Sintered Nd-Fe-B magnets are the most promising material for a driving motor of the electric vehicle (EV). In this kind of application, very large value of room-temperature coercivity H_c is required owing to the high-temperature circumstance of the EV-motor. Current Nd-Fe-B magnets commercially available for such high H_c applications, therefore, contains huge amount (~10 wt%) of Dy to enhance H_c. But, this method has a crucial disadvantage. That is, the magnetization must be sacrificed owing to the antiparallel coupling of Dy and Fe moments, resulting in smaller energy product values.

It is well known that a heat treatment around 600 C is indispensable to obtain a high coercivity in the Nd-Fe-B sintered magnets. We have been investigating an effect of high magnetic field in this annealing process on the magnetic properties of Nd-Fe-B sintered magnets. We already reported that the 37% increase of H_c occurs, when the sample containing 3.1 wt% Dy and 0.1 wt% Al was annealed at T_a = 550 C under a magnetic field of H_a = 140 kOe [1]. Here, we report on the high-magnetic-field processing work for a series of Dy-free Nd-Fe-B magnets.

The annealing treatments were carried out for samples with the dimensions 5x5x5 mm³ by using a furnace installed in a cryocooled superconducting magnets which generate magnetic fields of up to 140 kOe. Six different values of T_a were selected between 500 and 625 C. In the sample containing small amount of Al and Cu, highest H_c value of 14.7 kOe was obtained for T_a = 550 C and H_a=140 kOe. This value of H_c is 16% larger than that of the control sample (T_a=550 C and H_a=0). These results were discussed in terms of a field-induced change in the interface matching between the main Nd₂Fe₁₄B and intergranular Nd-rich phases.

[1] H. Kato, M. Sagawa, K. Koyama and T. Miyazaki, Appl. Phys. Lett. 84 (2004) 4230.

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1. Small angle neutron scattering study of interface nanostructure in sintered Nd-Fe-B magnets processed under high magnetic fields
2. Magnetostructural Phase Transformation and Shape Memory Effect of Fe-added Ni₂MnGa Films