New Acta Materialia publication

Direct observation of nanoscale pinning centers in Ce(Co0.8Cu0.2)5.4 permanent magnets

2026/02/02

(a) Hysteresis and initial magnetization curves for the low and high Hc samples. (b) 3D atom probe tomography (APT) data comparing low Hc and high Hc samples. Top: 3D APT reconstructions of low Hc and high Hc samples, respectively, with Co (dark blue), Ce (red), Cu(light blue); Middle: 2D Cu concentration maps from 10 nm thick slices; Bottom: 1D concentration profiles extracted from cylinders (5 nm diameter, 100 nm length). (c) Magnetic imaging of Ce(Co0.8Cu0.2)5.4 high Hc sample: Fresnel Lorentz Transmition Electron Microscopy image, showing black and white contrast at the positions of zigzag domain walls (DW), nucleated under applied field B = 1T. The image is recorded in magnetic-field-free conditions at a defocus of 1 mm. (d) Magnetic induction map of a DW region indicated by a yellow dotted square in (c). Arrows and colours indicate projected in-plane magnetic field directions. (e) Schematic representations of the coercivity mechanism in Ce(Co0.8Cu0.2)5.4 obtained by micromagnetic simulation: High Hc sample shows pronounced chemical contrast and preferred orientation of Cu-poor cylindrical cells leads to strong pinning at Cu-rich cell boundaries. Low Hc sample demonstrates weaker compositional contrast and random cell orientation reduce pinning effectiveness. The DW (dashed line) propagates more easily through the less segregated structure.

In our recent work, published in Acta Materialia, we resolve a long-standing question about the physical mechanism responsible for coercivity (the resistance to demagnetization) in Ce(Co_1-xCux)₅ permanent magnets. This problem has been debated in the scientific literature for several decades, and now, by combining the expertise of several research groups specializing in complementary advanced microscopy techniques, including magnetic imaging and atomic-resolution electron microscopy, we were able to provide a definitive explanation for the origin of magnetic hysteresis in this material system. In parallel, we employed micromagnetic modeling to link our experimental observations directly to the underlying magnetic behavior, revealing the physical basis of the unique properties of Ce(Co_1-xCux)₅ magnets.

Achieving a high-coercivity state in Ce(Co_1-xCux)₅ alloys requires a carefully controlled annealing process that produces a complex nanostructure. During heat treatment, a copper-depleted CeCo5 matrix forms, within which copper-rich Ce(Co_1-xCux)₅ nanoinclusions develop. These nanoscale inclusions hinder the motion of magnetic domain walls, thereby stabilizing the magnetized state.

In our study, we investigated two Ce(Co_0.8Cu_0.2)_5.4single crystals: one subjected to optimized annealing and another annealed at 1000 °C and rapidly quenched in water, allowing us to directly compare their nanostructure and magnetic properties. Using transmission electron microscopy and atom probe tomography, we identified a nanoscale cellular structure formed through spinodal decomposition. In the optimally annealed magnet, Cu-poor cylindrical cells (~5–10 nm in diameter, ~20 nm long) have a disordered CeCo₅-type structure and a composition Ce(Co_0.9Cu_0.1)_5.3. Cu-rich cell boundaries are ~ 5 nm thick and exhibit a modiﬁed CeCo₅ structure, with Cu ordered on the Co sites and a composition Ce (Co_0.7Cu_0.3)_5.0. Micromagnetic simulations show that steep copper concentration gradients (up to 12 atomic percent per nanometer) create strong spatial variations in magnetocrystalline anisotropy and domain-wall energy, leading to effective domain-wall pinning and high coercivity.

Compared to commercial high-temperature Sm₂Co₁₇-type magnets, Ce(Co_0.8Cu_0.2)_5.4 exhibits a finer-scale version of the conventional pinning mechanism, with more subtle structural and chemical variation at the nanoscale. The identification of this nanoscale chemical segregation in an otherwise nearly single-phase magnet provides a microstructural explanation for the long-observed phenomenon of “giant intrinsic magnetic hardness” in systems such as SmCo5-xMx, highlighting new opportunities for designing rare-earth permanent magnets through controlled nanoscale chemical engineering.

This work is the result of very close scientific cooperation between various institutes namely: Institute of Materials Science, Technische Universität Darmstadt; Max Planck Institute for Sustainable Materials, Düsseldorf; Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich; Faculty of Physics and Center for Nanointegration (CENIDE), Universität Duisburg-Essen, Duisburg. The manuscript was published in the framework of Collaborative Research Centre/Transregio (CRC/TRR) 270, funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), Project ID No 405553726.

N. Polin, S. Shen, F. Maccari, A. Aubert, E. Adabifiroozjaei, T. Smoliarova, Y. Yang, X. Chen, Y. Skourski, A. Saxena, A. Kovács, R. E. Dunin-Borkowski, M. Farle, B. Xu, L. Molina-Luna, O. Gutfleisch, B. Gault and K. Skokov.
Direct observation of nanoscale pinning centers in Ce(Co0.8Cu0.2)5.4 permanent magnets
Acta Mater. 307, 121906 (2026).
DOI: 10.1016/j.actamat.2026.121906