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

Abstract.

The structural and magnetic properties of $\gamma$-Fe₂O₃ nanoparticles dispersed on silica spheres prepared by sol-gel method were investigated. The properties of $\gamma$-Fe₂O₃ 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 $\gamma$-Fe₂O₃ 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 $\gamma$-Fe₂O₃ 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 $\gamma$-Fe₂O₃ 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.