MICROMAGNETIC ANALYSIS APPLIED TO MELT-SPUN NDFEB MAGNETS WITH SMALL ADDITIONS OF GA AND MO

This paper presents results of a systematic investigation of the relat ionship between microstructure and magnetic properties in Ga- and GaMo -doped melt-spun NdFeB-based permanent magnets according to a micromag netic description. Fundamental problems concerning the coercivity mech anism and the grain size dependence of the coercive field are discusse d. It is shown that the theory of nucleation in single domain particle s is able to explain the temperature dependence of the magnetic revers al process in rapidly solidified magnets. The additives Ga and Mo both improve the coercive field. Mo-doped samples show small magnetization reversal field intervals and a high reversibility of the demagnetizat ion curve. This is connected with the narrow grain size distribution. Furthermore, the concept of microstructural parameters alpha(K) and N- eff is applied to an extensive investigation of the temperature depend ence of the coercive field. For different microstructures it turns out that alpha(K) reaches saturation as required by the theory whereas a wide spectrum of N-eff is found. Nevertheless the strong correlation o f both alpha(K) and N-eff restricts the further improvement of the coe rcive field and leads to optimal coercive fields at room temperature f or annealed Ga- and Mo-doped samples with a fine-grained and uniform m icrostructure.