Atomic structure and magnetic properties of MnFe2O4 nanoparticles produced by reverse micelle synthesis

Using the aqueous cores of reverse micelles as nanoreactors, nanoparticles $\text{(d∼10\hspace{0.05em}}\mathrm{nm})$ of the mixed ferrite ${\mathrm{MnFe}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}}$ were produced. Seven processing trials were performed where the concentration of ammonium hydroxide, reaction temperature, and the oxidizing agent were varied. All trials result in Mn-ferrite particles with varying chemistry and structure. The Mn concentration in the resulting ferrite is strongly enhanced by both the presence of ${\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$ as an oxidizing agent and a surplus of ammonium hydroxide. The increased Mn concentration correlates with a higher fraction of octahedrally coordinated Mn cations. When near-stoic amounts of ammonium hydroxide are used, the resulting ferrites are nearly stoichiometric with a more equitable distribution of Mn cations on the octahedral and tetrahedral sublattices. In all ferrite nanoparticles, the Mn cations have a preference for octahedral site occupancy that is larger than the 20% measured in bulk Mn-ferrite. We attribute the cation filling trends to the stabilization of excess trivalent Mn during processing.