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

Abstract

To investigate the demagnetization process of R-Fe-B permanent magnets (R = rare earths), magnetic hysteresis loops for Nd 15 Fe 77 B 8 and-Pr 15 Fe 77 B 8 magnets 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 Nd 15 Fe 77 B 8 magnets coated with Nd by sputtering to examine the nucleation mechanism at surface defects. The demagnetization curve for Nd 15 Fe 77 B 8 magnet exhibits a stepwise decrease in a temperature range from 4.2 to 150 K where a spin reorientation occurs. On the contrary, those of Pr 15 Fe 77 B 8 show 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.