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.

Spherical core–shell Co x Fe1–x O@Co y Fe3‑yO4 nanoparticles (NPs) as well as spherical and cubic shaped CoFe2O4 NPs were synthesized through a thermal decomposition method by adjusting parameters such as the nature of precursors and ligands. The use of metal (iron and/or cobalt) oleates and stearates as precursors in the presence of oleic acid as ligand leads to core–shell NPs, due to the reducing environment provided by oleate groups from the oleic acid and precursors. By contrast, the use of oleylamine as ligand favored the decomposition of precursors and less reducing medium, which allows obtaining NPs with homogeneous composition. In addition, cobalt ferrite cubic-shaped NPs were synthesized using mixed oleate formed in situ from metal iron chloride and cobalt chloride in the presence of sodium oleate. The as-synthesized NPs were carefully characterized by combining several techniques including TEM, XRD, 57Fe Mössbauer spectrometry, STEM-EELS, and XMCD. The correlation between the crystalline structure and the magnetic properties was investigated by carrying out magnetic measurements as a function of an applied field and of temperature. The CoFe2O4 NPs were found to display high coercivity due to their homogeneous composition, while the core–shell NPs show higher blocking temperature and exchange bias properties originating from the interaction between the antiferromagnetic (AFM) core and the ferrimagnetic (FIM) layer at the surface.

Core−shell nanoparticles (NPs), which consist in a ferrimagnetic (FIM)/antiferromagnetic (AFM) interface and result in exchange bias coupling, became recently of primary importance in the field of magnetic nanoparticles. The enhancement of some applications such as hyperthermia or magnetic storage media based on the miniaturization of devices is correlated to the size reduction of NPs, which results in the decrease of the magnetocrystalline anisotropy and of the blocking temperature. We present here the synthesis of Fe 3−δ O 4 @CoO core−shell NPs by a one-pot seedmediated growth process based on the thermal decomposition of metal complexes at high temperature. A 2 nm thick CoO shell was grown homogeneously from the starting Fe 3−δ O 4 core surface. The Fe 3−δ O 4 @CoO core−shell NP structure has been deeply investigated by performing XRD and advanced techniques based on TEM such as EELS and EFTEM. The high quality of the core−shell interface resulted in the large exchange bias coupling at 5 K (H E ≈ 4.1 kOe) between the FIM and the AFM components. In comparison to starting Fe 3−δ O 4 NPs, the dramatic enhancement of the magnetic properties such as a high coercive field (at 5 K, H C ≈ 15 kOe) were measured. Furthermore, the core−shell structure resulted in the enhancement of the magnetocrystalline anisotropy and the increase of the blocking temperature to 293 K.

The chemical preparation, crystal structure, and infrared absorption spectrum are given for a new cyclotetraphosphate, KGdP4O12. X-ray investigations showed that the newly synthesized compound crystallizes in a monoclinic structure, space group C2/c with the following unit-cell dimensions:  a = 7.875(1), b = 12.431(2), c = 10.537(2) Å, β = 110.94(1)°, V = 963.4(3) Å3, Z = 4, and ρ = 3.532 g cm-3. The structure was solved from 1390 independent reflections with R1 = 2.99 and WR2 = 7.37%, refined with 84 parameters. As in all atomic arrangements, we observe the formation of an infinite network of P4O12 4- cyclotetraphosphate anions connected with GdO8 polyhedra to form a three-dimensional framework which delimits interesting tunnels where the K+ cations are located. In this structure, the P4O12 ring develops around an inversion center. The reported IR study, recorded at room temperature in the frequency range 400−4000 cm-1, shows some characteristics bands of cyclotetraphosphates.

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