Polarization-analyzed small-angle neutron scattering (PASANS) methods are used to determine the spin arrangements and experimental length scales of magnetic correlations in ordered three-dimensional assemblies of $\sim 7.6$ nm diameter chemically homogeneous manganese ferrite nanoparticles. While the perpendicular magnetic scattering at remanence mostly follows single particle behavior at high temperature, the scattering deviates upon cooling the particles, growing maximally at intermediate temperatures ($\approx$50–100 K) with intermediate field values also displaying canted magnetic structures. The data are fit with a sphere-based mass fractal model and are interpreted further via a micromagnetic simulation approach. The unusual temperature and field dependencies are explained in terms of a delicate balance between magnetic anisotropy and demagnetization. These results highlight the ability of PASANS to extract key features in nanoscale systems and the importance of correlating micromagnetic simulations.

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• Magnetic nanoparticles
• Micromagnetic modeling
• Neutron scattering