High Coercivity of Oleic Acid Capped CoFe₂O₄ Nanoparticles at Room Temperature

Abstract

High coercivity (9.47 kOe) has been obtained for oleic acid capped chemically synthesized CoFe₂O₄ nanoparticles of crystallite size ∼20 nm. X-ray diffraction analysis confirms the formation of spinel phase in these nanoparticles. Thermal annealing at various temperatures increases the particle size and ultimately shows bulk like properties at particle size ∼56 nm. The nature of bonding of oleic acid with CoFe₂O₄ nanoparticles and amount of oleic acid in the sample is determined by Fourier transform infrared spectroscopy and thermogrvimetric analysis, respectively. The Raman analysis suggests that the samples are under strain due to capping molecules. Cation distribution in the sample is studied using Mössbauer spectroscopy. Oleic acid concentration dependent studies show that the amount of capping molecules plays an important role in achieving such a high coercivity. On the basis of above observations, it has been proposed that very high coercivity (9.47 kOe) is the result of the magnetic anisotropy, strain, and disorder of the surface spins developed by covalently bonded oleic acid to the surface of CoFe₂O₄ nanoparticles.