Rare‐earth Intermetallics for Permanent Magnet Applications

Abstract

Abstract Modern high‐performance permanent magnets are based on rare‐earth (RE) intermetallic compounds with excellent intrinsic magnetic properties as well as on optimized microstructures and alloy compositions. Outstanding hard magnetic properties such as energy density product and coercive field are obtained in binary and ternary RE (Sm, Nd, Pr) compounds with transition metals (TM = Co, Fe), in particular, based on RECo 5 and RE 2 Co 17 and RE 2 Fe 14 B. The complex microstructure considerably influences the magnetic reversal process. Intergranular phases change the coupling behavior between the hard magnetic grains. Nonmagnetic phases eliminate the direct exchange interaction and reduce the long‐range magnetostatic coupling. Both effects increase the coercive field in nucleation‐controlled magnets. The RE‐rich intergranular phase is necessary for the liquid phase sintering process, but deteriorates the corrosion stability and reduces the remanence. In order to improve coercive field, remanence and energy density, it is necessary to increase the volume fraction of the hard magnetic phase by reducing the amount of oxygen, nonmagnetic phases and pores, and to improve the degree of alignment of the hard grains. The difference in magnetic domain wall energy in precipitation hardened multiphase magnets determines the coercive field in domain wall pinning controlled magnets. The inhomogeneous magnetization behavior near the intergranular regions of nanocrystalline RE magnets, which is directly responsible for remanence and coercivity, is strongly influenced by the microstructural parameters. By using numerical micromagnetic simulations based on the finite element technique the correlation between microstructure and magnetic hysteresis properties can be quantitatively predicted, thus providing a powerful tool for the further development of optimized high‐performance Nd 2 Fe 14 B and Sm(Co,Cu) 5 /Sm 2 (Co,Fe) 17 ‐type magnets.