Rare-earth magnet

A rare-earth magnet is a strong permanent magnet made from alloys of rare-earth elements. Developed in the 1970s and 1980s, rare-earth magnets are the strongest type of permanent magnets made, producing significantly stronger magnetic fields than other types such as ferrite or alnico magnets. The magnetic field typically produced by rare-earth magnets can exceed 1.2 teslas, whereas ferrite or ceramic magnets typically exhibit fields of 0.5 to 1 tesla.

There are two types: neodymium magnets and samarium–cobalt magnets. Rare-earth magnets are extremely brittle and also vulnerable to corrosion, so they are usually plated or coated to protect them from breaking, chipping, or crumbling into powder.

The development of rare-earth magnets began around 1966, when K. J. Strnat and G. Hoffer of the US Air Force Materials Laboratory discovered that an alloy of yttrium and cobalt, YCo₅, had by far the largest magnetic anisotropy constant of any material then known.^[1]^[2]

The term "rare earth" can be misleading, as some of these metals can be as abundant in the Earth's crust as tin or lead,^[3] but rare earth ores do not exist in seams (like coal or copper), so in any given cubic kilometre of crust they are "rare".^[4]^[5] The major source is currently China.^[6] Some countries classify rare earth metals as strategically important,^[7] and Chinese export restrictions on these materials have led other countries, including the United States, to initiate research programs to develop strong magnets that do not require rare earth metals.^[8]

^ Cullity, B. D.; Graham, C. D. (2008). Introduction to Magnetic Materials. Wiley-IEEE. p. 489. ISBN 978-0-471-47741-9.
^ Lovelace, Alan M. (March–April 1971). "More Mileage Than Programmed From Military R&D". Air University Review. 22 (3). US Air Force: 14–23. Archived from the original on February 24, 2013. Retrieved July 4, 2012.
^ Bobber, R. J. (1981). "New Types of Transducers". Underwater Acoustics and Signal Processing. pp. 243–261. doi:10.1007/978-94-009-8447-9_20. ISBN 978-94-009-8449-3.
^ McCaig, Malcolm (1977). Permanent Magnets in Theory and Practice. US: Wiley. p. 123. ISBN 0-7273-1604-4.
^ Sigel, Astrid; Helmut Sigel (2003). The lanthanides and their interrelations with biosystems. US: CRC Press. pp. v. ISBN 0-8247-4245-1.
^ Walsh, Bryan (March 13, 2012). "Raring to Fight: The U.S. Tangles with China over Rare-Earth Exports". Time Magazine. Retrieved November 13, 2017.
^ Chu, Steven (2011). Critical Materials Strategy. Diane Publishing. pp. 96–98. ISBN 978-1437944181. China rare earth magnets.
^ "Research Funding for Rare Earth Free Permanent Magnets". ARPA-E. Archived from the original on 10 October 2013. Retrieved 23 April 2013.

[Cullity-1] Cullity, B. D.; Graham, C. D. (2008). Introduction to Magnetic Materials. Wiley-IEEE. p. 489. ISBN 978-0-471-47741-9.

[Lovelace-2] Lovelace, Alan M. (March–April 1971). "More Mileage Than Programmed From Military R&D". Air University Review. 22 (3). US Air Force: 14–23. Archived from the original on February 24, 2013. Retrieved July 4, 2012.

[3] Bobber, R. J. (1981). "New Types of Transducers". Underwater Acoustics and Signal Processing. pp. 243–261. doi:10.1007/978-94-009-8447-9_20. ISBN 978-94-009-8449-3.

[4] McCaig, Malcolm (1977). Permanent Magnets in Theory and Practice. US: Wiley. p. 123. ISBN 0-7273-1604-4.

[5] Sigel, Astrid; Helmut Sigel (2003). The lanthanides and their interrelations with biosystems. US: CRC Press. pp. v. ISBN 0-8247-4245-1.

[Walsh-6] Walsh, Bryan (March 13, 2012). "Raring to Fight: The U.S. Tangles with China over Rare-Earth Exports". Time Magazine. Retrieved November 13, 2017.

[Chu-7] Chu, Steven (2011). Critical Materials Strategy. Diane Publishing. pp. 96–98. ISBN 978-1437944181. China rare earth magnets.

[arpa-e-8] "Research Funding for Rare Earth Free Permanent Magnets". ARPA-E. Archived from the original on 10 October 2013. Retrieved 23 April 2013.

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