Suppression of metastable phases in Nd₂Fe₁₄B/<i>α</i>-Fe nanocomposite magnets through laser-heated annealing with ultrafast heating rates

Abstract Nd 2 Fe 14 B/ α -Fe nanocomposite magnets are known for their superior theoretical energy products, but a significant challenge lies in eliminating harmful metastable phases, which often form during the crystallization process of amorphous materials. This study presents a strategy to simultaneously eliminate metastable phases and reduce grain sizes in Nd 2 Fe 14 B/ α -Fe nanocomposites through non-equilibrium laser-heated processing with ultrafast heating rates. The technique is applied to ternary amorphous Nd (7.5+x) Fe (86.5-x) B 6 (x = 0, 1.5, 4.3) alloys. In Nd 9 Fe 85 B 6 alloys, an ultrafast heating rate (∼400 °C s −1 ) during laser-heated annealing increases the short-range order ( R s ) of the amorphous matrix from 0.52 nm to 0.57 nm. This also lowers the crystallization temperature and promotes a direct transformation from the amorphous phase to the stable phase, effectively suppressing the formation of metastable phases during crystallization. As a result, the saturation magnetization ( M s ) increases from 1.59 T to 1.64 T. The refined grain sizes improve coercivity ( H ci ) and the squareness of the demagnetization curve, which are attributed to the reduced crystallization temperature from 750 °C to 700 °C necessary to produce the Nd 2 Fe 14 B/ α -Fe nanostructure. Further investigations into the crystallization behavior of amorphous alloys with varying Fe contents reveal that the inhibition of harmful metastable phases formation becomes more pronounced as the Fe content increases. These findings demonstrate an effective approach for mitigating metastable phase formation during the amorphous crystallization process, thereby enhancing the saturation magnetization of Nd 2 Fe 14 B/ α -Fe nanocomposite magnets. This approach offers a pathway for developing high-performance nanocomposite magnets with high Fe content.