OVERVIEW OF ND-FE-B MAGNETS AND COERCIVITY

High performance Nd2Fe14B-based permanent magnets are produced with di fferent composition and various processing techniques. The composition and the processing route influence the complex, multiphase microstruc ture of the magnets, such as grain size, alignment, and distribution o f phases. Grain sizes in the range between 10 and 500 nm are obtained by melt spinning, mechanical alloying, and the HDDR process. Sintered and hot worked magnets exhibit grain sizes above 1 mu m. The coercive field is determined by the high uniaxial magnetocrystalline anisotropy as well as the magnetostatic and exchange interactions between neighb oring hard magnetic grains. The dipolar interactions between misaligne d gains are more pronounced in large-grained magnets, whereas exchange coupling reduces the coercive field in small grained magnets. Transmi ssion electron microscopy has been used to study the influence of subs tituent and dopant elements on microstructure, coercivity, and corrosi on resistance of advanced (Nd,S1)-(Fe,S2)-B:(M1,M2) magnets. The repla cement of the Nd-rich intergranular phase by secondary phases formed a fter doping by M1 and M2 type elements improves the corrosion resistan ce, especially in large-grained magnets. Secondary, nonmagnetic phases reduce the remanence and the energy product. In addition to the chara cterization of the microstructure, special attention has been paid to the computer modeling of the interaction between microstructure and co ercivity. The simulation of the magnetization reversal process based o n the real microstructure reveals a good agreement with experimental v alues. It is shown that the coercive field depends on grain size, dist ribution, and misorientation of grains. A strong exchange coupling bet ween hard magnetic grains is desired in nanostructured magnets in orde r to improve the remanence. This effect is further increased by second ary, soft magnetic phases. Nanocrystalline, composite Nd-Fe-B based ma gnets show a remanence enhancement, both in experiments and in model c alculations. (C) 1996 American Institute of Physics.