Preparation and magnetic properties of γ-Fe₂O ₃@SiO₂ core shell ellipsoids with different aspect ratios

Beginning from spindle α-Fe₂O₃, monodisperse α-Fe₂O₃@SiO₂ ellipsoids with different shell thicknesses were prepared by the Stöber method using tetraethyl orthosilicate (TEOS) as the precursor. Subsequently, the monodisperse γ-Fe₂O₃@SiO₂ ellipsoids were fabricated by reduction-oxidation treatments under H₂/O₂ atmospheres. The SiO₂ shell thickness could be easily controlled by changing the amounts of TEOS. Their structures and optical properties were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. The hysteresis loops of these nanoparticles were comparatively investigated. Furthermore, the relationships of the magnetic parameters and shell thicknesses were analysed in depth.