Effect of Co²⁺ ion on the magnetic properties of sol-gel cobalt ferrite thin films

The effect of Co²⁺ ion on the magnetic properties of the sol-gel derived cobalt ferrite thin films (400 nm thickness) was studied. Films with different solution composition ([Co²⁺]/[Fe³⁺] = 0.5-1.0) were deposited on a Si substrate from the precursor containing an aqueous solution of FeCl₃·6H₂O and CoCl₂· 6H₂O. Deposited films were heat treated at different temperatures (700-1100 °C) and were characterized by X-ray diffraction using Cu Kα radiation. The films containing a nonstoichiometric α-(Fe _1-xCo_x)₂O₃ phase were obtained in addition to a small amount of CoO at relatively lower temperatures. With increasing heat treatment temperature, nonstoichiometric α-(Fe _1-xCo_x)₂O₃ was decomposed to stoichiometric α-Fe₂O₃ and the CoFe ₂O₄ phase was intensified. Only the films obtained from the solution enriched with Co showed the single phase of CoFe₂O ₄, though the Co to Fe content ratio analyzed by inductive coupled plasma spectroscopy (ICP) and energy dispersive X-ray spectroscopy (EDX) was nearly consistent with that of the starting solutions. The study of surface morphology of the films by using field emission scanning electron microscope (FESEM) showed that films were homogeneously distributed throughout the surface of the substrate and the grain sizes were in the range of 60 to 140 nm. Magnetic properties of the films, including the Curie temperature, were studied by using a vibrating sample magnetometer (VSM) and conversion electron Mössbauer spectroscopy (CEMS). The Curie temperature and the magnetic hyperfine fields of the films slightly decreased with increasing molar ratio of the Co²⁺ and Fe³⁺ ions while the saturation magnetization reached a maximum value with the film composition of x = 0.28.