Magnetic-field-assisted solute partitioning in 2:17-type Sm-Co-Fe-Fe-Zr magnets towards enhancing coercivity

Simultaneously achieving high remanence and high coercivity in hard magnets is highly desired for applications, but is difficult because they are intrinsically trade-off. This work presents magnetic-field-assisted solute partitioning in the pinning-controlled Sm-Co-Fe-Cu-Zr magnets to enhance coercivity without deteriorating remanence. As exhibited in a model commercially-sintered magnet Sm₂₅Co_44.9Fe_21.5Cu_5.6Zr_3.0 (wt.%), applying an external magnetic field during post-aging slow cooling process (defined as magnetic slow cooling, MSC) can effectively enhance coercivity H_cj from 18.50 kOe for the magnet with conventional Non-MSC processing to 21.30 kOe, well retaining the remanence B_r above 11.6 kG. Detailed microstructural and microchemical investigations revealed that MSC accelerates solute partitioning between rhombohedral Sm₂Co₁₇ (2:17R) nanocells and hexagonal SmCo₅ (1:5H) cell boundaries, by enriching Fe/Co contents inside the former that improves its saturation magnetization and enriching Cu content inside the later that strengthens the pinning force to hinder domain wall motions. In addition, MSC can also effectively reduce the cell-edge 2:17R’ phase (alternatively an incompletely-decomposed product with one faulted basal layer in 2:17R lattice, or an intermediate phase of 2:17R equilibrium phase) that was harmful for both remanence and coercivity. Thermodynamically, the beneficial role played by MSC can be attributed to the associated magnetostatic energy, which provides an extra driving force to promote phase transition and solute partitioning. Further study has shown that optimizing MSC cooling rate can further enhance H_cj to 23.30 kOe without deteriorating B_r, demonstrating that the magnetic-field-assisted solute partitioning method is effective to overcoming the remanence-coercivity trade-off in 2:17-type Sm-Co magnets.