Enhancing Coercivity of La/Ce‑Rich RE‑Fe‑B Magnets via Dy₂O₃–Ca Co‑Doping

Abstract

Rare‑earth‑rich RE‑Fe‑B permanent magnets traditionally suffer from low coercivity, limiting their commercial appeal despite the cost and abundance advantages of La and Ce. In this study we employed an industrial alloy (RE₁₀₀ = La₃₀.₆Ce₅₀.₂Pr₆.₄Nd₁₂.₈) and fabricated RE‑Fe‑B magnets through high‑energy ball milling followed by vacuum annealing. Co‑doping with 7 wt.% Dy₂O₃ and 2.3 wt.% Ca produced a dramatic coercivity increase from 2.44 kOe to 11.43 kOe. X‑ray diffraction and TEM reveal that Dy atoms incorporate into the RE₂Fe₁₄B lattice, boosting magnetocrystalline anisotropy, while Ca reduces Dy₂O₃, promoting the formation of Dy‑rich nanocrystals that hinder domain‑wall motion. These synergistic effects underpin the superior magnetic performance achieved with highly abundant rare earths.

Background

La and Ce are attractive for permanent magnet production due to their abundance and low cost, yet their reduced magnetocrystalline anisotropy in La₂Fe₁₄B and Ce₂Fe₁₄B compromises performance. Extensive research has focused on substituting Nd with La/Ce in Nd‑Fe‑B magnets and optimizing microstructure to mitigate losses. Heavy rare earths such as Dy or Tb are well‑known enhancers of coercivity and thermal stability; however, their high cost can negate the benefits of La/Ce usage. Low‑cost Dy and Tb oxides, combined with a reducing agent, offer a viable alternative. Calcium has emerged as an effective reductant in the Ca‑reduction‑diffusion process, enabling the transformation of rare‑earth oxides into active alloying elements during mechanical milling and subsequent annealing.

Methods

We arc‑melted the industrial RE alloy (RE₁₀₀ = La₃₀.₆Ce₅₀.₂Pr₆.₄Nd₁₂.₈, 99.5 wt.%) together with Fe (99.9 wt.%) and an Fe–B alloy (RE₁₃.₆Fe₇₈.₄B₈, 99.5 wt.%) to produce a nominal composition of RE₁₃.₆Fe₇₈.₄B₈. The ingot was pulverized and milled in an argon‑filled glove box using steel balls (12 mm) at 700 rpm for 5 h. Dopants were added before milling: 2.3 wt.% Ca (sample MC), 3 wt.% Dy₂O₃ (M3D), 7 wt.% Dy₂O₃ (M7D), and the co‑dopant combination 7 wt.% Dy₂O₃ + 2.3 wt.% Ca (M7DC). A reference sample without dopants (RM) was also prepared. Post‑milling annealing was carried out at 620–780 °C for 10 min under high vacuum (<1.3 × 10⁻³ Pa). Phase identification employed Cu‑Kα X‑ray diffraction (λ = 0.15406 nm). Magnetic hysteresis was measured at room temperature with a LakeShore 7404 VSM; samples were pressed into 2 mm × 4 mm cylinders and corrected for demagnetization (factor 0.28). Temperature‑dependent magnetization (300–700 K) and microstructure were characterized by PPMS and JEM‑2100F TEM, respectively.

Results and Discussion

Figure 1 shows that all annealed samples predominantly contain the RE₂Fe₁₄B phase. Detailed XRD analysis reveals lattice shrinkage with Dy₂O₃ doping and both a‑ and c‑axis contraction with the co‑dopants, indicating Dy incorporation into the 2:14:1 lattice. Ca alone induces a modest cell contraction, suggesting partial substitution of RE sites. The total volume reduction in the co‑doped sample exceeds the sum of the individual effects, confirming Ca’s role in promoting Dy entry.

Magnetization versus temperature curves (Figure 2) demonstrate a modest Curie temperature rise from 551.5 K (RM) to 564.5 K (Ca‑doped) and 566.1 K (co‑doped), consistent with Dy and Ca incorporation. Coercivity measurements (Figure 3) illustrate a pronounced enhancement: M7D reaches 7.65 kOe, while M7DC attains 11.43 kOe—an increase of 9.1 kOe over RM. The co‑doped sample displays a dual nucleation–domain‑wall‑pinning reversal mechanism, as evidenced by the initial magnetization curve.

Microstructural investigation (Figures 5 and 6) reveals a nanocrystalline matrix interspersed with coarse Dy/Ca‑rich grains. Energy‑dispersive spectroscopy confirms high Dy concentrations in these grains, which serve as effective pinning sites due to their elevated anisotropy. The Ca‑driven reduction of Dy₂O₃ enhances Dy diffusion during annealing, facilitating the formation of these beneficial microstructural features.

Conclusions

Co‑doping La/Ce‑rich RE‑Fe‑B magnets with Dy₂O₃ and Ca dramatically raises coercivity from 2.44 kOe to 11.43 kOe. Ca promotes Dy incorporation into the RE₂Fe₁₄B lattice through a reductive reaction, yielding a nanocrystalline structure with Dy‑rich grains that impede domain‑wall motion. This approach offers a cost‑effective pathway to high‑performance permanent magnets dominated by abundant rare earths.