Size-dependent magnetic properties of CoFe 2O 4 nanoparticles prepared in polyol

Highly crystalline CoFe 2O 4 nanoparticles with different diameters ranging from 2.4 to 6.1nm have been synthesized by forced hydrolysis in polyol. The size can be controlled through adjusting the nominal water/metal molar ratio. X-ray diffraction, transmission electron microscopy, x-ray absorption spectroscopy and 57Fe Mössbauer spectrometry were employed to investigate the structure and the microstructure of the particles produced. Magnetic measurements performed on these particles show that they are superparamagnetic with a size-dependent blocking temperature. At 5K, high saturation magnetization (85emug 1) approaching that of the bulk was found for the larger particles, whereas a very large coercivity (14.5kOe) is observed for the 3.5nm sized particles. © 2011 IOP Publishing Ltd.