Rare-earth magnet

Rare-earth magnets are strong permanent magnets made from alloys of rare earth elements (elements in the lanthanide series, plus scandium and yttrium). Developed in the 1970s and '80s, 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 1.4 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. Magnetostrictive rare-earth magnets such as Terfenol-D also have applications—e.g. in loudspeakers. 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 term "rare earth" can be misleading as these metals are not particularly rare or precious; they are about as abundant as tin or lead. 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.

The rare earth (lanthanide) elements are metals that are ferromagnetic, meaning that like iron they can be permanently magnetized, but their Curie temperatures are below room temperature, so in pure form their magnetism only appears at low temperatures. However, they form compounds with the transition metals such as iron, nickel, and cobalt, and some of these have Curie temperatures well above room temperature. Rare earth magnets are made from these compounds.

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