Rare earth permanent magnets have been widely commercialized for their excellent magnetic properties. However, they contain a rare earth element and iron as main constituents, which are easily oxidized, and have relatively low corrosion resistance, so that their magnetic properties are deteriorated by oxidation. Therefore, their magnet main bodies are coated with a various protective film.
Patent Document 1, for example, discloses a permanent magnet comprising a magnet main body with surface irregularities, coated with an electroplated layer and an electrolessly plated layer. Patent Document 1 discusses that adhesion between the magnet main body and the plated layers formed thereon is improved by limiting its Rmax value (index of surface roughness, defined by JIS B-0610, according to the cited document 1) to a range from 3 to 50 μm.
Patent Document 2 discloses that a magnet main body can be coated with a protective film of high peel resistance, when it has a 10-point average surface roughness of 5 to 100 μm, defined by JIS B-0610, on the grounds that adhesion is insufficient at a roughness of 5 μm or less, and no improvement in adhesion is observed and product value is conversely deteriorated at 100 μm or more.    Patent Document 1: Japanese Patent Laid-Open No. 2-185004 (Claims, page 5)    Patent Document 2: Japanese Patent Laid-Open No. 7-66032 (Claims)
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