Microstructural and magnetic studies on hydrothermally prepared hematite

Hematite particles of various shapes and sizes have been prepared hydrothermally at different pH values ranging from 3 to 10 from ferric chloride solution at 180 degrees C. Particle size decreases with an increase in the pH of precipitation as observed from Transmission Electron Micrographs (TEM). A comparison of TEM and mean crystallite diameter (MCD) from XRD data reveals that the particles prepared are polycrystalline in nature. The polycrystallinity decreases with an increase in the pH of precipitation, and at pH 10 almost single crystalline nature of particles is obtained. Polycrystalline monodispersed pseudocubic particles obtained at pH 3 exhibit very high coercive force and remanent magnetization which decreases with an increase in pH, whereas Morin transition temperature, T-m, shows an opposite trend. Magnetic a.c. susceptibility increases with an increase in pH up to pH 7 and then decreases. A probable mechanism for the formation of polycrystalline particles has been suggested in order to explain the above trends in magnetic behavior. (C) 1997 Academic Press