Superparamagnetic Behavior of MFe2O4 Nanoparticles and MFe2O4/SiO2 Composites (M: Co, Ni)

Two kinds of nanosized ferrite systems have been prepared: MFe2O4 nanoparticles and MFe2O4/SiO2 (M: Co, Ni) nanocomposites with different ferrite particle sizes. Magnetic measurements have been done for both ferrite systems in the 5-700 K temperature range, and the silica matrix effect on the magnetic behavior has been studied. Whereas CoFe2O4 samples are characterized by high anisotropy values, NiFe2O4 particles seem to be magnetically soft, which may favor the dipolar interactions. The silica matrix avoids all kinds of particle interactions that modify the magnetic behavior. Therefore, in 2-5 nm embedded Co-ferrite particles, a decreasing H-C value with the increasing particle size is not observed. This almost constant H-C value indicates that interactions between surface spins of different particles are absent for this particle size range. In the case of Ni-ferrite particles, the dipolar interactions between bulk particle moments are minimized due to the presence of the SiO2 matrix.