Magnetic and Structural Properties of Nanocrystalline Iron Oxides

Hematite (α- Fe2O3) and magnetite (Fe3O4) powders, milled for various times up to 10 hours by a high energy ball milling technique, were structurally and magnetically characterized by X-ray diffraction and magnetic measurements in magnetic field up to 10 T in the 2 K–300 K temperature range. From X-ray diffraction line analyses, it is found that when the milling time is increased, the grain size of α- Fe2O3 particles falls down to about 15 nm, and the induced strains and distortions increase up to 0.9% at 10 hours of milling. The ZFC-FC magnetization curves of the milled samples for 1 hour, with an applied field of 5 mT, show that superparamagnetism is present, but with a blocking temperature well above 300 K. The magnetization at saturation increases to a maximum of 0.78 Am2/kg, while the coercive force falls rapidly to a constant value after a milling time of 5 hours. Magnetite powders, milled up to 10 hours, transform partially to hematite, leading to a decrease of the magnetization at 300 K. A correlation between the microstructural evolution of the milled sample can be made with its magnetic properties.