Magnetic properties of ultrafine cobalt ferrite particles

We have studied magnetic properties of a diluted system of ultrafine cobalt ferrite nanoparticles (d∼3.3 nm). From the peak of the zero-field-cooled measurements, we obtained the blocking temperature TB of about 90.5 K and it is virtually independent of the applied magnetic field up to 5 kOe. At the superparamagnetic region T>TB, the system follows the modified Curie-law variation of the magnetic susceptibility χ=χo+C/T. We observed that the saturation magnetization follows a spin-wavelike temperature dependence at temperature above 10 K. In spite of the cubic structure for cobalt ferrite, at 2 K, the reduced remanence Mr/Ms is equal to 0.46 which is close to the theoretical value of 0.5 expected for noninteracting uniaxial single-domain particles with the easy axis randomly oriented. From the ac susceptibility measurements at different frequencies, we obtained a linear dependence of the logarithm of the experimental time window τex as function of inverse blocking temperature (1/TB). The fitting results in the anisotropy constant value K of 3.15×107 erg/cm3 that is one order of magnitude higher than 1.8–3.0×106 erg/cm3 in bulk CoFe2O4 materials.