On the reversible and fully repeatable increase in coercive field of sintered Nd–Fe–B magnets following post sinter annealing

The effect of different heat treatments on coercivity (Hc) has been investigated in three different Nd–Fe–B magnets. These were: commercially available Dy-free and Dy-containing Nd–Fe–B sintered magnets, both with minor additions of Al and Cu, and a third Nd–Fe–B sintered magnet which contained no measurable Cu but a similar amount of Al to the other materials. Heat treating the materials in the as-sintered state at 500 °C for 1 h resulted in an increase in Hc of ≈30% in all cases. Taking these annealed materials and heat treating them at 1050 °C for 1 h resulted in a reduction in Hc of a similar magnitude. This process was repeated several times and Hc changed by ≈30% each time. The Hc increase was therefore not only reversible but also fully repeatable. The remanence and oxygen content were stable throughout. Experiments with annealing duration showed that the beneficial effects to Hc were complete after <5 min at 500 °C. A simple mechanism to explain the results was proposed, which implied that differences in the distribution of the Nd-rich phases between the low and high Hc states should be present. Scanning and transmission electron microscopy confirmed the presence of such differences. In the Cu doped materials, the increase in Hc was linked to the melting of a Nd–Cu eutectic. This reaction was not observed in the low-Cu material, indicating that it was not essential for the increase in Hc.