First-order reversal curve analysis of a Nd-Fe-B sintered magnet with soft X-ray magnetic circular dichroism microscopy

First-order reversal curve (FORC) diagram, which visualizes the variation of magnetic susceptibility on a field plane, has been applied to a Nd-Fe-B sintered magnet. The FORC diagram exhibits the characteristic behavior of two remarkable spots in low-field and high-field regions. The high-field FORC spot corresponds to the irreversible magnetization reversal at a coercive field, whereas the low-field FORC spot indicates the appearance of a large magnetic susceptibility state during the demagnetization process. Moreover, this low-field FORC spot becomes dominant at high temperature, accompanied by a significant reduction in coercivity. These results suggest that the low-field FORC spot has a strong correlation with the degradation of magnetic properties of a Nd-Fe-B sintered magnet. To clarify the actual magnetization reversal processes corresponding to these two FORC spots, soft X-ray magnetic circular dichroism (XMCD) microscopy observation was employed with similar field sequences of the FORC measurements. Consequently, the low-field FORC spot is mainly attributed to the domain wall motion in multi-domain grains, whereas the high-field FORC spot corresponds to the magnetization reversal of single-domain grains. These indicate that a FORC diagram is a powerful evaluation method for the magnetization reversal processes of permanent magnets.