Fe3O4 magnetic nanocrystals were prepared by coprecipitating Fe2＋ and Fe3＋ ions in an ammonia solution with ultrasonic enhancement and modification by the anionic surfactant sodium dodecyl sulfate (SDS). The product was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), high resolution transmission electron microscopy (HRTEM), N2 absorption-desorption, and thermogravimetric-differential scanning calorimetry (TG-DSC), respectively. The surface electrical properties and magnetic properties of the Fe3O4 magnetic nanocrystals were systematically investigated. The growth mechanism of the Fe3O4 magnetic nanocrystals was discussed. The results show that the Fe3O4 magnetic nanoparticles obtained are well-crystallized particles, with an average size of 10 nm, which have good dispersivity. The specific surface area of particles can reach as much as 91.6 m2•g－1. The obtained nanoparticles also have good thermostability, and the isoelectric point (IEP) under water conditions is at pHpzc＝5.7. Magnetic measurement reveals the magnetic nanoparticles are superparamagnetic with a saturation magnetization of 65.0 emu•g－1. The ultrasonic enhancement and the surfactant modification play an important role in the growth mechanism of the Fe3O4 superparamagnetic nanocrystals. These superparamagnetic nanocrystals might be applied to biological and medical fields such as cell or enzyme immobilization.

Key words: Fe3O4 superparamagnetic nanocrystal, ultrasonic enhancement, coprecipitation process, surface electrical property, magnetic property