Demagnetization process and magnetic seeds for R-Fe-B permanent magnets

To investigate the demagnetization process of R-Fe-B permanent magnets (R = rare earths), magnetic hysteresis loops for Nd15Fe77B8and-Pr15Fe77B8magnets were measured at temperatures from 4.2 to 300 K by using a vibrating sample magnetometer and a 100 kOe superconducting magnet. The hysteresis loops were also measured for Nd15Fe77B8magnets coated with Nd by sputtering to examine the nucleation mechanism at surface defects. The demagnetization curve for Nd15Fe77B8magnet exhibits a stepwise decrease in a temperature range from 4.2 to 150 K where a spin reorientation occurs. On the contrary, those of Pr15Fe77B8show a less remarkable stepwise decrease, because the Pr magnet exhibits no spin reorientation. This stepwise decrease is quantitatively explained in terms of a spin reorientation, together with the misorientation of the c-axis in each grain about the aligned axis. The magnetization reversal for the Nd-Fe-B magnet without Nd-coating occurs with many large Barkhausen jumps. The Nd-coating is effective in increasing the coercive force significantly. This effect is discussed in terms of magnetic seeds for the nucleation of magnetization reversal.