Permalloy
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'''Permalloy''' is the term for a [[nickel]] [[iron]] magnetic [[alloy]]. Generically, it refers to an alloy with about 20% iron and 80% nickel content. Permalloy has a high [[Permeability (electromagnetism)|magnetic permeability]], 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. Permalloy's electrical [[resistivity]] generally varies within the range of 5% depending on the strength and the direction of an applied [[magnetic field]]. Permalloys typically have the [[face centered cubic]] cystal structure with a lattice constant of approximately 0.355 nm in the vicinity of a nickel concentration of 80%.
Permalloy is used in transformer laminations and magnetic [[recording head]] [[sensor]]s. In its initial application, Permalloy was wrapped around the insulated copper conductors of telecommunication cables, reducing signal distortion through improved inductive compensation of cable [[capacitive reactance]].
The right conditions for transmitting signals through cables without distortion were first worked out mathematically by [[Oliver Heaviside]].<ref>Bragg, L. ''Electricity'' (London: G. Bell & Sons, 1943) p. 212-3.</ref> After a prolonged search, Permalloy was discovered in [[1914]] by [[Gustav Elmen]] of [[Bell Laboratories]], who found it had higher permeability than [[silicon steel]]. Later, in [[1923]], he found its permeability could be greatly enhanced by heat treatment.
Other compositions of Permalloy are available, designated by a numerical prefix denoting the percentage of nickel in the alloy, for example ''45 Permalloy'' containing 45% nickel, and 55% iron. ''Molybdenum Permalloy'' is an alloy of 81% nickel, 17% iron and 2% [[molybdenum]]. The latter was invented at [[Bell Labs]] in 1940. At the time, when used in long distance copper telephone lines, it allowed a tenfold increase in maximum line working speed.<ref>Bragg, L. ''Electricity'' (London: G. Bell & Sons, 1943) p. 212-3.</ref>
==See also==
* [[Mu-metal]].
* [[Sendust]].
==References==
{{Reflist}}
* Richard M. Bozorth, ''Ferromagnetism,'' Wiley-IEEE Press (1993 reissue), ISBN 0-7803-1032-2.
* P. Ciureanu and S. Middelhoek, eds., ''Thin Film Resistive Sensors,'' Institute of Physics Publishing (1992), ISBN 0-7503-0173-2.
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*[http://www.arnoldmagnetics.com/mtc/pdf/molyperm_vs_sendust1.pdf Comparison of molybdenum permalloy with sendust] as energy storage inductors
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[[Category:Nickel alloys]]
[[Category:Magnetic alloys]]
[[Category:Ferromagnetic materials]]