Synthesis of $\mathsf{\gamma}$ -Fe $\mathsf{_2}$ O $\mathsf{_3}$ nanoparticles coated on silica spheres: Structural and magnetic properties

The structural and magnetic properties of $\gamma$ -Fe 2 O 3 nanoparticles dispersed on silica spheres prepared by sol-gel method were investigated. The properties of $\gamma$ -Fe 2 O 3 nanoparticles without silica were compared with those on silica spheres. Both the nanoparticle assemblages were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), Mössbauer (20, 80 and 300 K) and electron paramagnetic resonance (EPR) (80, 300 K) measurements. The XRD spectra clearly indicated the formation of pure $\gamma$ -Fe 2 O 3 nanoparticles and the absence of any other form of iron oxide. TEM images showed a uniform distribution of the nanoparticles of size $\sim$ 6 nm on the surfaces of silica spheres (diameter $\sim$ 35-60 nm). The size of the individual nanoparticles (without silica) varied within 5-6 nm. The low temperature (20 K) Mössbauer spectra consisted of a partially split sextet superimposed on a doublet. The partial magnetic splitting of the sextet at 20 K revealed the effect of surface magnetization and surface modifications of the $\gamma$ -Fe 2 O 3 nanoparticles coated on silica spheres. The gradual collapse of the partially split sextet into a doublet with increasing temperature indicated the superparamagnetic relaxation exhibited by the $\gamma$ -Fe 2 O 3 nanoparticles with/without silica. The surface magnetization arising out of mis-aligned spins at the surface as evidenced by Mössbauer spectra was further confirmed by electron paramagnetic resonance (EPR) studies. Copyright Springer-Verlag Berlin/Heidelberg 2003