Ji-Bing Sun

Nanocomposite magnets combining hard Nd2Fe14B and soft α-Fe phases are promising for surpassing the energy product limit of single-phase permanent magnets, but conventional routes using external Fe powders suffer from poor interfacial compatibility and uncontrolled soft-phase coarsening. Here, an in situ precipitation strategy coupled with an ultrahigh melt-spinning speed of 50 m/s is employed to fabricate (Nd2Fe14B)1−xFex (x = 0.1–0.9) ribbons. Increasing Fe content raises both the α-Fe fraction and the amorphous content, with amorphization occurring mainly in the Nd2Fe14B matrix. Strikingly, at an optimal composition of x = 0.5, ultrafine α-Fe grains (12–17 nm) are homogeneously embedded within the Nd2Fe14B nanograins (∼20 nm), yielding a coercivity of 8.6 kOe and enhanced remanence even with high amorphous and α-Fe content. In contrast, excessive Fe (x = 0.7) triggers abnormal growth of some α-Fe particles (120–200 nm), deteriorating the coercivity despite increased maximum magnetization. Micromagnetic simulations corroborate that the exchange coupling is maximized when the soft grain size is confined to ∼10 nm, and the hard grains are appropriately refined. This work demonstrates that synergistic control of composition and rapid solidification kinetics enables the realization of ideal nanostructures, providing a robust pathway for designing high-performance rare-earth permanent magnets.

SmCo₄B-based alloys with high magnetocrystalline anisotropy are expected to be used as raw materials or constituent phases for new permanent magnets. In this work, we develop Sm(Co, Fe, Ni)₄B alloys with excellent hard magnetic properties by tuning the contents of Fe, Co, and Ni. The addition of Fe enhances the amorphous formation ability of the as-spun ribbons, whereas Ni addition improves the structural stability of the crystalline phases. During annealing, the amorphous phase crystallizes into different Sm-Co-B phases in stages. A high coercivity of 5.68–6.71 MA·m⁻¹ is obtained in the annealed SmCo_4–x–yFe_xNi_yB (x = 1.0–2.0, y = 0.8–1.0) ribbons composed of platelet-shaped and equiaxed grains in comparison with the coercivity of 2.89–5.18 MA·m⁻¹ in the x = 1.0–1.2 and y = 0–0.8 ribbons with equiaxed grains. Here, we show the strong correlations between the microstructure and magnetic properties and provide insights for the future development of high-performance SmCo₄B-based magnets.