Field-induced alignment of Nd-rich phase and coercivity in sintered Nd-Fe-B magnets

摘要

These is an imperative need to develop a nature-friendly electric vehicle (EV) as an alternative transportation. Sintered Nd-Fe-B magnets are the most promising materials for a driving motor of the hybrid EV and the forthcoming pure EV. Since the operating temperature of magnets in these high-power motor applications reaches beyond 200°C, a very large value of coercivity H{sub}c is necessary at room temperature. Current Nd-Fe-B magnets commercially available for such high H{sub}c applications, therefore, contains a huge amount (～10wt.%) of Dy to enhance H{sub}c. But, this method has at least two crucial disadvantages. First problem is an estimated short supply of Dy in the EV mass-production stage, owing to its low natural abundance as compared with that of Nd. The second inferiority in using Dy arises from the antiparallel coupling of Dy and Fe moments, which results in smaller magnetization and much-reduced energy product values. It is well known that a heat treatment around 600°C is indispensable to obtain a high coercivity in the sintered Nd-Fe-B magnets. We have been investigating an effect of high magnetic field in this annealing process on the magnetic properties. The characteristic value of this experiment is the Field-enhanced Coercivity Ratio (FCR), which is defined to be a difference of H{sub}c with and without annealing field, divided by an original H{sub}c value. We already reported that the FCR value of 37% can be achieved, when the sample containing 3.1wt.% Dy and 0.13wt.% Al was annealed at T{sub}a=550°C under a magnetic field of H{sub}a=140kOe [1]. However, detailed mechanism of such coercivity enhancement phenomena has been left unclarified. In this work, we present a more systematic study for a series of Nd-Fe-B magnets with and without Dy, containing a small amount of Al and Cu. We propose a possibility of field-induced alignment of Nd-rich grains during the annealing, which would make a better lattice matching in the interface structure.