Thermal excitation studies on the silica coating magnetite composite microspheres with Mössbauer spectra and first principle calculations

Fe3O4 nanomaterials have attracted lastest interests due to their excellent properties and potential applications. In this paper, the thermal excitation is investigated by Mossbauer spectra and first principle calculations. Fe3O4 nanoparticles with uniform size, regular shape and good dispersibility are prepared by the solvothermal methods, and Fe3O4@SiO2 composite magnetic particles with layers of mesostructured SiO2 structure are assembled by improved Stober method. The particle size is 500 nm and the shell thickness of SiO2 layer is 80 nm. Fe3O4@SiO2 composite magnetic particles are then annealed at different temperature of 200 °C, 400 °C and 600 °C. The experimental measurements exhibit that the grain size grows up, and the magnetization is enhanced significantly, with increasing heat treatment temperature. However, Mossbauer spectra show that the relative absorption intensity in B-site fast decreases as increasing heat treatment temperature, from 64% at room temperature to 27% at 600 °C, indicating that large amount of Fe vacancies may form in B-site, and correspondingly, the net magnetic moment due to the difference of amount of Fe in B and A sites would be reduced, which strongly disagrees with the results of magnetization measured by vibrating sample magnetometer. In order to find the mechanism behind magnetization, the first-principle calculations are performed and reveal that the oxygen ions in the SiO2 shell may go into Fe3O4 core, which causes number of Fe2+ ions to decreases and transfer to Fe3+ ions, leading to the increase of magnetic moment. So we classify the difference between the amount of Fe3+ and Fe2+ ions in B-site as γ-Fe2O3. Based on this, Mossbauer spectra analysis presents quantitatively the concentration of various phase and their hyperfine interactions. This work provides a method to analyze the magnetic moment of Fe3O4.