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

Permanent magnets containing rare earth elements are essential components for the electrification of society. Ce(Co 1-x Cu x ) 5 permanent magnets are a model system known for their substantial coercivity, yet the underlying mechanism remains unclear. Here, we investigate Ce(Co 0.8 Cu 0.2 ) 5.4 magnets with a coercivity of ∼1 T. Using transmission electron microscopy (TEM) and atom probe tomography (APT), we identify a nanoscale cellular structure formed by spinodal decomposition. Cu-poor cylindrical cells (∼5-10 nm in diameter, ∼20 nm long) have a disordered CeCo 5 -type structure and a composition Ce(Co 0.9 Cu 0.1 ) 5.3 . Cu-rich cell boundaries are ∼ 5 nm thick and exhibit a modified CeCo 5 structure, with Cu ordered on the Co sites and a composition Ce(Co 0.7 Cu 0.3 ) 5.0 . Micromagnetic simulations demonstrate that the intrinsic Cu concentration gradients up to 12 at.% Cu/nm lead to a spatial variation in magnetocrystalline anisotropy and domain wall energy, resulting in effective pinning and high coercivity. Compared to Sm 2 Co 17 -type magnets, Ce(Co 0.8 Cu 0.2 ) 5.4 displays a finer-scale variation of conventional pinning with lower structural and chemical contrast in its underlying nanostructure. The identification of nanoscale chemical segregation in nearly single-phase Ce(Co 0.8 Cu 0.2 ) 5.4 magnets provides a microstructural basis for the long-standing phenomenon of "giant intrinsic magnetic hardness" in systems such as SmCo 5-x M x , highlighting avenues for designing rare-earth-lean permanent magnets via controlled nanoscale segregation.