  Fe$_3$O$_4$ magnetic nanoparticles (MNPs) were synthesized by a
co-precipitation method using sodium citrate and oleic acid as modifiers. Phase
composition and microstructure analysis indicate that the sodium citrate and
oleic acid have been successfully grafted onto the surface of Fe$_3$O$_4$ MNPs.
The magnetic behaviors reveal that the modification can decrease the saturation
magnetization of Fe$_3$O$_4$ MNPs due to the surface effect. Fe$_3$O$_4$ MNPs
modified by sodiumcitrate and oleic acid show excellent dispersion capability,
which should be ascribed to the great reduction of high surface energy and
dipolar attraction of the nanoparticles.
