High-energy NdFeB magnets and their applications

Abstract

In sintered NdFeB magnets, additive elements for increasing coercivity decrease residual magnetization. Also, fine magnetic powder oxidization prevents identification of the stoichiometry composition (Nd ₂Fei₁₄B).To improve the magnetic properties of a sintered NdFeB magnet, the authors have developed a method involving two alloys. Magnetic Nd ₂Fei₁₄Balloy and a rare- earth- rich alloy (including richer dysprosium content) are melted individually and mixed together after coarse pulverization. After the sintering process, dysprosium in the sintered body is enriched in each grain region near the grain boundary. The two- alloy method minimizes the liquid phase necessary to keep the coercive force at a useful level, and thus results in compositions closer to stoichiometry. The energy product of magnets having an inhomogeneous dysprosium distribution is typically 360 kj - m ^{- 3} (45 MG.Oe) at production level. The corrosion characteristics of cobalt- substituted NdFeB magnets also were investigated. These magnets are now used in voice coil motors for hard disk drives and contribute to shortening access time and hard- drive downsizing.