Tuning the Thermal Stability of Molecular Precursors for the Nonhydrolytic Synthesis of Magnetic MnFe₂O₄ Spinel Nanocrystals

Variations on thermal stabilities were studied when the ligands of coordination compounds of manganese (II) acetylacetonate (Mn(acac)2) and iron (III) acetylacetonate (Fe(acac)3) were substituted with benzoylacetonate (bzac). By replacing the acetylacetonate ligand with the benzoylacetonate ligand, the difference of onset thermal decomposition temperatures can be brought within 15° between Fe(bzac)3 and Mn(bzac)2, which shows the feasibility of tuning thermal stability to produce more suitable molecular precursors for the materials syntheses. The closeness in the decomposition temperatures of these two compounds enables the synthesis of high-quality manganese ferrite, MnFe2O4, nanocrystals with tunable sizes from 3 to 12 nm through a combination of a non-hydrolytic reaction and a seed-mediated growth process. Studies on magnetic properties clearly indicate that superparamagnetic properties such as blocking temperature (TB) and coercivity (HC) are strongly dependent on the size of these MnFe2O4 nanocrystals, which is consistent with the Stoner−Wohlfarth single domain theory.