Micromagnetic analysis of nanocrystalline Ga- and Nb/Mo-doped NdFeB permanent magnets

Abstract

Nanocrystalline as well as microcrystalline melt-spun ribbons of NdFeB alloys with small additions of Ga and Nb or Mo were prepared using the single-roller technique. Average grain sizes of 25 nm up to 500 nm were obtained by varying the wheel speed and the parameters of the heat treatment after rapid quenching. Systematic investigations of the relationship between microstructural effects and magnetic properties were carried out according to micromagnetic concepts. The theory of nucleation in single-domain particles is able to explain the magnetization reversal process, if several deteriorating effects of the microstructure are taken into account. The additives Ga and Nb/Mo both improve the coercive field. Ga is known to form new intergranular phases leading to a better decoupling of the grains. High-melting Nb/Mo-containing precipitates act as nucleation centers. Ga- and Nb/Mo-doped-samples show therefore small magnetization reversal intervals and a high reversibility of the first part of the demagnetization curve. Furthermore the concept of microstructural parameters α K and N eff is applied to an extensive investigation of the temperature dependence of coercivity. For different microstructures it turns out that α K reaches saturation as required by theory whereas a wide spectrum of N eff is found. Nevertheless a strong correlation of both α K and N eff restricts the improvement of the coercive field. Best critical fields at room-temperature up to 2.5 T have been obtained for annealed Ga-and Nb-doped samples with a fine-grained nanocrystalline and uniform microstructure.