Magnetic alignment in powder magnet processing

Abstract

A strong magnetic field is used to align single-crystal powder particles in the process of producing sintered powder permanent magnets, including hard ferrites and rare-earth permanent magnets. The applied magnetic field aligns the easy direction of magnetization of each particle, owing to strong crystalline anisotropy. Shape anisotropy, existence of particles containing multigrains, and physical interlock between particles reduce the degree of alignment. This study provides a quantitative analysis of magnetic alignment in powder magnet processing. We assume (1) the powder particle is a single crystal; (2) it has the shape of an oblate spheroid and its short axis is the easy direction of magnetization; and (3) the applied magnetic field is strong enough to overcome the resistance of alignment. By applying the minimum-energy principle, it was concluded that the necessary and sufficient condition for a complete magnetic alignment is that the magnetocrystalline anisotropy constant K1 of the particles is greater than its shape anisotropy constant Ks, provided the applied magnetic field is strong enough. When Ks≳K1+2K2, the angle between the short axis of the oblate particle and the direction of applied magnetic field is 90°, and when K1≤Ks≤K1+2K2, the angle is arcsin√(Ks−K1)/2K2.