Giant magnetodielectric effect in composites of nanodimensional spin glass of system CoO-SiO2 and mesoporous silica SBA-15

Abstract

Nanodimensional silica-based spin glasses of compositions xCoO-(100-x)SiO2 with x having values 20 and 30 were synthesized by sol-gel method within mesoporous silica SBA-15 (Santa Barbara Amorphous-15) having a pore diameter ~5.5 nm. X-ray photoelectron spectroscopy (XPS) analysis showed the presence of both Co2+ and Co3+ species within the glasses. This caused electronic conduction in this amorphous system. The nanoglasses exhibited resistivity values at room temperature which were about three orders of magnitude lower than those of the corresponding bulk glasses. The resistivity data for the nanodimensional glasses on analysis confirmed the conduction to arise due to small polaron hopping between the localized states represented by these ions with an activation energy in the range 0.08–0.07 eV. The inter-site separation values extracted from the analysis of the resistivity data were found to lie in the range 6.7–6.6 Å. The values of magnetodielectric (MD) parameter for the different nanocomposites were rather large with the highest value found to be in the range 523–49% for the frequencies 1 kHz and 1 MHz in case of nanocomposite with a glass composition 30CoO-70SiO2. The dissipation factor for the samples was, however, in the range 0.3–1.6. By using suitable measuring frequency the dissipation factor could be brought down to a value less than 1.0 with the M.D. parameter exhibiting values in the range 10–49%. The results were fitted to Catalan’s model based on two dielectrics with different resistivity values connected in series. The satisfactory fit of the experimental data to the theoretical model based on a negative magnetoresistance of the nanoglasses leads to the conclusion that an enhancement of spin polarized electron hopping is the reason behind this effect. The results obtained should be further explored to find applications of these materials as magnetic sensors as well as magnetically controlled supercapacitors.