Overview of Nd–Fe–B magnets and coercivity (invited)

Abstract

High performance Nd2Fe14B-based permanent magnets are produced with different composition and various processing techniques. The composition and the processing route influence the complex, multiphase microstructure of the magnets, such as grain size, alignment, and distribution of 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 μm. The coercive field is determined by the high uniaxial magnetocrystalline anisotropy as well as the magnetostatic and exchange interactions between neighboring hard magnetic grains. The dipolar interactions between misaligned grains are more pronounced in large-grained magnets, whereas exchange coupling reduces the coercive field in small grained magnets. Transmission electron microscopy has been used to study the influence of substituent and dopant elements on microstructure, coercivity, and corrosion resistance of advanced (Nd,S1)–(Fe,S2)–B:(M1,M2) magnets. The replacement of the Nd-rich intergranular phase by secondary phases formed after doping by M1 and M2 type elements improves the corrosion resistance, especially in large-grained magnets. Secondary, nonmagnetic phases reduce the remanence and the energy product. In addition to the characterization of the microstructure, special attention has been paid to the computer modeling of the interaction between microstructure and coercivity. The simulation of the magnetization reversal process based on the real microstructure reveals a good agreement with experimental values. It is shown that the coercive field depends on grain size, distribution, and misorientation of grains. A strong exchange coupling between hard magnetic grains is desired in nanostructured magnets in order to improve the remanence. This effect is further increased by secondary, soft magnetic phases. Nanocrystalline, composite Nd–Fe–B based magnets show a remanence enhancement, both in experiments and in model calculations.