The fine control of iron oxide nanocrystal sizes within the nanometre scale (diameters range from 2.5 to 14 nm) allows us to investigate accurately the size-dependence of their structural and magnetic properties. A study of the growth conditions of these nanocrystals obtained by thermal decomposition of an iron oleate precursor in high-boiling point solvents has been carried out. Both the type of solvent used and the ligand/precursor ratio have been systematically varied, and were found to be the key parameters to control the growth process. The lattice parameters of all the nanocrystals deduced from X-ray diffraction measurements are consistent with a structure of the type Fe3-xO4, i.e. intermediate between magnetite and maghemite, which evolves toward the maghemite structure for the smallest sizes (x=1/3). The evolution of the magnetic behavior with nanoparticle sizes emphasizes clearly the influence of the surface, especially on the saturation magnetization Ms and the magneto-crystalline anisotropy K. Dipolar interactions and thermal dependence have been also taken into account in the study on the nanoscale size-effect of magnetic properties.

Iron oxide nanoparticles (NPs) with average sizes in the range 4−28 nm have been obtained by varying different synthesis parameters of the thermal decomposition of an iron precursor (iron stearate) in the presence of surfactants in high boiling solvents. The synthesis parameters affect the NPs nucleation and growth steps, by modifying the stability of iron stearate on which depend the monomer formation and concentration, in agreement with the LaMer model. The monomer formation, which is reaction time and/or temperature dependent, is thus found to vary mainly as a function of the nature of solvents and ligands. The structural and magnetic characterizations of NPs with sizes in the range 5−20 nm confirm that the composition of NPs evolves from the maghemite for small sizes (typically <8 nm) up to a core of rather stoichiometric magnetite surrounded by an oxidized shell for large sizes (>12 nm) via a perturbed oxidized state for intermediate sizes. The values of saturation magnetization lower than those of bulk magnetite and maghemite were found to be related to this composition evolution and to the presence of oxidation defects, surface spin canting and volume spin canting as a function of NPs diameter. Small NPs presented mainly a surface spin canting. NPs with large sizes display M s which depends on their oxidized shell thickness, defects and surface spin canting. NPs with intermediate sizes display a surface and in particular a volume spin canting due to a disordered structure induced by a perturbed oxidation state in these NPs.