Micromagnetic analysis applied to melt-spun NdFeB magnets with small additions of Ga and Mo

Abstract

This paper presents results of a systematic investigation of the relationship between microstructure and magnetic properties in Ga- and GaMo-doped melt-spun NdFeB-based permanent magnets according to a micromagnetic description. Fundamental problems concerning the coercivity mechanism and the grain size dependence of the coercive field are discussed. It is shown that the theory of nucleation in single domain particles is able to explain the temperature dependence of the magnetic reversal 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 demagnetization curve. This is connected with the narrow grain size distribution. Furthermore, the concept of microstructural parameters α K and N eff is applied to an extensive investigation of the temperature dependence of the coercive field. For different microstructures it turns out that α K reaches saturation as required by the theory whereas a wide spectrum of N eff is found. Nevertheless the strong correlation of both α K and N eff restricts the further improvement of the coercive field and leads to optimal coercive fields at room temperature for annealed Ga- and Mo-doped samples with a fine-grained and uniform microstructure.