Increasing the size of Fe3-δO4 Nanoparticles by Performing a Multistep Seed-Mediated Growth Approach

Iron oxide nanoparticles were synthesized by an original multistep seed-mediated growth approach. The thermal decomposition of an iron stearate precursor was performed successively up to 5 times to produce nanoparticles with a narrow size distribution from 6.4 to 15.0 nm. The chemical composition and crystal structure of each set of nanoparticles was characterized by TEM, FT-IR, XRD, and Mossbauer spectrometry. Each layer was successively grown at the surface of a pristine Fe3-delta O4 nanoparticle by epitaxial relationship and resulted in a single crystal structure. An intermediate wash after each thermal decomposition step resulted in the surface oxidation of each layer. Therefore, the maghemite phase increased relative to the magnetite phase as the nanoparticle expanded. Finally, the study of the magnetic properties by SQUID magnetometry showed the trend of the magnetic anisotropy energy to increase as a function of the nanoparticle size. In contrast, the coercive field and the magnetization saturation display nonmonotonic variations that may result from the interplay of intrinsic and collective properties.