Spindly cobalt ferrite nanocrystals: Preparation, characterization and magnetic properties

In this paper we describe the preparation of homogeneously needle-shaped cobalt ferrite (CoFe2O4) nanocrystals on a large scale through the smooth decomposition of urea and the resulting co-precipitation of Co2+ and Fe3+ in oleic acid micelles. Furthermore, we found that other ferrite nanocrystals with a needle-like shape, such as zinc ferrite (ZnFe2O4) and nickel ferrite (NiFe2O4), can be prepared by the same process. Needle-shaped CoFe2O4 nanocrystals dispersed in an aqueous solution containing oleic acid exhibit excellent stability and the formed colloid does not produce any precipitations after two months, which is of prime importance if these materials are applied in magnetic fluids. X-ray diffraction (XRD) measurements were used to characterize the phase and component of the co-precipitation products, and demonstrate that they are spinel ferrite with a cubic symmetry. Transmission electron microscopy (TEM) observation showed that all the nanocrystals present a needle-like shape with a 22 nm short axis and an aspect ratio of around 6. Varying the concentration of oleic acid did not bring about any obvious influence on the size distribution and shapes of CoFe2O4. The magnetic properties of the needle-shaped CoFe2O4 nanocrystals were evaluated by using a vibrating sample magnetometer (VSM), electron paramagnetic resonance (EPR), and a Mossbauer spectrometer, and the results all demonstrated that CoFe2O4 nanocrystals were superparamagnetic at room temperature.