An inner asymmetric PSt/Fe₃O₄ composite microspheres, in which most of the inorganic particles encapsulated by polymers gather into cluster in one side of the microsphere and only few particles distribute in the other part of the microsphere, were prepared in one step miniemulsion polymerization. TEM (Transmission Electron Microscopy), FTIR (Fourier Infrared Spectrum), VSM (Vibrating Sample Magnetometer), TGA (Thermogravimetry) and laser particle analyzer were used to characterize the inner structure, magnetic intensity and particle sizes of the novel microspheres. The average particle size of these new microspheres is 0.8 μm, the magnetic content of this new asymmetric composite microspheres is 46.67% and the saturation magnetization is 23.2 emu/g. The influencing factors such as the amounts of oleic acid on the surface of Fe₃O₄, emulsifier, co-emulsifier and hyper-dispersant are discussed in this article. And then the mechanism of this process is also investigated. Finally, the surface properties of Fe₃O₄ particles were considered as the key factor leading to the inner asymmetric structure.

Key words: inner asymmetric structure, Fe₃O₄, magnetic composite microspheres