Showing papers by "Chandran Sudakar published in 2004"

Effect of cationic substituents on particle morphology of goethite and the magnetic properties of maghemite derived from substituted goethite

Abstract: Preparation of nanoparticles with the desired shape and size by the wet-chemical precipitation process is a challenging task. Thus, the effects of different substitutional impurities such as Al, Cr, Co or Ni on the particle morphology and phase stability of goethite have been investigated. Goethite is prepared by air oxidation of $Fe(OH)_2 · xH_2O$ gel under near neutral conditions. Below certain concentration levels of dopants $(Al^{3+} \leq 10; Cr^{3+} \leq 5; Co^{2+} \leq 10$ and $Ni^{2+} \leq 5 at.%)$ the samples remained monophasic as revealed by XRD, TEM, and IR studies. Above these levels, the substituents produce traces of secondary phases such as lepidocrocite, spinel ferrite and $M^n(OH)_n$. The goethite structure is stable in spite of the iso- or aliovalent substitutions. The individual additives have divergent influence on the particle morphology; $Al^{3+}$ and $Cr^{3+}$ decreases the particle size to <50 nm and aspect ratio (AR) <2. Co-substitution produces slender particles with AR as high as 25. Whereas, $Ni^{2+}$ does not have any influence on the particle morphology. The attributable factors in morphology control are the increased nucleation rate, restricted growth along needle axes, and the strain induced in the goethite lattice as a result of difference in ionic radii. Maghemite, $\gamma -Fe_2O_3– \delta$, particles are obtained from goethite wherein the topotactic conversion renders the retention of the particle morphology of the precursor. Maghemite with substituted impurities showed substantial differences in magnetic properties. Saturation magnetization $(\sigma s)$ and coercivity $(H_ c)$ go down to very low values due to relaxation of spins on the surface atoms as revealed by Mossbauer spectroscopy. Decrease in coercivity is by way of the presence of diamagnetic ion $(Al^{3+})$. Whereas, Co-substituted maghemite has enhanced $H_ c$ as a result of high magnetocrystalline anisotropy accompanied by the shape anisotropy.

Electrical and magnetoresistance properties of composites consisting of iron nanoparticles within the hexaferrites

Abstract: Nanocomposites containing Fe or FeCo (Fe-rich) dispersed in hexaferrites (M, W, or Y phase) are realized by the heterogeneous solid-gas reduction under H2 + N2. Transmission electron microscopy (TEM) studies show that metal nanoparticles precipitate coherently as thin flakes along the a-b planes of the hexaferrite lattice above the characteristic reduction temperature, TR >375°C. The electrical resistivity measurements reveal that the charge transport mechanism in the composites is by tunneling, whereas samples having higher fractions of the alloy particles show metallic behavior. Controlled reduction at TR leads to apparent insulator-metal changeover in the ρ versus T plot. This changeover persists even in the presence of a high magnetic field (7 T) and is ascribed to the percolation of metal particles caused by the difference in the coefficient of thermal expansion between the constituents. In the insulator regime, negative magnetoresistance (MR) of ∼5–9% is observed at 25°C. Further, ρ-T curves by the two-probe method exhibit hysteretic behavior caused by large inhomogeneity in the distribution of metal content and the time-dependent charge accumulation (Coulomb blockade) at the metal granules for these composites. They also exhibit nonlinearity in the current-voltage (I-V) characteristics with the nonlinearity coefficient ranging from 1.2 to 1.4 at different temperatures.