Spindly cobalt ferrite nanocrystals: preparation, characterization and magneticproperties

Abstract

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-precipitationof Co2+ and Fe3+ in oleic acid micelles. Furthermore, we found that other ferritenanocrystals with a needle-like shape, such as zinc ferrite(ZnFe2O4) and nickelferrite (NiFe2O4), can be prepared by the same process. Needle-shapedCoFe2O4 nanocrystals dispersed in an aqueous solution containing oleic acid exhibit excellentstability and the formed colloid does not produce any precipitations after twomonths, which is of prime importance if these materials are applied in magneticfluids. X-ray diffraction (XRD) measurements were used to characterize the phaseand component of the co-precipitation products, and demonstrate that they arespinel ferrite with a cubic symmetry. Transmission electron microscopy (TEM)observation showed that all the nanocrystals present a needle-like shape with a 22 nmshort axis and an aspect ratio of around 6. Varying the concentration of oleic aciddid not bring about any obvious influence on the size distribution and shapes ofCoFe2O4. The magnetic properties of the needle-shapedCoFe2O4 nanocrystals were evaluated by using a vibrating sample magnetometer (VSM), electronparamagnetic resonance (EPR), and a Mössbauer spectrometer, and the results all demonstrated thatCoFe2O4 nanocrystals were superparamagnetic at room temperature.