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Fe3O4 magnetic nanoparticles have been synthesized successfully in chemical co-precipitation method and coated with oleic and citric acids, separately. The crystalline sizes are determined by the Debye-Scherer equation with X-ray powder diffraction data. As a result, the mean crystalline size and lattice constant of Fe3O4 nanoparticles respectively increases from ~11 to ~13.8 nm and ~8.352 to ~8.3696 Å when the oleic acid surfactant is replaced by citric acid. The superparamagnetic behavior of the nanoparticles has been documented by the hysteresis loop measured at 300 K. The Fe3O4 nanoparticles show the magnetic saturation values of ~8.136 and ~17.7 emu g–1 for oleic and citric acid coated, respectively. IR spectroscopy has revealed that oleic acid is chemisorbed via a carboxyl group onto the surface of the iron oxide nanoparticle and is coordinated symmetrically to the oxide atoms. Furthermore, there is a ligand exchange between –OH groups and –COO groups on the Fe3O4 nanoparticle surfaces. [ABSTRACT FROM AUTHOR]