Recent developments in hard magnetic bulk materials

J Fidler, T Schrefl, S Hoefinger, M Hajduga
J Phys Condens Matter 16 (2004) S455-S470

The importance of newly developed permanent magnetic materials in many electromechanical, magneto mechanical and electronic applications is attributed to the drastic improvement in microstructure related properties, such as the remanence, the magnetic energy density product and the coercive field. The influence of the microstructure on the magnetic properties of the magnets will be discussed, where special emphasis is laid on rare earth permanent magnets. Highest performance, anisotropic Nd-Fe-B magnets with J(r) > 1.5 T, (BH)max > 450 kJ m(-3) and H-J(c) > 750 kA m(-1), which are produced by the powder metallurgy route, show a strong influence of composition and processing parameters on the magnetic properties. The magnetic properties of Sm(Co, Cu, Fe, Zr), sintered magnets, which are used nowadays for high temperature applications between 300 and 500degreesC, are determined by the cellular precipitation microstructure, which is developed during a complex heat treatment and by the microchemistry. Special hard magnetic powder materials, such as Sm2Fe17N3 and nanocrystalline, composite Nd2Fe14B/(alpha-Fe, Fe3B) materials have been developed especially for usage in bonded magnetic materials, which show the strongest annual increase in the production of permanent magnets. The phenomenon of the enhancement of remanence, occurring in single phase and composite Nd2Fe14B based magnets with isotropic grain alignment, is attributed to intergrain exchange interactions.

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