Spherical (ZndeltaNi1-deltaFe2O4)gamma nanoparticles in an amorphous (SiO2)1-gamma)1-gamma matrix, prepared with the sol-ge1 method

By using the sol-gel method we have obtained spherical nanoparticles (NPs) of stoichiometric (ZndeltaNi1-deltaFe2O4)gamma mixed ferrite (*d = 0.15; 0.35; 0.65 and *g= 0.15), dispersed in an amorphous (SiO2)1-gamma, matrix (nanocomposites). The nanocomposites were investigated at room temperature by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), electron diffraction (ED), Mossbauer spectroscopy and quasi-static magnetic measurements. We have studied in detail the magnetic behavior of ferrite NPs in a silica matrix, both under quasi-static (50 Hz) and dynamic (20 MHz) conditions. The results show a pure superparamagnetic (SPM) behavior of the sample that has a concentration of the Zn ions of *d = 0.15. At higher concentrations, there is a significant deviation from this behavior and this deviation increases with the increase of 8. We attribute the observed behavior to the Neel-type magnetic relaxation processes. After processing the TEM image and performing magneto-granulometric measurements, it has resulted that on the surface of NPs embedded in the silica matrix a layer is formed; this layer has a thickness in the size range of nanometers and it has no magnetic ordering.