Measuring and modeling extended magnetic structures in chemically homogeneous manganese ferrite nanoparticle assemblies

Magnetic nanoparticles are key to many important biomedical and sensor applications, but it is difficult to determine the underlying interactions that dictate their macroscopic behavior. Here, we discuss results on ordered assemblies of 7.6 nm diameter chemically homogeneous MnFe₂O₄ nanoparticles. The particle assemblies have been investigated using polarization-analyzed small angle neutron scattering (PASANS) methods to allow for clean separation of magnetic vs. structural features over a range of temperatures (5-400 K) and magnetic fields (0 to 1.4 T). While the magnetic behavior in remanence mostly follows a single particle form factor at high temperature, the scattering deviates as the sample is cooled and grows maximally at intermediate temperatures (~50-100 K) versus at the lowest temperature (5 K). In addition, intermediate field values display a magnetic structure canted from field alignment. The data are interpreted through models ranging from a Lorentzian-squared correlation to a mass fractal to a micromagnetic simulation approach coupled with a neutron scattering calculator. In the latter case, magnetic anisotropy is found to play a critical role in dictating the balance against competing exchange, dipolar coupling, and Zeeman energy terms. These results illustrate the ability of PASANS to extract important three-dimensional magnetic features in nanoscale systems.