For the purpose of increasing the coercivity of the Nd Fe B sintered magnets without Dy substitution, we explored a possibility of enhancing the coercivity by grain size reduction of the sintered body by employing the starting powder with an ultimately small particle size. To produce Nd Fe B alloy powder with such a small particle size, and to handle it to produce sintered bodies without oxidation, we developed a special jet mill and a special process called pressless process (PLP). By utilizing these techniques, it has become feasible to produce Nd Fe B sintered magnets with a coercivity of 1.59 MA/m without Dy substitution which is compared with 0.87 MA/m for the commercially available Nd Fe B sintered magnets containing no Dy.

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.

In order to trace the annealing temperature dependence of the coercivity in sintered Nd-Fe-B magnets systemtically, we examined a single successive annealing (SSA) method, in which the coercivity value of a single sample is successively recorded after each annealing cycle at T a = 400˚C -710˚C with increments of ∆T a = 10˚C. In addition to the coercivity values, we paid special attention to the shapes of demagnetization curves, in terms of the differential susceptibility versus magnetic field plot, which provides information on the nucleation field distribution (NFD). Samples with small amounts of Cu additives showed a more significant temperature dependence of the coercivity than those without Cu additives.

The Dy free and Dy substituted (0 100) Nd Fe B sintered magnets based on the NEOMAX 50 type magnet were prepared, and the magnetic properties such as the saturation magnetizations, the coercivities and the magnetic anisotropy constants (K 1 , K 2 ) were determined by the obtained data using an high field magnetometer. The domain structures, especially domain widths were measured using MOKE and SEM, and the sizes of magnetic interacted regions were calculated based on the resulted values. The coercivity of the magnets is subjected to, first: the magnetic anisotropy field, second: crystal grain sizes, third: size of group of crystal grains behaving as a cooperated region that reflects the magnetic interaction through the grain boundary (GB).