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Characterization of Cobalt-Substituted Ferrite Nanoparticles for Use in Sensors

Victoria L. Calero, University of Puerto Rico, Mayaguez, PO Box 9046, Mayaguez, PR 00680 and Carlos Rinaldi, Chemical Engineering, University of Puerto Rico Mayguez Campus, Mayaguez, PR 00680.

Cobalt-substituted ferrite nanoparticles with FCC structure have been synthesized using reverse micelles. Because the reverse micelles act as templates, we obtained nanoparticles with approximately ~3 nm diameter and a geometric deviation of ~0.2. Fe:Co ratios of 3:1, 4:1, and 5:1 were used in the synthesis, obtaining cobalt-substituted ferrites (CoxFe3-xO4). Inductively coupled plasma mass spectroscopy (ICP-MS) verified the presence of cobalt in all samples. Fourier transform infrared (FTIR) spectra show bands at ~560 and ~400 cm-1, confirming the metal-oxygen bond characteristic of ferrites. Despite the advantages posed by the reverse micelles, the method produces particles that are chemically bound to AOT. Bands observed at ~1725 cm-1, ~1459 cm-1, and ~1218 cm-1 were attributed to AOT-particle bond. A lattice parameter value of a=8.388 was obtained by X-ray for Co0.61Fe0.39O4, which is near that of CoFe2O4(a=8.39). Magnetic measurements showed coercivities (Hc) higher than 8 kOe at 5K, whereas at 300K the particles showed superparamagnetic behavior. A rigorous analysis based on the Debye model for a magnetic dipole in an oscillating field was used to determine the anisotropy constant from AC susceptibility measurements. Assuming magnetic relaxation proceeds through the Nel mechanism, a relation between χ' and the temperature of the AC in-phase susceptibility curve peak was obtained. Using this relation, anisotropy constant values in the order of ~106 kerg/cm3 were determined, whereas anisotropy constants in the order of ~107 kerg/cm3 were calculated assuming Ωτ=1 at the temperature peak of the in-phase component of the susceptibility curve.