Temperature dependence of superparamagnetism in CoFe2O4 nanoparticles and CoFe2O4/SiO2 nanocomposites.

CoFe2O4 particles of 16 nm and 17 nm embedded in a silica matrix have been prepared through the hydrothermal method and the sol-gel method, respectively. From neutron powder diffraction a cation distribution of (Fe(0.72)Co(0.28))[Fe(1.28)Co(0.72)]O4 has been determined for Co-ferrite particles of 17 nm, which is in agreement with its particle size taking into account the reported x values for other nanometric Co-ferrite particles. Magnetic measurements were performed up to 700 K as the prepared ferrite samples present blocking temperatures above room temperature. The temperature dependence of the superparamagnetic moment has been analyzed and presents for both samples an abrupt drop in the magnitude once the blocking temperature is overcome. The temperature dependence of the calculated magnetic field needed to reach the magnetic saturation of the samples allows us to determine the temperature range for which the nanoparticles show superparamagnetic behaviour. The ordering temperature is in both cases lower than the tabulated one for bulk Co-ferrite (793 K) which has been ascribed mainly to two factors: a different cation distribution and the nanometric particle size, both contributing to lowering of the strength of the superexchange interactions.