Fabrication of superparamagnetic spinel calcium ferrite nanoparticles by the auto-combustion sol-gel method and investigation of their structural, microstructural, and magnetic properties

In this research, calcium ferrite nanoparticles (CaFe₂O₄) were first synthesized and structurally characterized by the self-combustion sol-gel method at temperatures of 300–700 ˚C and a time of 8 h, and then at a temperature of 400 ˚C and times of 2–10 h. Their microstructure and magnetism were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), atomic force microscope (AFM), and vibrating sample magnetometer (VSM). X-ray diffraction analysis showed that the manufactured samples have a spinel structure and the size of the nanocrystals was estimated between 7 and 12 nm and 4–10 nm at different temperatures and sintering times using Scherer’s equation. This study showed that a pure calcium ferrite sample (CaFe₂O₄) is formed at 400 ˚C temperature and 8 h time respectively. The presence of metal-oxide vibrational modes was confirmed by an infrared spectrometer (FT-IR) in calcium ferrite nanoparticles at all temperatures and sintering times. Field emission scanning electron microscope and transmission electron microscope analyses confirmed the nanostructure of calcium ferrite. Also, it showed that the shape of the nanoparticles is spherical and the size of the particles increased with increasing temperature and sintering time. To check the topography of the surface and study the microstructure of the samples, analysis (AFM) was performed. The results of this analysis showed that calcium ferrite nanoparticles have a uniform distribution and also, have a spherical shape with an average size of 18 nm. Magnetic properties of nanoparticles such as saturation magnetization (Ms) and magnetic coercive field (Hc) were investigated by vibrating sample magnetometer (VSM) at room temperature and the results showed that calcium ferrite nanoparticles have superparamagnetic behavior at different temperatures and sintering times. In addition, by measuring the Curie temperature (Tc) of the samples, it was observed that the Curie temperature decreases with the increase of the sintering temperature. In contrast the Curie temperature increases with the increase of the sintering time.