Coexistence of superparmagnetism and spin-glass like magnetic ordering phenomena in a CoFe2O4-SiO2 nanocomposite

The effect of interparticle interactions on the magnetic behavior of cobalt ferrite nanoparticles was investigated in a CoFe2O4/SiO2 nanocomposite with 50 wt % of magnetic phase. A sample of pure cobalt ferrite nanoparticles prepared by the same method was also studied, as reference. Both samples show irreversible magnetic behavior, when the DC magnetization, M(T), is observed by the zero field cooling-field cooling (ZFC-FC) measurements. For a field higher than 0.2 T, the ZFC curves of the two samples show a maximum and the corresponding temperature is proportional to the magnetic field raised to the 2/3 power (T-max proportional to H-2/3). This study shows that in both samples the M-ZFC(T) maxima are due to magnetic ordering phenomena rather than superparamagnetic blocking, pointing out that strong interactions among particles are present also in the nanocomposite. However, the presence of the silica matrix gives rise to the coexistence of two different magnetic behaviors. In fact, a secondary maximum at low temperature is observed in the M-ZFC(T) curves measured for the silica supported sample. The study of the T-max vs H-2/3 plot of such secondary MZFC(T) maximum shows that it is not due to collective magnetic ordering but rather to a superparamagnetic transition of a fraction of small nanoparticles. The coexistence of two populations of nanoparticles is in agreement with Mossbauer spectroscopy and TEM analysis.