The effect of cation distribution on the magnetic properties of CoFe2O4 nanoparticles

CoFe2O4 nanoparticles were synthesized via a co-precipitation method at pH values of 9, 11 and 14. All synthesized samples were characterized using XRD, TEM, XANES and VSM. The XRD results indicate a pure phase of a cubic spinel CoFe2O4 structure at pH values of 11 and 14. They were found to contain a Fe2O3 impurity phase at pH 9. The crystallite size decreased with increased pH values and it was found to be in the range of 41–48 nm. The lattice constants also decreased with increased pH values corresponding to the theoretical lattice calculation. The average particle sizes are found to be in the range of 47.6–51.6 nm, which were observed in TEM images. The valence states and cation distribution of Fe and Co ions were examined using XANES and XPS techniques. XPS results show that the valences of Fe and Co ions in the samples are 3+ and 2+, respectively. The migration of Co2+ and Fe3+ ions between octahedral and tetrahedral sites fluctuated and the trend of those results depended on particle size. The saturation magnetization results depend on the cation distribution, particle size and an impurity phase. The sample at pH 11 showed the highest migration of Co2+ ions from octahedral to tetrahedral sites and obtained the maximum saturation magnetization value (71 emu/g). The law of approach to saturation magnetization was employed to find the cubic anisotropy constant (K1) of all samples.