Nanocrystalline Formation and Magnetic Properties in Fe₂O₃-C System by Mechanical Alloying

The effect of mechanical alloying (MA) on the solid state reaction of hematite and graphite system with a positive reaction heat was investigated using a mixture of elemental Fe₂O₃-C powders. The solid state reduction of hematite to Fe₃O₄ has been obviously observed after 3 hours of MA by a vibrating ball mill. A two-phase mixture of Fe₃O₄ and remaining Fe₂O₃ is obtained after 5 hours of MA. Saturation magnetization gradually increases with MA time due to the formation of Fe₃O₄ and then reaches 23 emu/g after 5 hours of MA. In addition, a Fe₃O₄ single phase is obtained by MA after 3 hours and subsequently heat treated up to 700°C. X-ray diffraction result shows that the average grain size of Fe₃O₄ prepared by MA for 5 hours and heat treatment to be in the range of 92 nm. The saturation magnetization of Fe₃O₄ prepared by MA and heat treatment reaches a maximum value of 56 emu/g for 5 hours MA sample. It is also observed that the coercivity of 5 hours MA sample annealed at 700 °C is still high value of 113 Oe, suggesting that the grain growth of magnetite phase during annealing process tends to be suppressed.