Iron Oxide Nanoparticles: Tunable Size Synthesis and Analysis in Terms of the Core–Shell Structure and Mixed Coercive Model

Iron oxide nanoparticles (NPs) are currently a very active research field. To date, a comprehensive study of iron oxide NPs is still lacking not only on the size dependence of structural phases but also in the use of an appropriate model. Herein, we report on a systematic study of the structural and magnetic properties of iron oxide NPs prepared by a co-precipitation method followed by hydrothermal treatment. X-ray diffraction and transmission electron microscopy reveal that the NPs have an inverse spinel structure of iron oxide phase (Fe3O4) with average crystallite sizes (DXRD) of 6–19 nm, while grain sizes (DTEM) are of 7–23 nm. In addition, the larger the particle size, the closer the experimental lattice constant value is to that of the magnetite structure. Magnetic field-dependent magnetization data and analysis show that the effective anisotropy constants of the Fe3O4 NPs are about five times larger than that of their bulk counterpart. Particle size (D) dependence of the magnetization and the non-saturating behavior observed in applied fields up to 50 kOe are discussed using the core–shell structure model. We find that with decreasing D, while the calculated thickness of the shell of disordered spins (t ∼ 0.3 nm) remains almost unchanged, the specific surface areas Sa increases significantly, thus reducing the magnetization of the NPs. We also probe the coercivity of the NPs by using the mixed coercive Kneller and Luborsky model. The calculated results indicate that the coercivity rises monotonously with the particle size, and are well matched with the experimental ones.