Magnetic properties of ultrafine cobalt ferrite particles synthesized by hydrolysis in a polyol medium

Abstract

Fine CoFe₂O₄ powders with monodisperse, almost equi-axial nanometer-sized particles were synthesised in a polyol medium by forced hydrolysis of ionic Co(II) and Fe(III) salts at 160°C. K(Co) XANES and ⁵⁷Fe Mössbauer spectroscopy show that the structure of this ferrite is slightly deviated from an inverse spinel structure: 16% of cobalt atoms are in tetrahedral sites. The particles are superparamagnetic above 300 K and ferrimagnetic below this blocking temperature with, at low temperature, strong coercivity, a saturation magnetisation value close to the bulk value and high reduced remanence. The saturation magnetisation measured at 5 K is clearly enhanced with respect to CoFe₂O₄ nanometer-sized particles previously prepared by other methods. These magnetic characteristics suggest that these particles have a high crystallinity which may result from this novel synthesis route.