Identifying grain boundary and intragranular pinning centres in Sm2(Co,Fe,Cu,Zr)17 permanent magnets to guide performance optimisation

Permanent magnets draw their properties from a complex interplay of chemical composition and phase, each with their associated intrinsic magnetic properties. Gaining an understanding of these interactions is the key to deciphering the origins of a permanent magnets’ magnetic performance and facilitate the engineering of much improved-performing magnets. Here, we use advanced multiscale microscopy and microanalysis on a bulk Sm2(Co,Fe,Cu,Zr)17 pinning-type high-performance magnet with outstanding thermal and chemical stability. Comparison of the microstructure in regions of different composition, we demonstrate that the pinning of magnetic domains, imaged by nanoscale magnetic induction mapping, is controlled by the composition and atomic arrangement of copper. This is confirmed by micromagnetic simulations. Contrary to the belief that grain boundaries are “weak links” in magnetic materials, we demonstrate grain boundaries undergo magnetization reversal at relatively low fields (0.1-0.3 T), but this remains confined to these regions and does not significantly impact the magnet’s coercivity. Our results showcase that it is the optimal microstructure within the grain itself that is crucial for achieving the desired magnetic properties.

💡 Research Summary

The paper presents a comprehensive multiscale investigation of the microstructural origins of magnetic performance in high‑temperature Sm₂(Co,Fe,Cu,Zr)₁₇ permanent magnets. Two industrially produced samples from the same batch were examined: sample A, which exhibits a reduced coercivity (μ₀Hc ≈ 2.2 T) due to a slightly higher homogenisation temperature, and sample B, the reference magnet with optimal coercivity (μ₀Hc ≈ 3 T). By combining scanning electron microscopy, Kerr microscopy, magnetic force microscopy, transmission electron microscopy, atom probe tomography, Lorentz microscopy, electron holography, and micromagnetic simulations, the authors correlate magnetic domain behaviour with chemical composition and nanoscale structure.

Key findings include: (1) The pinning of magnetic domains is governed primarily by the copper content and its atomic arrangement. In high‑coercivity regions, a continuous Cu‑rich 1:5 cell‑boundary phase of roughly 10 nm thickness surrounds the 2:17 matrix, and Cu‑rich coating layers form on the Z‑platelet lamellae. These “functionalized defects” provide strong exchange‑coupled pinning, as confirmed by micromagnetic modelling. (2) Low‑coercivity regions display a thinner, more discontinuous 1:5 network, a higher density of Z‑platelets, and a larger fraction of the intermediate 2:17R’ phase, which is known to degrade coercivity. Energy‑dispersive X‑ray spectroscopy and atom probe data reveal Cu depletion and higher Sm concentration in these zones. (3) Grain boundaries, traditionally considered weak links, indeed reverse at low fields (0.1–0.3 T) but the reversal is confined to a thin outer layer and does not significantly affect the overall coercivity. The kink observed in hysteresis loops originates from this localized reversal, which slightly reduces the squareness factor and the maximum energy product but is a secondary effect compared with intragranular pinning. (4) Increasing Fe content above ~20 wt % accelerates diffusion, leading to more pronounced segregation of Cu and Zr, a higher residual 2:17R’ fraction, and consequently lower coercivity—consistent with earlier literature.

The study demonstrates that the decisive factor for achieving high coercivity and energy density in Sm‑Co‑Fe‑Cu‑Zr magnets is the optimal microstructure inside the grains, not the grain‑boundary phase. By controlling Cu distribution, ensuring a continuous Cu‑rich 1:5 network, and minimizing the 2:17R’ residual phase, manufacturers can retain high performance even when substituting critical Co with more abundant Fe. These insights provide a science‑driven pathway for designing next‑generation rare‑earth permanent magnets with reduced reliance on critical elements while maintaining superior high‑temperature stability.