Magnetic Iron Oxide Nanoparticles: Reproducible Tuning of the Size and Nanosized-Dependent Composition, Defects, and Spin Canting

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.