Mu-metal is a

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...

–

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

soft ferromagnetic

alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductilit ...

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 Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

, 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 Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...

, and the remaining 12 to 15% iron. The name came from the Greek letter mu ( μ) which represents permeability in physics and engineering formulas. 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 In multiphase flow in porous media, the relative permeability of a phase is a dimensionless measure of the effective permeability of that phase. It is the ratio of the effective permeability of that phase to the absolute permeability. It can be ...

values of 80,000–100,000 compared to several thousand for ordinary steel. It is a "soft" ferromagnetic material; it has low

magnetic anisotropy In condensed matter physics, magnetic anisotropy describes how an object's magnetic properties can be different depending on direction. In the simplest case, there is no preferential direction for an object's magnetic moment. It will respond ...

and

magnetostriction Magnetostriction (cf. electrostriction) is a property of magnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of materials' magnetization due to the applied magnetic field chan ...

, giving it a low

coercivity Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...

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 Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

, malleable and workable, allowing it to be easily formed into the thin sheets needed for magnetic shields. Mu-metal objects require

heat treatment Heat treating (or heat treatment) is a group of industrial process, industrial, thermal and metalworking, metalworking processes used to alter the physical property, physical, and sometimes chemical property, chemical, properties of a material. ...

after they are in final form— annealing in a magnetic field in

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

atmosphere, which increases the magnetic permeability about 40 times. The annealing alters the material's

crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns ...

, aligning the

grains A grain is a small, hard, dry fruit (caryopsis) – with or without an attached hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and legumes ...

and removing some impurities, especially

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...

, which obstruct the free motion of the

magnetic domain A magnetic domain is a region within a magnetic material in which the magnetization is in a uniform direction. This means that the individual magnetic moments of the atoms are aligned with one another and they point in the same direction. When c ...

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.

Magnetic shielding

Mu-metal is a soft magnetic alloy with exceptionally high magnetic permeability. The high permeability of mu-metal provides a low

reluctance Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is defined as the ratio of magnetomotive force (mmf) to magnetic flux. It represents the opposition to magnetic flux, and depends on the geo ...

path for magnetic flux, leading to its 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 providing a path for the

magnetic field line A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

s around the shielded area. Thus, 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 Saturation, saturated, unsaturation or unsaturated may refer to: Chemistry * Saturation, a property of organic compounds referring to carbon-carbon bonds **Saturated and unsaturated compounds ** Degree of unsaturation **Saturated fat or fatty aci ...

, at high field strengths. Thus, mu-metal shields are often made of several enclosures one inside the other, each of which successively reduces the field inside it. Because mu-metal saturates at such low fields, sometimes the outer layer in such multilayer shields is made of ordinary steel. Its higher saturation value allows it to handle stronger magnetic fields, reducing them to a lower level that can be shielded effectively by the inner mu-metal layers. 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.

Superconducting Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...

materials can also expel magnetic fields by the

Meissner effect The Meissner effect (or Meissner–Ochsenfeld effect) is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state when it is cooled below the critical temperature. This expulsion will repel a ne ...

, but require cryogenic temperatures. The alloy has a low coercivity, near zero magnetostriction, and significant anisotropic magnetoresistance. The low magnetostriction is critical for industrial applications, where variable stresses in thin films would otherwise cause a ruinously large variation in magnetic properties.

History

Mu-metal was developed by British scientists Willoughby S. Smith and Henry J. GarnettUS Patent 1582353
Willoughby Statham Smith, Henry Joseph Garnett, ''Magnetic Alloy'', filed January 10, 1924, granted April 27, 1926US Patent 1552769
Willoughby Statham Smith, Henry Joseph Garnett, ''Magnetic Alloy'', filed January 10, 1924, granted September 8, 1925 and patented in 1923 for inductive loading of

submarine telegraph cable A submarine communications cable is a cable laid on the sea bed between land-based stations to carry telecommunication signals across stretches of ocean and sea. The first submarine communications cables laid beginning in the 1850s carried tel ...

s by The Telegraph Construction and Maintenance Co. Ltd. (now Telcon Metals Ltd.), a British firm that built the Atlantic undersea telegraph cables. The conductive seawater surrounding an undersea cable added a significant

capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized ar ...

to the cable, causing distortion of the signal, which limited the

bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...

and slowed signaling speed to 10–12 words per minute. The bandwidth could be increased by adding inductance to compensate. This was first done by wrapping the conductors with a helical wrapping of metal tape or wire of high magnetic permeability, which confined the magnetic field. Telcon invented mu-metal to compete with permalloy, the first high-permeability alloy used for cable compensation, whose patent rights were held by competitor Western Electric. Mu-metal was developed by adding copper to permalloy to improve

ductility Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile str ...

. of fine mu-metal wire were needed for each 1.6 km of cable, creating a great demand for the alloy. The first year of production Telcon was making 30 tons per week. In the 1930s this use for mu-metal declined, but by World War II many other uses were found in the electronics industry (particularly shielding for

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

s and cathode-ray tubes), as well as the fuzes inside

magnetic mines A naval mine is a self-contained explosive device placed in water to damage or destroy surface ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered by the approach of, or contact with, any v ...

. Telcon Metals Ltd. abandoned the trademark "MUMETAL" in 1985. The last listed owner of the mark "MUMETAL" is Magnetic Shield Corporation, Illinois.

Uses and properties

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

s, which are built with mu-metal shells to prevent them from affecting nearby circuitry. * Hard disks, which have mu-metal backings to the magnets found in the drive to keep the magnetic field away from the disk. * Cathode-ray tubes used in analogue

oscilloscopes An oscilloscope (informally a scope) is a type of electronic test instrument that graphically displays varying electrical voltages as a two-dimensional plot of one or more signals as a function of time. The main purposes are to display repetiti ...

, which have mu-metal shields to prevent stray magnetic fields from deflecting the electron beam. * Magnetic phonograph cartridges, which have a mu-metal case to reduce interference when LPs are played back. * Magnetic resonance imaging (MRI) equipment. * The magnetometers used in

magnetoencephalography Magnetoencephalography (MEG) is a functional neuroimaging technique for mapping brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain, using very sensitive magnetometers. Arrays of SQUIDs (s ...

and

magnetocardiography Magnetocardiography (MCG) is a technique to measure the magnetic fields produced by electrical currents in the heart using extremely sensitive devices such as the superconducting quantum interference device (SQUID). If the magnetic field is mea ...

. * Photomultiplier tubes. *

Vacuum chamber A vacuum chamber is a rigid enclosure from which air and other gases are removed by a vacuum pump. This results in a low-pressure environment within the chamber, commonly referred to as a vacuum. A vacuum environment allows researchers to con ...

s for experiments with low-energy

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

s, for example, photoelectron spectroscopy. *

circuits and especially

Josephson junction In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mec ...

circuits. * Fluxgate magnetometers and compasses as part of the sensor. *

Proximity sensors A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact. A proximity sensor often emits an electromagnetic field or a beam of electromagnetic radiation (infrared, for instance), and looks for ...

(inductive type)

Similar materials

Other materials with similar magnetic properties include Co-Netic, supermalloy, supermumetal, nilomag, sanbold, molybdenum permalloy,

Sendust Sendust is a magnetic metal powder that was invented by Hakaru Masumoto at Tohoku Imperial University in Sendai, Japan circa 1936 as an alternative to permalloy in inductor applications for telephone networks. Sendust composition is typically 85% ...

, M-1040, Hipernom, HyMu-80 and Amumetal. In recent times pyrolytic graphite (also incidentally used in some OLED panels as a heat sink) has been used as it also shows useful magnetic field exclusion properties.

References

External links

Mu-Metal Properties

Zero gauss chambers
{{Webarchive, url=https://web.archive.org/web/20130217134030/http://www.abschirmungen.eu/english/products/zero-gauss-chambers/ , date=2013-02-17
Info about using mu metal shields
Nickel alloys Magnetic alloys