Superparamagnetic relaxation in CoFe2O4 nanoparticles

CoFe2O4 nanoparticles synthesized by a microemulsion method have been studied by X-ray diffractometry, transmission electron microscopy, SQUID magnetometry and Mossbauer spectroscopy. All peaks of X-ray diffraction patterns can be attributed to a cubic spinel structure with the lattice constant a(0) = 8.39 Angstrom. The average size of the particles, determined by transmission electron microscopy, is 7.8 nm. Superparamagnetic behavior of the particles is confirmed by the coincidence of plots of the magnetization versus field divided by temperature. As the temperature increases toward the Neel point, Mossbauer line broadening and a pronounced central doublet appear, suggesting superparamagnetic relaxation. As the temperature increases, the relaxation rate increases rapidly as the seventh power of the absolute temperature. (C) 2003 Elsevier Science B.V. All rights reserved.