Synthesis and magnetic characterization of cobalt-substituted ferrite (CoxFe3-xO4) nanoparticles

Cobalt-substituted ferrite nanoparticles were synthesized with a narrow size distribution using reverse micelles formed in the system water/AOT/isooctane. Fe:Co ratios of 3:1, 4:1, and 5:1 were used in the synthesis, obtaining cobalt-substituted ferrites (CoxFe3-xO4) and some indication of gamma-Fe3O4 when 4:1 and 5:1 Fe:Co ratios were used. Inductively coupled plasma mass spectroscopy (ICP-MS) verified the presence of cobalt in all samples. Fourier transform infrared (FTIR) showed bands at similar to 560 and similar to 400cm(-1), characteristic of the metal-oxygen bond in ferrites. Transmission electron microscopy showed that the number median diameter of the particles was similar to 3 nm with a geometric deviation of similar to 0.2. X-ray diffraction (XRD) confirmed the inverse spinel structure typical of ferrites with a lattice parameter of a = 8.388 angstrom for Co0.61Fe0.39O4, which is near that of CoFe2O4(a = 8.394 angstrom). Magnetic properties were determined using a superconducting quantum interference device (SQUID). Coercivities higher than 8 kOe were observed at 5 K, whereas at 300 K the particles showed superparamagnetic behavior. The anisotropy constant was determined based on the Debye model for a magnetic dipole in an oscillating field and an expression relating chi' and the temperature of the in-phase susceptibility peak. Anisotropy constant values in the order of similar to 106 erg/cm(3) were determined using the Debye model, whereas anisotropy constants in the order of similar to 10(7) erg/cm(3) were calculated assuming Omega tau = 1 at the temperature peak of the in-phase component of the susceptibility curve as commonly done in the literature. Our analysis demonstrates that the assumption Omega tau = 1 at the temperature peak of chi' is rigorously incorrect. (C) 2007 Published by Elsevier B.V.