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

Uniform gamma-Fe2O3 ellipsoidal particles were obtained by reduction-oxidat ion of different alpha-Fe2O3 precursors with similar particle axial ratio ( approximate to 5) and different lengths, between 600 and 300 nm. The magnet ic properties of the gamma-Fe2O3 particles showed no dependence on the part icle size but a strong dependence on the preparation method of the alpha-Fe 2O3 precursor particles, obtained by hydrolysis of Fe(iii) salts in the pre sence of phosphates. TEM, EDSX, X-ray diffraction, Auger and infrared spect roscopy have been used to characterise the samples as well as to identify t he nature of the defects in the hematite particles, which seem to be respon sible for the different magnetic properties of the resulting gamma-Fe2O3 pa rticles. 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 m agnetic properties of the resulting gamma-Fe2O3 particles has been establis hed. The elimination of the OH groups during the transformation from alpha- Fe2O3 to gamma-Fe2O3 leads to a mesoporosity in the particles, which must l imit the particle volume where the magnetisation reversal takes place by co herent rotation and therefore, the value of the coercivity.