Magnetic nanoparticles of cobalt ferrite have been synthesized by wet chemical method using stable ferric and cobalt salts with oleic acid as the surfactant. X-ray Diffraction (XRD) and Transmission Electron Microscope (TEM) confirmed the formation of single phase cobalt ferrite nanoparticles in the range 15-48nm depending on the annealing temperature and time. The size of the particles increases with annealing temperature and time while the coercivity goes through a maximum, peaking at around 28nm. A very large coercivity (10.5kOe) is observed on cooling down to 77K while typical blocking effects are observed below about 260K. The high field moment is observed to be small for smaller particles and approaches the bulk value for large particles. PACS: 75.50Gg; 75.50Tt; 75.70Rf

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