Size-dependent x-ray photoelectron spectroscopy and complex magnetic properties of CoMn2O4 spinel nanocrystals

A facile nonaqueous route has been developed for the preparation of tetragonal CoMn2O4 nanocrystals. In a simple reaction process, cobalt (II) acetylacetonate and manganese (II) acetylacetonate were dissolved in oleylamine and allowed to decompose at 230 degrees C. This process led to nearly monodisperse nanocrystals with sizes ranging from 5.3 to 12 nm, as shown by x-ray diffraction analysis and transmission electron microscopy. An x-ray photoelectron spectroscopy study revealed that the lower oxidation state Mn2+ ions increased with decreasing particle size, accompanied by an increase in higher oxidation state cations (such as: Mn1+, CO3+ and CO4+ ions) due to charge balancing. The broad metastable cation distribution reveals that the nanoparticles have potential applications for catalysts. Magnetic measurements revealed that the nanoparticles were ferrimagnetic at low temperature and paramagnetic at room temperature, and possess blocking temperatures of 30-40 K. The blocking temperature did not increase when the samples' size increased from 6.7 to 9.0 nm, partly because of the broad metastable cation distribution and partly due to the dipolar interactions between the nanoparticles. In addition, the nanoparticles showed exchange bias behaviour due to the ferrimagnetic-ferromagnetic exchange coupling between the ferrimagnetic core and ferromagnetic shell.