Thermal annealing of nanocrystalline Fe3S4 films deposited on Si substrates by dc-magnetron sputtering at room temperature

Thermal annealing of nanocrystal Fe3S4 films are studied using x-ray diffraction (XRD), scanning-electron microscopy, micro-Raman scattering and vibrating sample magnetometry. The evolution of XRD patterns due to thermal annealing provides evidence for the conversion of Fe3S4 to Fe7S8 and Fe3O4 at low temperatures and an increase in the annealing temperature leads to a single phase of Fe3O4. Raman spectroscopy reveals a large electron-phonon coupling strength for the Fe3O4 nanocrystals, which also exhibits an apparent reduction with an increase in annealing temperature. This behaviour, together with the magnetization evolutions, most likely originates from the coalescence of Fe3O4 grains as well as the annihilation of crystal disorders/defects, such as antiphase boundaries, presented in the Fe3O4 nanocrystals.