Raúl Pozas

THE NATURE OF Co IN SYNTHETIC Co-SUBSTITUTED GOETHITES

The crystallochemical features of Co in Co-substituted goethite solid-solutions prepared by two different procedures have been studied using infrared, X-ray photoelectron and electron energy loss spectroscopies. It was found that the path followed for the synthesis of Co-substituted goethite determines the oxidation state of Co in the goethite structure. Thus, in the solid-solution prepared by precipitation with Na2CO3 of an Fe(II) aqueous solution containing Co(II) cations, followed by the aerial oxidation of the precipitate, the Co cations were found to be divalent, whereas trivalent Co was incorporated into the goethite obtained by ageing a solution containing Fe(III) and Co(II) cations precipitated by the addition of KOH. This different behavior is explained by the higher pH of goethite formation in the latter case, which favors the oxidation of the Co(II) cations.

Phase delay and group velocity determination at a planar defect state in three dimensional photonic crystals

Phase sensitive optical transmission measurements have been performed on three dimensional opal-based photonic crystals containing a planar defect. From numerical derivation of the measured phase, the group velocity has been retrieved. Strong modulations in the group velocity are seen to correlate with a recovery in the transmission inside the forbidden spectral interval, demonstrating the presence of a localized defect state. Accordingly, the phase change measured across the forbidden interval doubles in the lattice containing a planar defect with respect to the defect-free crystal, as expected when introducing a localized state inside the pseudogap. All results have been modeled with a scalar wave approximation in a two band model including extinction.

FeCo magnetic nanoneedles obtained by Co-coating haematite

Uniform FeCo magnetic nanoneedles of ?nm in width and axial ratio have been obtained by Co-coating haematite and subsequent hydrogen reduction in two steps. By this method, FeCo needle-like nanoparticles were obtained with Co contents up to 10% evenly distributed inside the particles. Higher Co contents up to 30% were incorporated by coating the Co-doped magnetite nanoneedles. No segregation of a secondary phase was observed in any case and the morphology of the particles was preserved without adding any extra element. Structural changes during the reduction process have been followed by several techniques. Although protected by an oxide layer, the final metal nanoparticles behave as single crystals, but they are composed of sub-crystals with the same crystallographic orientation and a mean diameter that decreases as the Co content increases. We found the highest reported saturation magnetization values for samples of similar size (180?emu?g?1). The evolution of the coercivity with the cobalt modification could arise from the changes of the microstructure and the contributions of shape and crystalline anisotropies. Time dependence magnetization measurements showed the thermal activation to be related to the presence of crystallographic sub-units within the nanoneedles.

The influence of protective coatings on the magnetic properties of acicular iron nanoparticles

Core?shell acicular nanoparticles of ~90?nm length and ~5 axial ratio consisting of an iron core coated with an oxidized layer of different composition have been obtained by thermal reduction with hydrogen of coated goethite precursors. Uniform goethite particles were prepared by oxidation in air of FeSO4 solutions previously precipitated with Na2CO3. Al or Y oxide layers were deposited on the goethite particle surface by heterocoagulation. The efficiencies of both compounds as protecting agents for preventing particle sintering and corrosion and their effects on the magnetic properties of the final ?-Fe nanoparticles have been evaluated. Yttrium oxide coating was found to be more effective than alumina coating, giving rise to iron nanoparticles with larger coercivity and higher corrosion resistance. This behaviour is mainly discussed in terms of the observed differences in location of the two sintering prevention agents during the transformation from iron oxide to metal, which give rise to different particle microstructures.

Determination of the Orientation of the Crystallographic Axes in Anisometric Particles by Infrared Spectroscopy

The applicability of IR spectroscopy for the determination of the orientation of the crystallographic axes in uniform anisometric particles is discussed for the case of monocrystalline hematite particles. Two samples consisting of uniform particles of similar size (∼300 nm in length) and shape (axial ratio ∼5) obtained by different preparation methods have been used in this study. It is shown that in spite of the similar composition, crystalline structure, and morphological characteristics of the samples, their IR spectra present important variations in the position and relative intensity of the absorption bands due to the different orientations of the crystallographic axes. Such behavior can be accounted for by the effective medium theory using the optical constants of hematite. Therefore, from the analysis of the IR spectrum of a powder, it is possible to identify the orientation of the crystallographic axes of the constituent anisometric particles. These results have been corroborated by electron diffraction carried out on individual particles.

Improving Co distribution in acicular Fe-Co nanoparticles and its effect on their magnetic properties

We report on a procedure for increasing the Co content in acicular Fe–Co nanoparticles (70 nm length and ~5 axial ratio) obtained by thermal reduction of Co-doped goethite nanoparticles coated with Co(OH)2 layers. It has been found that the diffusion of Co cations located onto the particle surface of the precursor to the inner part can be promoted by separating reduction in two steps, first from the precursor to Co-doped magnetite and, finally, from this phase to metal. Furthermore, the uniformity in the Co distribution in the final metallic alloy could be improved by annealing the Co-doped magnetite obtained from the first reduction step, resulting in an important increase of coercivity. The enhancement of the magnetic properties is mainly discussed in terms of the reversal magnetization mechanism into the metallic alloy. The hard magnetic properties resulting from the higher Co content and its homogeneous distribution in the final Fe–Co alloy, along with the reduction in the particle size, make our acicular Fe–Co nanoparticles suitable for high-density magnetic recording applications.

Full processing of colloidal photonic crystals by spin coating

Herein we show how to use spin coating, a technique widely employed in current optoelectronics technology, to fully process colloidal photonic crystals. We demonstrate that control over every stage of the lattice growth and post-treatment is achievable by using this technique.

Full processing of Colloidal Photonic Crystals by Spin-Coating

Herein we show how to use spin coating, a technique widely employed in current optoelectronics technology, to fully process colloidal photonic crystals. We demonstrate that control over every stage of the lattice growth and post-treatment is achievable by using this technique.