Atomic structure and magnetic properties of MnFe2O4 nanoparticles producedby reverse micelle synthesis

Using the aqueous cores of reverse micelles as nanoreactors, nanoparticles (d similar to 10 nm) of the mixed ferrite MnFe2O4 were produced. Seven proc essing trials were performed where the concentration of ammonium hydroxide, reaction temperature, and the oxidizing agent were varied. All trials resu lt in Mn-ferrite particles with varying chemistry and structure. The Mn con centration in the resulting ferrite is strongly enhanced by both the presen ce of H2O2 as an oxidizing agent and a surplus of ammonium hydroxide. The i ncreased 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 equitabl e distribution of Mn cations on the octahedral and tetrahedral sublattices. In all ferrite nanoparticles, the Mn cations have a preference for octahed ral site occupancy that is larger than the 20% measured in bulk Mn-ferrite. We attribute the cation filling trends to the stabilization of excess triv alent Mn during processing. (C) 1999 American Institute of Physics. [S0021- 8979(99)17708-9].