Low-temperature annealing and demagnetization curves in sintered Nd-Fe-B magnets high-field annealed at temperatures above ternary-eutectic point

Effect of magnetic-field direction applied during annealing at 800°C 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°C annealing, which suggests an existence of soft magnetic grains. We then annealed this sample in zero field successively from T_a = 400°C to 720°C with the temperature increment of 10°C, 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°C 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 Nd₂Fe₁₄B small particles in the grain boundary, which would act as a nucleation site on the large Nd₂Fe₁₄B host matrix. These results suggest that the magnetic coupling between Nd₂Fe₁₄B grains is one of the important factors to understand the coercivity mechanism in sintered Nd-Fe-B magnets.