High performance and exchange coupling in magnetization reversal of sintered (Nd,Dy)-Fe-B magnets

Abstract

Improving the coercivity in sintered Nd-Fe-B magnets would sacrifice the remanence and the energy product. In this work it develops a method of preparing the composite (Nd,Dy)-Fe-B magnets to enhance the coercivity with a slighter decrease of remanence. The high performance (Nd,Dy)-Fe-B magnets were prepared via adding Dy-Fe-B into Nd-Fe-B powders in blending, followed by compacting and sintering. The addition of Dy-Fe-B powders gave rise to the enhancement of coercivity in the sintered magnets. The investigation of thermal activation indicates that the fluctuation field in thermal activation doesn’t obey the law of increasing with the coercivity. The grains of Dy-Fe-B crystal phase coexist in (Nd,Dy)-Fe-B magnets, which could increase the domain wall size by exchange coupling and decrease the fluctuation field in magnetization reversal. Some of Dy atoms diffuse into the grain outer-layer of Nd2Fe14B crystal phase, the grain boundary structure is probably optimized, the anisotropy increases at grain shell, and so the domain wall size would be reduced, leading to an increase of the fluctuation field. Though their effects of the exchange coupling and the anisotropy increasing are opposite on the changes of the domain wall size and fluctuation field, they are beneficial to improve the nucleation field in magnetization reversal, and the coercivity increases significantly in the sintered magnets. Since Dy-Fe-B powders mainly evolve into the main phase of RE2Fe14B crystal instead of the intergranular phase, the mass fraction of ferromagnetic phase doesn’t decrease evidently. Owing to the exchange coupling effect, the core-shell morphology of grains and the no remarkable increase in the amount of intergranular phase, the high performance with high energy product of 46.6 MGOe and high coercivity of 20.5 kOe could be obtained in the sintered (Nd,Dy)-Fe-B composite magnets.