Re-formation of metastable ε-Fe2O3 in post-annealing of Fe2O3/SiO2 nanostructure: Synthesis, computational particle shape analysis in micrographs and magnetic properties

Several Fe2O3/SiO2 nanostructures were synthesized by the combination of the microemulsion and a sol-gel methods. Based on X-ray powder diffraction (XRD) and magnetic measurements (giant coercivity ~2.13 T) we identified ε-Fe2O3 (hard magnet) as the dominant crystalline phase. TEM analysis showed a wide size distribution of iron oxide nanoparticles (from 4 to 50 nm) with various morphologies (spherical, ellipsoidal and rod-like). We quantitatively described (computational analysis, MATLAB code) morphological properties of nanoparticles using the ellipticity of the shapes. The as-synthesized hard magnetic material was subjected to a post-annealing treatment at different temperatures (200, 500, 750, 1000 and 1100 °C) in order to investigate stability, formation and transformation of the ε-Fe2O3 polymorph. We found decreasing coercivity in the thermally treated samples up to the temperature of 750 °C (Hc=1245 Oe), followed by an observation of a surprising jump in coercivity HC~1.5 T after post-annealing at 1000 °C. We conclude that the re-formation of the ε-Fe2O3 structure during post-annealing at 1000 °C is the origin of the observed phenomena. The phase transformation ε-Fe2O3→α-Fe2O3 and crystallization of amorphous silica in quartz and cristobalite were observed in the sample treated at 1100 °C.