Effect of precursor impurities on the magnetic properties of uniform γ-Fe2O3 ellipsoidal particles

Uniform γ-Fe2O3 ellipsoidal particles were obtained by reduction–oxidation of different α-Fe2O3 precursors with similar particle axial ratio (≈5) and different lengths, between 600 and 300 nm. The magnetic properties of the γ-Fe2O3 particles showed no dependence on the particle size but a strong dependence on the preparation method of the α-Fe2O3 precursor particles, obtained by hydrolysis of Fe(III) salts in the presence of phosphates. TEM, EDSX, X-ray diffraction, Auger and infrared spectroscopy have been used to characterise the samples as well as to identify the nature of the defects in the hematite particles, which seem to be responsible for the different magnetic properties of the resulting γ-Fe2O3 particles. Both hydroxy and phosphate groups have been identified as the main defects on the surface and inside the hematite particles. Thus, a relation between the amount of structural hydroxy groups in the precursor and the magnetic properties of the resulting γ-Fe2O3 particles has been established. The elimination of the OH groups during the transformation from α-Fe2O3 to γ-Fe2O3 leads to a mesoporosity in the particles, which must limit the particle volume where the magnetisation reversal takes place by coherent rotation and therefore, the value of the coercivity.