三元共晶温度以上で強磁場熱処理したNd-Fe-B系焼結磁石の低温熱処理と減磁曲線

摘要

Effect of magnetic-field direction applied during annealing at 800℃ on the demagnetization process was investigated systematically in sintered Nd-Fe-B magnets. We observed a stepwise decrease in the demagnetization curve when the perpendicular field was applied during the 800℃ annealing, which suggests an existence of soft magnetic grains. We then annealed this sample in zero field successively from T_a = 400℃ to 720℃ with the temperature increment of 10℃, and traced a change in demagnetization curves. The volume fraction V_(soft) of the soft magnetic grains was estimated from the step width of the demagnetization curves. The V_(soft) value showed a maximum around T_a = 500℃ with V_(soft) = 25%, and decreased rapidly with increasing T_a. The coercivity versus T_a plot exhibited a similar tendency to that of V_(soft) versus T_a plot. Origin of such soft magnetic grains is considered to be a precipitation of perpendicularly-aligned Nd_2Fe_(14)B small particles in the grain boundary, which would act as a nucle-ation site on the large Nd_2Fe_(14)B host matrix. These results suggest that the magnetic coupling between Nd_2Fe_(14)B grains is one of the important factors to understand the coercivity mechanism in sintered Nd-Fe-B magnets.%Nd-Fe-B系焼結磁石ほ，現在世界最強の希土類磁石であり，その用途は電気自動車の駆動用モーターなどへ拡大しつつある．ところが，高負荷条件下では200℃近くまで磁石の温度が上昇するため，不可逆的な熱減磁が起こることが問 題となっている．その解決のためには，保磁力を上昇させることが有効であり，現在様々な指針に基づいた多くの研究が行われている．