Editing Mu-metal

{{Short description|Nickel-iron alloy with high magnetic permeability}}
{{distinguish|nu metal}}
[[File:Mu-metal assortment 1951.jpg|thumb|Assortment of mu-metal shapes used in electronics, 1951]]
[[File:Mumetal box by Zureks.jpg|thumb|Five-layer mu-metal box. Each layer is about 5 mm thick. It reduces the effect of the Earth's magnetic field inside by a factor of 1500.]] 
'''Mu-metal''' is a [[nickel]]–[[iron]] soft [[ferromagnetic]] [[alloy]] with very high [[Magnetic permeability|permeability]], which is used for shielding sensitive electronic equipment against static or low-frequency [[magnetic field]]s.

==Properties==
Mu-metal has several compositions. One such composition is approximately
: 77% nickel,
: 16% iron,
: 5% [[copper]], and
: 2% [[chromium]] or [[molybdenum]].<ref name="Jiles">{{cite book |last=Jiles |first=David |title=Introduction to Magnetism and Magnetic Materials |publisher=CRC Press |year=1998 |page=354 |url=https://books.google.com/books?id=axyWXjsdorMC&dq=mu+metal&pg=PA354 |isbn=978-0-412-79860-3}}</ref><ref name="Weast">{{cite book |last=Weast |first=Robert |title=Handbook of Chemistry and Physics |edition=64th |publisher=CRC Press |year=1983 |page=E-108 |isbn=978-0-8493-0463-7}}</ref>
More recently, mu-metal is considered to be ASTM A753 Alloy&nbsp;4 and is composed of approximately
: 80% nickel,
: 12-15% iron,
: 5% molybdenum,
: and small amounts of various other elements such as [[silicon]].<ref>{{cite web |title=MuMetal Home |website=mu-metal.com |publisher=Josh Wickler |url=http://www.mu-metal.com |access-date=2015-07-06}}</ref>
The name came from the Greek letter mu ([[Mu (letter)|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 [[Permeability (electromagnetism)|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]],<ref name="Jiles" /> giving it a low [[coercivity]] so that it saturates at low magnetic fields. This gives it low [[hysteresis loss]]es 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.<ref name="Jiles" />

Mu-metal objects require [[heat treatment]] after they are in final form—[[Annealing (metallurgy)|annealing]] in a magnetic field in [[hydrogen]] atmosphere, which increases the [[magnetic permeability]] about 40 times.<ref name="Murby">{{cite web| title = Mu Metal specifications| work = Shielding Specifications| publisher = Nick Murby| url = http://mumetal.co.uk/?p=100#more-100| access-date = 2013-01-21| date = 2009-03-25}}</ref>  The annealing alters the material's [[crystal structure]], aligning the [[grain boundary|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.{{Citation needed|date=February 2023}}

==Application==
[[File:Mu metal CRT shields.jpg|thumb|Mu-metal shields for [[cathode-ray tube]]s (CRTs) used in [[oscilloscope]]s, from a 1945 electronics magazine]]
Mu-metal is a soft magnetic alloy with exceptionally high magnetic permeability. The high permeability of mu-metal provides a low [[reluctance]] path for [[magnetic flux]], leading to its use in [[Magnetic shielding|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]]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 [[Magnetic saturation|saturation]], 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 relatively 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.<ref>{{Cite web |date=2019-10-22 |title=MuMetal Shielding Performance - MuMETAL® High Permeability Magnetic Shielding Alloy ASTM A753 |url=https://web.archive.org/web/20191022014848/https://www.mu-metal.com/shielding-fundamentals.html |access-date=2025-04-07 |website=web.archive.org}}</ref><ref>{{Cite web |title=MuMETAL® Technical Data Sheet {{!}} Magnetic Shield Corporation |url=https://www.magnetic-shield.com/mumetal-technical-data/ |access-date=2025-04-07 |website=www.magnetic-shield.com}}</ref>

[[Radio frequency|RF]] magnetic fields above about 100 [[Kilohertz|kHz]] can be shielded by [[Faraday cage|Faraday shields]]: ordinary conductive metal sheets or screens which are used to shield against [[electric field]]s.<ref>{{cite web| title = Magnetic Fields and Shields| work = FAQ| publisher = Magnetic Shield Corp.| url = http://www.magnetic-shield.com/faq/interference.html| access-date = 2008-12-14| archive-date = 2008-12-18| archive-url = https://web.archive.org/web/20081218190613/http://magnetic-shield.com/faq/interference.html| url-status = dead}}</ref> [[Superconducting]] materials can also expel magnetic fields by the [[Meissner effect]], 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.

==Examples==
Mu-metal is used to shield equipment from magnetic fields. For example:
* [[Electric power]] [[transformer]]s, which are built with mu-metal shells to prevent them from affecting nearby circuitry.
* [[Hard disk]]s, which have mu-metal backings to the magnets found in the drive to keep the magnetic field away from the disk.<ref>{{Cite book |last1=Daniels |first1=Ryan J. |last2=McIntyre |first2=Timothy |last3=Kisner |first3=Roger |last4=Killough |first4=Stephen |last5=Lenarduzzi |first5=Roberto |title=SoutheastCon 2015 |chapter=Design and implementation of a Hall Effect sensor array applied to recycling hard drive magnets |date=April 2015 |chapter-url=https://ieeexplore.ieee.org/document/7132879 |pages=1–6 |doi=10.1109/SECON.2015.7132879|isbn=978-1-4673-7300-5 |s2cid=7196422 }}</ref>
* [[Cathode-ray tube]]s used in analogue [[oscilloscopes]], which have mu-metal shields to prevent stray magnetic fields from deflecting the electron beam.
* [[Magnetic cartridge|Magnetic phonograph cartridges]], which have a mu-metal case to reduce interference when [[Gramophone record|LP]]s are played back.
* [[Magnetic resonance imaging]] (MRI) equipment.
* The [[magnetometer]]s used in [[magnetoencephalography]] and [[magnetocardiography]].
* [[Photomultiplier tubes]].
* [[Vacuum chamber]]s for experiments with low-energy [[electron]]s, for example, photoelectron spectroscopy.
* [[Superconducting]] circuits and especially [[Josephson junction]] circuits.
* [[Magnetometer#Fluxgate magnetometer|Fluxgate magnetometers and compasses]] as part of the sensor.
* [[Proximity sensors]] (inductive type)

==Similar materials==
Other materials with similar magnetic properties include Co-Netic, [[supermalloy]], supermumetal, nilomag, sanbold, molybdenum [[permalloy]], [[Sendust]], M-1040, Hipernom, HyMu-80 and Amumetal.  [[Electrical steel]] is used similarly in some transformers as a cheaper, less permeable option.

Ceramic [[Ferrite (magnet)|ferrites]] are used for similar purposes, and have even higher permeability at high frequencies, but are brittle and nearly non-conductive, so can only replace mu-metals where conductivity and pliability aren't required.

==History==
[[File:Mu-metal_cable.svg|thumb|Mu-metal submarine cable construction]]
Mu-metal was developed by British scientists Willoughby S. Smith and Henry J. Garnett<ref name="Patent279549">{{Cite patent|number=GB279549A|title=New and improved magnetic alloys and their application in the manufacture of telegraphic and telephonic cables|gdate=1927-10-27|url=https://patents.google.com/patent/GB279549A/en}}</ref><ref name="Patent1582353" >[https://patents.google.com/patent/US1582353 US Patent 1582353] Willoughby Statham Smith, Henry Joseph Garnett, ''Magnetic Alloy'', filed January 10, 1924, granted April 27, 1926</ref><ref name="Patent1552769" >[https://patents.google.com/patent/US1552769 US Patent 1552769] Willoughby Statham Smith, Henry Joseph Garnett, ''Magnetic Alloy'', filed January 10, 1924, granted September 8, 1925</ref> and patented in 1923 for [[inductance|inductive]] loading of [[submarine telegraph cable]]s by The Telegraph Construction and Maintenance Co. Ltd. (now Telcon Metals Ltd.), a British firm that built the Atlantic undersea telegraph cables.<ref>{{cite web| last = Green| first = Allen| title = 150 Years Of Industry & Enterprise At Enderby's Wharf| work = History of the Atlantic Cable and Undersea Communications| publisher = FTL Design| date = 2004| url = http://atlantic-cable.com/Article/EnderbyAG/index.htm| access-date = 2008-12-14}}</ref> The conductive seawater surrounding an undersea cable added a significant [[capacitance]] to the cable, causing distortion of the signal, which limited the [[Bandwidth (signal processing)|bandwidth]] 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]]. {{Convert|80|km|mi}} of fine mu-metal wire were needed for each 1.6&nbsp;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]]s and [[cathode-ray tube]]s), as well as the [[fuze]]s inside [[naval mine|magnetic mines]]. Telcon Metals Ltd. abandoned the trademark "MUMETAL" in 1985.<ref>{{Cite web|url=http://tsdr.uspto.gov/#caseNumber=73431306&caseType=SERIAL_NO&searchType=statusSearch|title=Trademark Status & Document Retrieval|website=tsdr.uspto.gov|language=en|access-date=2017-07-28}}</ref> The last listed owner of the mark "MUMETAL" is Magnetic Shield Corporation, Illinois.<ref>{{Cite web|url=http://tsdr.uspto.gov/#caseNumber=73410422&caseType=SERIAL_NO&searchType=statusSearch|title=Trademark Status & Document Retrieval|website=tsdr.uspto.gov|language=en|access-date=2017-07-28}}</ref>

==References==
{{reflist}}

== External links ==
* [http://www.mu-metal.com/technical-data.html Mu-Metal Properties]
* [http://www.abschirmungen.eu/english/products/zero-gauss-chambers/ Zero gauss chambers] {{Webarchive|url=https://web.archive.org/web/20130217134030/http://www.abschirmungen.eu/english/products/zero-gauss-chambers/ |date=2013-02-17 }}
* [https://web.archive.org/web/20130416124524/http://www.abschirmungen.eu/english/products/faq Info about using mu metal shields]

[[Category:Nickel alloys]]
[[Category:Magnetic alloys]]
[[Category:Ferromagnetic materials]]