Magnetite and maghemite have the same crystal structure. The iron and oxygen ions form a face-centered cubic crystal system, and the oxygen ions are in the cubic close-packed arrangement. Magnetite has an inverse spinal structure, and Fe3+ ions occupy all the tetrahedral sites and both Fe3+ and Fe2+ ions occupy all the octahedral sites. Maghemite has a spinal structure. Differing with magnetite, vacancies exist in the octahedral sites in maghemite, and Fe3+ ions occupy two-third of the sites. One unit cell of maghemite contains 32 oxygen ions, 211/3 Fe3+ ions, and 2 1/3 vacancies.57, 59
Increased spin-orbit coupling strength of the divalent ions in octahedral sites enhances magnetocrystalline anisotropy,60-61 where for instance an increasing strength can be observed for Mn2+ < Fe2+ < Co2+.60, 62-63 From this it follows that the incorporation of Co2+ in to the ferrite structure (CoFe2O4) results in an increased TB.63 This means that magnetic hysteresis and the occurance of remanence and coercivity can be even observed at room temperature for very small particles. In ferrites (MxFe3-xO4) the composition also strongly affects the magnetic properties.63-65 It strongly depends on the magnetic moment of the M2+ cation, e.g. in increasing order for Ni, Co, Fe, Mn, m = n·?B with n= 2, 3, 4 and 5, respectively.
As we mentioned earlier, compared to bulk ferrite MNPs, surface effects play a more important role. On the one hand, the stoichiometry on the surface differs from the center and thereby the surface can be regarded amorphous due to non-periodicity of the crystal structure. Both result in a spin-disordered surface layer (“dead layer”) where spin-canting effects occur and magnetic properties deviate. Spin canting means a non-collinear coupling of surface spins, producing magnetically disordered shell. The effect of spin canting is higher at lower temperature due to the collectively frozen state of spins. The saturation magnetization in this layer is lowered due to harder alignment of magnetic moments with respect to each other (higher activation energy due to disorder).47-48