The effect of a high magnetic field on coercivity was investigated in the annealing process for sintered Nd-Fe-B magnets containing Dy 10 wt.%. In a sample containing Cu 0.13 wt.%, a coercivity value H c of 36 kOe was obtained for an annealing temperature T a of 475˚C under an annealing field H a of 140 kOe. This value of H c is 20% larger than that for the control sample (T a = 475˚C and H a = 0). In samples without Cu, H c values were considerably smaller (< 27 kOe) and no such enhancement of the coercivity by the magnetic fields was observed. The DSC curves exhibited a marked endothermic reaction just below 500˚C only for samples containing Cu. This temperature is attributed to a eutectic point of Nd 70 Cu 30. A possibility of field-induced alignment of Nd-rich grains in a matrix of Nd-Cu liquid is discussed; this would be closely related to the coercivity enhancement.
In order to understand the coercivity enhancement mechanism by annealing in sintered Nd Fe B magnets, we performed systematic annealing experiments. It was found that an addition of 0.13 of Cu not only enhance the coercivity but also ease the required cooling condition after annealing to obtain high coercivity. Using single successive annealing technique, we observed a 0.3 MA/m jump in coercivity at annealing temperature T a =640°C for Cu free Nd Fe B magnets and at T a =440 460°C for Cu containing ones. We also observed sharp reduction of 0.15 MA/m in coercivity for Cu doped sintered Nd Fe B magnets at T a = 600°C. Furthermore, we confirmed a reversible behavior of coercivity at the annealing temperature range which is lower than the optimal annealing temperature. This implies that some kinds of reversible structural natures depending on the temperature are closely related to the coercivity of sintered Nd Fe B magnets.
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