Micelles induced high coercivity in single domain cobalt-ferrite nanoparticles

We have prepared CoFe2O4 nanoparticles in micellar medium by wet chemical technique and obtained very high coercivity value of 4.4 kOe at room temperature for particle size similar to 16 nm. A large coercivity (similar to 20 kOe) is observed on cooling down to 2.5 K. We annealed the sample at different temperatures to check the role of micelles and particles size in the change in coercivity value. Here we observed micelles as capping agent playing an important role to enhance the coercivity, as after removal of micelles for the same particle size the coercivity drops from 4.4 kOe to small value similar to 350 Oe. But the coercivity again increases due to the increase in particle size with increase in annealing temperature from 873 K and above. To obtain structural information and size of particles, we have taken x-ray diffraction spectra from the samples before and after annealing at different temperatures which confirm the spinel phase only. (C) 2010 American Institute of Physics. [doi:10.1063/1.3525994]