Role of nanoscale interfacial defects on magnetic properties of the 2:17-type Sm–Co permanent magnets

Abstract

Development of miniaturized magnet-associated devices operated at elevated temperatures requires that the 2:17-type Sm(Co, M)z (M = Fe, Cu, Zr) permanent magnets have large magnetic energy product and high coercivity. Raising iron content is essential to enlarge the energy product, however it leads to serious deterioration in coercivity (Hcj) and squareness factor (SF). In order to uncover the underlying microstructural origins, here we performed a comparative study between two nanocell structured Sm(Co0.68Fe0.22Cu0.07Zr0.03)7.5 magnets with different magnetic properties. We found that the nanoscale interfacial defects, including the retained phase at the grain edges and the intermediate rhombohedral phase (2:17R’) at the cell edges, act as extra defects that are detrimental to the coercivity as well as the squareness, besides the formerly-reported incomplete cell walls and discontinuous platelets. These microstructural defects build up inhomogeneous pinning sites, at the interfaces between the cell walls and the cell interiors, and at the intersections of the cell walls and the Zr-rich platelets. The comparative results show that the magnet with Hcj = 34.39 kOe and SF = 62.08% contains much less microstructural defects than the one with Hcj = 24.42 kOe and SF = 47.08%. Our work highlights the importance of reducing microstructural defects for achieving high coercivity and large squareness in the iron-rich Sm(Co, M)z permanent magnets.