Magnetic properties of extremely small Nd-Fe-B sintered magnets

Magnetic properties of small Nd-Fe-B sintered magnets were investigated. Thickness of surface damaged layer, which shows low coercivity and causes the deterioration in magnetic properties, was found to be comparable to mean grain size of sintered bodies. Magnets with small grains of around 5 μm showed rather higher magnetic performance after machining into small pieces with a surface area/volume ratio of 20-35 mm^-1. Chemical etching with a nitric acid solution was effective in partial recovery of magnetic properties through bringing slightly higher coercivity of surface grains. Heat treatment at higher temperatures than the melting point of Nd-rich phase and subsequent aging enhanced the coercivity of surface grains. Coating with powders of Dy₂O₃, DyF₃, or TbF₃ before the heat treatment resulted in a great enhancement in coercivity without significant reduction in remanence.