Highly crystalline and colloidal $Mn_3O_4$ nanoparticles 11 nm in diameter have been prepared by thermal decomposition of $Mn(acac)_2$ (acac = acetylacetonate) in oleylamine( 1: 24 molar ratio) at 150℃ under inert atmosphere.
When 10 equiv. of water was introduced to the reaction slurry of $Mn(acac)_2$ in oleylamine( 1: 24 molar ratio) at 150℃, pure 29 nm MnO nanoparticles were obtained without contamination by $Mn_3O_4$. Smaller nanoparticles 24 nm in diameter were obtained by simply decreasing the temperature to 120℃. Bigger nanoparticles 37 nm in diameter were obtained by increasing the temperature to 180℃.
The crystalline nature of tetragonal $Mn_3O_4$ and cubic MnO nanoparticles was characterized by X-ray diffraction, electron diffraction, high-resolution transmission electron microscopy.
The MnO nanoparticles showed the ferromagnetic behavior at low temperatures, but unusual size dependency of magnetic properties was observed for MnO nanoparticles. Under ZFC measurements of 100 Oe, the observed blocking temperatures $T_B$ were 20K and 17K for 24 nm and 37 nm MnO nanoparticles, respectively.
