Revealing the intrinsic magnetic landscape of cobalt-doped ferrite nanoparticles

Cobalt ferrite nanoparticles are a benchmark among low-to-medium energy alternatives to rare-earth permanent magnets, although their intrinsic behavior is often obscured by surface disorder, finite-size effects, and superparamagnetic relaxation. Here, we overcome these limitations by synthesizing large, highly crystalline cobalt-doped ferrite nanoparticles (≈25 nm), which remain blocked at room temperature and thus provide a clean platform to disentangle the fundamental role of cobalt in the spinel lattice. By systematically varying the cobalt content, we reveal a complex interplay between cation distribution, oxygen vacancy formation, and magnetic response. Structural and compositional analysis confirms predominant Co2+ occupancy at octahedral sites, accompanied by a redistribution of Fe2+/Fe3+ and non-linear oxygen vacancy generation. We find that while saturation magnetization is largely governed by defect chemistry, the coercivity and effective anisotropy are primarily controlled by cobalt incorporation and saturate at intermediate compositions. In contrast, thermomagnetic analysis reveals an anomalous evolution of magnetization at intermediate temperatures for specific cobalt contents. This behavior is consistent with a change in the anisotropy landscape, suggestive of a growing contribution from higher-order anisotropy terms, rather than a simple uniform increase in magnetocrystalline anisotropy. These results indicate that cobalt doping tunes the balance between different anisotropy contributions in a composition- and temperature-dependent manner. Overall, our findings highlight the subtle interplay between cation distribution, anisotropy landscape, and thermal stability in spinel ferrites, providing fundamental insight for the design of high-coercivity rare-earth-free nanomagnets.