Silica Encapsulated Ni Nanoparticles: Variation of Optical and Magnetic Properties with Particle Size

Ni nanoparticles embedded in SiO₂ matrix were prepared by sol–gel process. The molar percentages of Ni were varied from 2 to 20% of total SiO₂ present in the matrix. Transmission electron microscope (TEM) images revealed that particle sizes varied from 8.0–15.7 nm at an annealing temperature of 773 K with variation of concentration. The optical absorption spectra revealed that the surface plasmon resonance (SPR) peak in the UV region of the spectrum shifted with the particle diameter (D) from that at 247.3 nm for D = 8.0 nm to 250.7 nm for D = 15.7 nm. In hysteresis loop measurements the magnetizations (M) of the nanocomposites also increased with higher Ni content in the matrix and did not saturate in the measuring limit of the magnetic filed (H) of 4 KOe. The anhysteric curves for different samples were analyzed with the law of approach to saturation (LAS). The zero field cooled (ZFC) and field cooled (FC) magnetization measurements at 50 Oe showed increasing broadening of the ZFC curve with the higher Ni content. To calculate the average blocking temperature (<T _B>) a distribution of the blocking temperatures (T _B) was assumed to initiate theoretical fittings and it was found to be increasing with the Ni concentration in the matrix.