Editing Mu-metal (section)

==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}}