Effects of Annealing on the Coercivity in Sintered Nd-Fe-B Magnets

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 degrees C for Cu-free Nd-Fe-B magnets and at T-a=440-460 degrees 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 degrees 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.