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 degrees 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 degrees C annealing, which suggests an existence of soft magnetic grains. We then annealed this sample in zero field successively from T-a = 400 degrees C to 720 degrees C with the temperature increment of 10 degrees 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 degrees 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.