Role of primary Zr-rich particles on microstructure and magnetic properties of 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets

Trace Zr addition is essential for achieving desired cellular nanostructure and large coercivity in the pinning-controlled 2:17-type Sm-Co-Fe-Cu-Zr magnets that have served as the strongest high temperature permanent magnets for over 40 years. However, accompanying this is the formation of Zr-rich particles that may deteriorate the hard magnetic properties. Besides the formerly-reported 1:3R Zr-rich platelets, in this work, 1–2 μm sized Zr₆(Co, Fe)₂₃ (6:23) particles (Fm 3¯ m) and 100–200 nm sized 1:3R Zr-rich particles were also found based on combined structural identifications and element mapping analysis. Around such particles, the desired 1:5H cell wall precipitates that provide the strongest pinning force of magnetic domain wall motions are rare, forming the precipitate-free-zones (PFZs). The 1:5H-PFZs and the soft magnetism of both 6:23 and 1:3R Zr-rich particles act as local weak pinning points, which are unfavorable to retain the large magnetization in strong opposite fields and lead to poor squareness. As observed in a Sm₂₅Co_45.9Fe_19.5Cu_5.6Zr_4.0 (wt.%) magnet, the co-existence of such Zr-rich particles and the associated 1:5H-PFZs leads to a pretty low squareness factor of only 52.89 % given the large coercivity of 29.04 kOe. Our findings suggest that careful controlling the Zr content and avoiding its aggregation to form harmful 6:23 and 1:3R Zr-rich particles are essential for achieving high squareness as well as large energy product in the Sm-Co-Fe-Cu-Zr permanent magnets.