REMANENCE AND COERCIVITY IN ISOTROPIC NANOCRYSTALLINE PERMANENT-MAGNETS

Numerical micromagnetic calculations rigorously describe the correlati on between the microstructure and the magnetic properties of nanocryst alline permanent magnets. In isotropic nanocrystalline permanent magne ts exchange interactions override the anisotropy of the individual gra ins. Therefore the spontaneous magnetic polarization deviates from the easy axes in a region along the grain boundaries. For a fine grain st ructure with a mean grain size d < 20 nm the remanence is considerably enhanced, since the volume fraction of the boundary regions where the spontaneous magnetic polarization points towards the direction of the applied field becomes significantly high. The inhomogeneous ground st ate, however, favors the nucleation of reversed domains leading to a r eduction of the coercive field with decreasing grain size. A uniform g rain structure with a very small range in grain size avoids large dema gnetizing fields and thus preserves a high coercivity. For a grain siz e of 10 nm isotropic two-phase permanent magnets based on Fe14Nd2B and alpha-Fe show remarkable high-energy products, because the volume fra ction of the magnetically soft phase can be increased up to 50% withou t a significant loss of coercivity.