Tuning of Synthesis Conditions by Thermal Decomposition toward Core–Shell Co_xFe_1–xO@Co_yFe_3–yO₄ and CoFe₂O₄ Nanoparticles with Spherical and Cubic Shapes

Spherical core–shell Co x Fe 1– x O@Co y Fe 3-y O 4 nanoparticles (NPs) as well as spherical and cubic shaped CoFe 2 O 4 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, 57 Fe 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 CoFe 2 O 4 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.