Magnetic transitions and structural characteristics of Mn-doped α-Fe2O3/silica nanocomposites

Hematite (α-Fe2O3) has become popular these days for their photocatalytic activities of water splitting. Metal-doped hematite materials are interesting as well for the bandgap engineering and for resolving fast charge–hole recombination. In this study, magnetism and ionic behaviors of rare manganese-doped α-Fe2O3/silica nanocomposites are investigated. These nanocomposites are prepared by the impregnation method with a mixture of metal halides, followed by rapid heating (30 °C/min) under air condition. When the molar ratio between FeCl3·6H2O and MnCl2·4H2O is 2.97, wasp-waisted hysteresis and ferromagnetism with the Curie temperatures of 56.1 and 58.0 K are observed for the nanocomposites annealed at 600 °C for the duration of 3 and 7 h, respectively, while dominant spin glass states are observed for the nanocomposites annealed at 500 °C. In x-ray diffraction patterns, mixed phases of α-Fe2O3 are identified, whereas crystalline metallic Mn or Mn oxides are hardly found. Electron energy-loss spectroscopy study indicates that Mn2+ is severely oxidized, and with this oxidation of Mn2+, Si becomes more metallic. When the molar ratio between Fe and Mn halides is 7.32, magnetism is affected by a small amount of γ-Fe2O3, and spin glass states and the competition between ferromagnetism and antiferromagnetism are observed in the long temperature range.