Synthesis and characterization of cobalt-substituted ferrite nanoparticles using reverse micelles

With the objective of developing a magnetic nanoparticle based sensor, we have synthesized cobalt-substituted ferrite particles using reverse micelles. Reverse micelles have been used to control the nanoparticle size. Cobalt ferrite was chosen due to its high anisotropy constant which assures that the relaxation mechanism is Brownian. Fe:Co ratios of 3:1, 4:1, and 5:1 were used in the synthesis, obtaining cobalt-substituted ferrites (CoxFe3-x O4). 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 ∼400cm-1, confirming the metal-oxygen bond characteristic of ferrites. Transmission electron microscopy shows that the average size of the particles was ∼3 nm with a geometric deviation of ∼0.2. X-ray diffraction (XRD) confirmed the inverse spinel structure typical of ferrites with a lattice parameter of a = 8.388A for Co0.61Fe0.39O4, which is near that of CoFe2O4 (a = 8.39A). Magnetic properties were determined using a Superconducting Quantum Interference Device (SQUID). Coercivities (Hc) higher than 8 kOe were observed at 5K, whereas at 300K the particles showed superparamagnetic behavior.; The anisotropy constant was determined based on the Debye model for a magnetic dipole in an oscillating field. We obtained an expression relating chi ' and the temperature of the in-phase susceptibility peak. Anisotropy constant values in the order of ∼106 kerg/cm3 were determined, using the Debye model, whereas anisotropy constants in the order of ∼107 kerg/cm3 were calculated assuming Otau = 1 at the temperature peak of the in-phase component of the susceptibility curve.