Shape factors dependence of magnetic features of CoFe_2−xGd_xO₄nanocrystals

In the first step, CoFe_2−xGd_xO₄(x = 0–0.05 in a step of 0.01) ferrite nanoparticles were synthesized by a hydrothermal process and without subsequent annealing. X-ray diffraction, field-emission scanning electron microscope, energy dispersive X-ray microanalyzer, and vibrating sample magnetometer were used to evaluate structural and magnetic properties of synthesized nanoparticles. The X-ray diffraction analysis showed that single phase spinel ferrites were obtained. The FE-SEM micrographs of the synthesized samples indicated the presence of roughly spherical morphology of ferrite nanoparticles. Based on the EDS analysis, it was noted that the applied technique for preparation of CoFe_2−xGd_xO₄nanoparticles is a suitable process for the synthesis of spinel ferrites with homogeneity in composition. The results of magnetic hysteresis at a room temperature indicated that the coercive field increased from 802.93 Oe for x = 0 to 965.48 Oe for x = 0.02 and then at higher contents in the composition (x > 0.02), decreased. In addition, it was observed that with substitutions of gadolinium cations, the values of saturation magnetization decreased from 69.34 emu/g for x = 0–49.31 emu/g for x = 0.05. In the second step, based on the magnetic hysteresis results, gadolinium substituted cobalt ferrite with composition of CoFe_1.98Gd_0.02O₄nanocrystals were prepared and the processing conditions of hydrothermal technique; especially the effect of temperature of reaction in the range of 100–200 °C and the cetyltrimethylammonium bromide (CTAB) as the surfactant evaluated. The FE-SEM micrographs of the synthesized samples showed that the temperature of reaction can be provoked modifications on the shape of the ferrite grains. At 100 °C the grains were mainly rods, while at 160 °C most of them exhibited roughly spherical shapes. At 200 °C the grains had the form of a cube. The usage of CTAB as the surfactant had no effect on the shape of the ferrite grains. The grains obtained in the presence of CTAB have fundamentally irregular shape. The possible formation mechanism for the synthesis of ferrites with different shapes has been preliminarily explained. The results of magnetic hysteresis at a room temperature indicated that with the change of temperature of reaction and shape of the ferrite grains, the coercive field can be altered from 656.25 Oe for T = 100 °C to 965.48 Oe for T = 160 °C. The coercive field of sample synthesized at T = 200 °C with the form of a cube was 793.26 Oe. It was interesting to find that the coercivity of sample obtained in the presence of CTAB was 252.47 Oe, which is attributed to the existence of very small particles.