Significant effect of ordered micro-domain on cell boundary phase distribution and demagnetization curve squareness of Sm2Co17-type magnet

Cu-rich cell boundary phase is difficult to precipitate evenly, resulting in a generally poor demagnetization curve squareness for Fe-rich Sm2Co17-type magnet, which is a key factor limiting the further improvement of magnetic energy product. In this study, we report that nanoscale strip-like ordered micro-domains distributed in 1:7H disordered matrix phase of the solid solution precursor is a new factor significantly affecting the precipitation and distribution of the cell boundary phase. Long strip-like and continuous micro-twin structure with twin boundaries neatly perpendicular to the C-axis is observed after sintering treatment. After solution treatment, sequential and long strip-like micro-twins gradually transform into disordered state along the basal plane, forming narrow disordered 1:7H (TbCu7-type structure) phase between the separated strip-like ordered micro-domains. This disordering transformation takes place via broken down of the long strip-like ordered micro-domains, which is accomplished by narrowing along the width direction followed by reduction of the length. Furthermore, a new model revealing the effect of the ordered micro-domains on the formation of the cell boundary phase is proposed. Antiphase boundaries enriched in Cu have already existed in the precursor with long strip-like ordered micro-domains. Therefore, the Cu-rich cell boundary phase acting as strong pinning centers cannot be precipitated homogeneously and distributed continuously after aging, resulting in a poor demagnetization curve squareness of Sm2Co17-type magnet. Our results indicate that significant broken down of the nanoscale ordered micro-domains in solution precursor is the key factor improving the distribution of cell boundary phase in Sm2Co17-type magnets.