The preparation of variable-sized (20.6～45.0 nm) Fe₃O₄ nanocrystals using oxidation- coprecipitation phase transformation, changing the pH values (9～11.5) and temperature (20～100 ℃) of the system was described. The effects of magnetic intensity and pH on the formation of the one-demensional (1D) Fe₃O₄ nano-particles were studied. The results showed that the magnetic fields and pH values can affect the assembly of the 1D Fe₃O₄ nano-particles. When the pH values were in the range of 9～11.5 under the external magnetic fields (＜75 mT), the Fe₃O₄ nano-particles showed the 1D chain structure. The tendency of the Fe₃O₄ 1D chain increased with the decreasing of pH. The formation of the 1D chain structure was attributed to the synergy of the exterior magnetic fields and the surface charge of the Fe₃O₄ nano-particles. The Fe₃O₄ nanocrystals displayed ferromagnetic behavior. Fe₃O₄ particles with 1D chain structure have the higher coercivity and remanence.

Key words: nano-Fe₃O₄, size, magnetic field, pH, one-dimensional chain