The performance characteristics of magnetic nanoparticles toward application, e.g., in medicine and imaging or as sensors, are directly determined by their magnetization relaxation and total magnetic moment. In the commonly assumed picture, nanoparticles have a constant overall magnetic moment originating from the magnetization of the single-domain particle core surrounded by a surface region hosting spin disorder. In contrast, this work demonstrates the significant increase of the magnetic moment of ferrite nanoparticles with an applied magnetic field. At low magnetic field, the homogeneously magnetized particle core initially coincides in size with the structurally coherent grain of 12.8(2) nm diameter, indicating a strong coupling between magnetic and structural disorder. Applied magnetic fields gradually polarize the uncorrelated, disordered surface spins, resulting in a magnetic volume more than 20% larger than the structurally coherent core. The intraparticle magnetic disorder energy increases sharply toward the defect-rich surface as established by the field dependence of the magnetization distribution. In consequence, these findings illustrate how the nanoparticle magnetization overcomes structural surface disorder. This new concept of intraparticle magnetization is deployable to other magnetic nanoparticle systems, where the in-depth knowledge of spin disorder and associated magnetic anisotropies are decisive for a rational nanomaterials design.

Size dependence of the total magnetized volume of nanoparticles in a magnetic field.

Understanding the complex link among composition, microstructure, and magnetic properties paves the way to the rational design of well-defined magnetic materials. In this context, the evolution of the magnetic and structural properties in a series of oleate-capped manganese-substituted cobalt ferrites (Mn_xCo_1–xFe₂O₄) with variable Co/Mn molar ratios is deeply discussed. Single-phase ferrites with similar crystallite and particle sizes (about 10 nm), size dispersity (14%), and weight percentage of capping oleate molecules (17%) were obtained by an oleate-based solvothermal approach. The similarities among the samples permitted the interpretation of the results exclusively on the basis of the actual composition, beyond the other parameters. The temperature and magnetic field dependences of the magnetization were studied together with the interparticle interactions by DC magnetometry. Characteristic temperatures (T_max, T_diff, and T_b), coercivity, anisotropy field, and reduced remanence were found to be affected by the Co/Mn ratio, mainly due to the magnetic anisotropy, interparticle interactions, and particle volume distribution. In addition, the cobalt and manganese distributions were hypothesized on the basis of the chemical composition, the inversion degree obtained by ⁵⁷Fe Mössbauer spectroscopy, the anisotropy constant, and the saturation magnetization.