Synopses & Reviews
In this dissertation, synthesis, structural and magnetic properties of nickel (Ni) nanoparticles (NPs) embedded in amorphous silica matrix are described in detail. These Ni NPs were prepared using the sol-gel technique. The percent composition of x-Ni/SiO2 was varied from 1, 5 and 15%. Further, the samples were annealed in a furnace at temperatures between 400 to 800C for the duration of 2 hours in a continuous flow of ultra high pure (UHP) nitrogen in order to obtain different particle sizes ranging from 3.8 to 23 nm for the 15% Ni/SiO 2 composition. Structural characterization of the Ni NPs was done using transmission emission microscopy (TEM) and x-ray diffraction (XRD). Average particle sizes were obtained from the TEM micrographs by fitting them to log-normal distribution giving 3.8, 11.7, 15, 21 and 23 nm. The particle sizes were compared to those calculated from XRD patterns using the Debye-Scherrer equation. Magnetic properties of these Ni NPs was studied using the superconducting quantum interference device (SQUID). The variations of the blocking temperature (TB) with measuring frequency (fm) and applied field (H) are reported for Ni NPs with the nominal composition Ni/SiO2 (15/85). Measurements from the variation in magnetization (M) vs. temperature (T) (2 to 350 K) in H enabled us to determine the TB from the peaks in the zero field cooled curves. Measurements from M vs. H data above T B was fit to the modified Langevin function to obtain the magnetic moment per particle (muP). The large value of the moment of the order of (103 muB) characterized these particles as superparamagnetic above TB. Hysteresis measurements on cooling the sample in H were also done as a check for the presence of anti-ferromagnetic/ferromagnetic layer leading to exchange bias. Temperature dependence of AC susceptibility measurements were done for frequencies varying from 0.1 to 997 Hz. The blocking temperatures TB, as determined by peaks in chi'' vs. T data, were fit to the Vogel-Fulcher law to determine the energy barrier and strength of the interparticle interaction. The temperature dependence (5 to 300 K) of the electron magnetic resonance (EMR) lines observed at 9.28 GHz in 15% Ni/SiO2 nanocomposites with different particle sizes are also reported. In EMR, three resonance lines are observed: (i) Line 1 with linewidth DeltaH≃50 Oe and g≃2, and Curie-like variation of the line-intensity, with DeltaH and g being temperature and size-independent; (ii) Line 2 with DeltaH≃50 Oe and g≃2.3 for D=3.8 nm at 294 K with both DeltaH and g increasing with decreasing T and DeltaH size-dependent; and (iii) weak line 3 with g∼4 at 300 K, with g also increasing with decreasing T. We argue that the line 1 is due to dangling bonds in SiO2 as a similar line with DeltaH≃9 Oe is also observed in SiO2 without Ni doping. Lines 2 and 3 are attributed to majority Ni NPs and large Ni clusters respectively whose anisotropy is both size and temperature dependent, leading to the observed DeltaH and g values of the lines.