Magnetic properties of ultrafine cobalt ferrite particles

We have studied magnetic properties of a diluted system of ultrafine cobalt ferrite nanoparticles (dsimilar to3.3 nm). From the peak of the zero-field-cooled measurements, we obtained the blocking temperature T-B of about 90.5 K and it is virtually independent of the applied magnetic field up to 5 kOe. At the superparamagnetic region T>T-B, the system follows the modified Curie-law variation of the magnetic susceptibility chi=chi(o)+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 M-r/M-s 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 tau(ex) as function of inverse blocking temperature (1/T-B). The fitting results in the anisotropy constant value K of 3.15x10(7) erg/cm(3) that is one order of magnitude higher than 1.8-3.0x10(6) erg/cm(3) in bulk CoFe2O4 materials. (C) 2003 American Institute of Physics.