Grain-size dependence of remanence and coercive field of isotropic nanocrystalline composite permanent magnets

Micromagnetic calculations using a finite element technique are useful to investigate magnetization processes in nanocrystalline ferromagnetic materials. In particular numerical calculations reveal the microstructural conditions required for high remanence and high coercivity isotropic permanent magnets. composite materials of Nd2Fe14B and alpha-Fe are excellent candidates for such high performance permanent magnets, The soft magnetic alpha-Fe grains cause a large spontaneous magnetization and the hard magnetic grains induce a large coercive field, provided that both phases are strongly exchange coupled. The numerical investigations on realistic three-dimensional grain arrangements suggest an optimal microstructure consisting of small soft magnetic grains (D approximate to 10 nm, V-soft approximate to 40%) embedded between hard magnetic grains with a mean grain diameter of about D approximate to 20 nm. Additionally, a microstructure with regular shaped grains improves the magnetic properties.