Permalloy
Permalloy is a nickel-iron magnetic alloy, with about 20% iron and 80% nickel content. It is notable for its very high magnetic permeability, which makes it useful as a magnetic core material in electrical and electronic equipment, and also in magnetic shielding to block magnetic fields. Commercial permalloy alloys typically have relative permeability of around 100,000, compared to several thousand for ordinary steel.[1]
In addition to high permeability, its other magnetic properties are low coercivity, near zero magnetostriction, and significant anisotropic magnetoresistance. The low magnetostriction is critical for industrial applications, allowing it to be used in thin films where variable stresses 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 crystal 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 heads.
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[edit] Development
Permalloy was initially developed in the early 20th century for inductive compensation of telegraph cables.[2] When the first transatlantic submarine telegraph cables were laid in the 1860s, it was found that excessive capacitive reactance caused distortion which reduced the maximum signalling speed to only 10 - 12 words per minute.[2] The right conditions for transmitting signals through cables without distortion were first worked out mathematically in 1885 by Oliver Heaviside.[3] It was discovered by Karup in Denmark that the capacitive reactance of the cable could be compensated by wrapping it with a wire or tape made of high permeability metal, increasing the inductance. 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. A wrapping of permalloy tape could reportedly increase the signalling speed of a telegraph cable fourfold.[2]
[edit] Other compositions
Other compositions of permalloy are available, designated by a numerical prefix denoting the percentage of nickel in the alloy, for example 45 Permalloy means an alloy 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.[3] Supermalloy, at 77 wt. percent Ni, 14% Fe, 5% Cu, and 4% Mo, is also well known for its high performance as soft magnetic material. It is characterized by high permeability and low coercivity.
[edit] See also
- Mu-metal
- Sendust
- Supermalloy (a material with even higher magnetic permeability)
[edit] Notes
- ^ Jiles, David (1998). Introduction to Magnetism and Magnetic Materials. CRC Press. p. 354. ISBN 0412798603. http://books.google.com/?id=axyWXjsdorMC&pg=PA354&dq=mu+metal.
- ^ a b c Green, Allen (2004). "150 Years Of Industry & Enterprise At Enderby's Wharf". History of the Atlantic Cable and Undersea Communications. FTL Design. http://atlantic-cable.com/Article/EnderbyAG/index.htm. Retrieved 2008-12-14.
- ^ a b Bragg, L. Electricity (London: G. Bell & Sons, 1943) p. 212-3.
[edit] References
- 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.
