Insight into surface heterogenity of SBA-15 silica: Oxygen related defects and magnetic properties

The magnetic properties of SBA-15 silica were investigated between liquid helium and room temperature by a SQUID magnetometry. The sample exhibits the ferromagnetic behaviour at 2 K, represented by weak hysteresis with coercivity of Hc = 60 Oe and the saturation of the magnetization. The magnetic behaviour is sensitive to the atmosphere of experiment. At 2 K the sample is ferromagnetic in air but diamagnetic in helium. The observed ferromagnetic interaction shows on the presence of unpaired electrons in the sample and the parallel spin alignment. It is suggested that the magnetic behaviour of the sample is the result of the adsorption of the oxygen molecules at the point defects, oxygen vacancies on the silica surface. The radical sites (≡Si–O• and ≡Si•), which are known to be generated by dehydroxylation of silica surface isolated hydroxyl groups at temperatures above 673 K, are important for the observed ferromagnetic alignment of the oxygen molecules. The value of magnetization was shown to depend on the temperature of the hydrothermal treatment during the synthesis of SBA-15. The magnetization of the SBA-15 sample increased as the synthesis temperature increased from 40 to 100 °C, with the corresponding values of the magnetization 0.25 and 0.89 emu/g, respectively. The magnetic behaviour observed for SBA-15 sample was compared with the magnetic properties of MCM-41 silica and quartz glass.