Iron Oxide Nanoparticles Co-Precipitated in Air Environment: Effect of [Fe $^{+2}$ ]/[Fe $^{+3}$ ]

Iron oxide nanoparticles (IONs) were synthesized from ferrous aqueous solutions in air atmosphere by co-precipitation. The effect of [Fe⁺²]/[Fe⁺³] ratio on the properties of the nanoparticles were investigated. X-ray diffraction (XRD) patterns showed the charac- teristic peaks of iron oxide for all samples. Fourier transform infrared spectroscopy analysis also confirmed the iron oxide formation. According to magnetic measurements, saturation magnetization, M_s of the nanoparticles increased from 37.6 emu/g to 59.4 emu/g with the increase of [Fe⁺²]/[Fe⁺³] ratio from 1/2 to 6/6 and the samples are superparamagnetic with zero coercivities. However, with the increase of the ratio above 6/6 the samples start to show coercivities (8 Oe, 22 Oe and 33 Oe) and M_s increases up to 74.3 emu/g. Particle sizes calculated from the XRD patterns, the images of the transmission electron microscope and magnetic data are consistent with each other and increased with the increase of [Fe⁺²]/[Fe⁺³] ratio. It is revealed that the particle size, M_s and magnetic character of IONs were significantly influenced by [Fe⁺²]/[Fe⁺³] ratio. Below the critical size of ~11 nm it is seen that IONs showed superparamagnetic character with zero coercivity.