Effect of silica coating on the structural, dielectric, and magnetic properties of maghemite nanoparticles

Influence of silica (SiO2) coating concentration (x) = 0, 15, 30, 45, and 60% on structural, dielectric, and magnetic properties of maghemite nanoparticles has been studied. X-ray diffraction analysis confirmed the cubic inverse spinel structure for the samples with x = 0, 15 and 30%, while x = 45 and 60% samples show amorphous behavior. Both average crystallite size and lattice constant were decreased with SiO2 concentration. Scanning electron microscopy images showed that the nanoparticles are spherical in shape and self-organized in leaf-like shape. Fourier transform infrared spectroscopy analysis confirmed the formation of maghemite and SiO2 phases for all the samples. At higher SiO2 concentration, vibrational bands of maghemite were suppressed and some bands overlapped with the SiO2 bands. Dielectric parameters such as dielectric constant and loss tangent were decreased with increasing frequency and became constant at higher frequencies. However, ac conductivity showed an opposite trend with higher values at higher frequencies. Dielectric properties have been explained using Koop's and Maxwell-Wagner models. Nanoparticles with x = 30% SiO2 concentration showed the highest dielectric constant with reduced magnetization. Dielectric properties also showed non-monotonic dependency on the SiO2 concentration. Bare nanoparticles got high saturation magnetization close to bulk value due to larger average crystallite size while, nanoparticles with higher SiO2 concentration exhibited smaller magnetization and higher coercivity which is attributed to their smaller average crystallite size. In summary, SiO2 concentration has significant effects on the structural, dielectric, and magnetic properties of maghemite nanoparticles and can be used to get suitable ferrite/SiO2 nanocomposites for different applications.