, the external field in question is well below this threshold, for example, the Earth’s field which is approximately 50uT. Where a shield is to be used in a much stronger field, for industrial or medical purposes for example, it is common to incorporate a buffer layer of Supra-50 which has a slightly different composition (50% nickel) and a saturation level of 1.6 T. This cuts down the high field by a significant amount before it reaches the MuMetal® layer so that the MuMetal® layer can work within its saturation threshold.
Radio Frequency shielding is required to stop high frequency fields (> 100 kHz). Copper, aluminium and metallised plastics are normally used in this case. Magnetic shielding is typically performed in the 30 – 300 Hz AC range. AC stands for alternating current which reverses its direction over a short period of time. These fields are generated by typical 50-60 Hz electric power equipment. DC stands for direct current which flows in one direction only, such as the fields emitted from the Earth or produced by magnets and some motors. Magnetic shielding is effective for both of these types of current.