Magnetic properties of nanocrystalline CoFe2O4 dispersed in amorphous silica

Almost spherical CoFe2O4 nanocrystals with a narrow size distribution centered around 2.5(5)nm, uniformly dispersed in an amorphous SiO2 matrix have been prepared by high energy ball milling followed by thermal treatment at 900°C. Mössbauer spectra show the presence of CoFe2O4 nanoparticles in the superparamagnetic state at 293K and mainly in the blocked state at 77K. This is confirmed by ZFC/FC measurements giving evidence of a distribution of blocking temperatures, centered at 50K.