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Rare-earth magnets are strong permanent magnets made from alloys of rare earth elements. 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 be in excess of 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. 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. ==Explanation of strength== 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. The advantage of the rare earth compounds over other magnets is that their crystalline structures have very high magnetic anisotropy. This means that a crystal of the material is easy to magnetize in one particular direction, but resists being magnetized in any other direction. Atoms of rare earth elements can retain high magnetic moments in the solid state. This is a consequence of incomplete filling of the f-shell, which can contain up to 7 unpaired electrons with aligned spins. Electrons in such orbitals are strongly localized and therefore easily retain their magnetic moments and function as paramagnetic centers. Magnetic moments in other orbitals are often lost due to the strong overlap with their neighboring electrons; for example, electrons participating in covalent bonds form pairs with zero net spin. High magnetic moments at the atomic level in combination with a stable alignment (high anisotropy) of those atoms results in a high magnetic field strength. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Rare-earth magnet」の詳細全文を読む スポンサード リンク
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