Abstract: The magnetic performance of Ce-Fe-B permanent magnets is compromised by the emergence of non-magnetic CeFe₂ phase during synthesis. In this study, partial substitution of Ce with La was employed to synthesize Ce_17-xLa_xFe_76.5Co₁Zr_0.5B₆ (x = 0-5) alloys using the melt-spinning technique. Among these, the alloy with x = 3 exhibited the best magnetic performance and was subsequently subjected to magnetic field annealing to investigate its effects on magnetic properties, phase constitution, and microstructural evolution. The results indicate that La addition effectively suppresses the precipitation of the CeFe₂ phase and promotes grain refinement. At x = 3, the alloy achieved optimal overall magnetic properties, with an intrinsic coercivity (H_ci) of 485.32 kA/m, remanence (B_r) of 0.55 T, squareness (H_k/H_ci) of 0.57, and a maximum energy product ((BH)_max) of 44.61 kJ/m³—representing improvements of 12.3 %, 34.2 %, 90.0 %, and 71.0 %, respectively, compared to the alloy without La addition. Transmission electron microscopy (TEM) and atom probe tomography (APT) analyses revealed that the as-spun sample consisted of the Ce₂Fe₁₄B phase, a Co-rich CeFe₂ phase, a Co-poor (Ce,La)Fe₂ phase, and a minor amorphous component. After annealing at 445 K under a 1 T magnetic field, the metastable (Ce,La)Fe₂ phase decomposes and the amorphous phase crystallizes, resulting in an alloy composed exclusively of the Ce₂Fe₁₄B and CeFe₂ phases. Precession electron diffraction (PED) analysis quantitatively verified that magnetic annealing treatment reduces residual strain within the alloy and suppresses the formation of the 1:2 phase. These microstructural improvements significantly enhanced ferromagnetic exchange coupling between hard magnetic phase grains, increasing the B_r to 0.59 T and the (BH)_max to 50.05 kJ/m³—representing gains of 7.3 % and 12.2 %, respectively, compared to the as-spun sample. The synergistic effect of La substitution and magnetic field annealing provides an effective strategy for suppressing CeFe₂ phase precipitation and developing high-performance Ce-Fe-B permanent magnets.

Key words: Ce-Fe-B magnet, La content, Magnetic field annealing, Hard magnetic properties, Microstructure