2011 Volume 75 Issue 4 Pages 193-197
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 Ta=400°C to 720°C with the temperature increment of 10°C, and traced a change in demagnetization curves. The volume fraction Vsoft of the soft magnetic grains was estimated from the step width of the demagnetization curves. The Vsoft value showed a maximum around Ta=500°C with Vsoft=25%, and decreased rapidly with increasing Ta. The coercivity versus Ta plot exhibited a similar tendency to that of Vsoft versus Ta 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.
