José F. Marco

Cation distribution and magnetic structure of the ferrimagnetic spinel NiCo2O4

The compound NiCo2O4, with spinel-related structure, has been prepared by thermal decomposition of metal nitrates and its bulk structural properties examined by means of magnetic measurements, neutron diffraction, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The results suggest a delocalised electron distribution on the octahedral sites with average oxidation states of +3.5 and +2.5 for nickel and cobalt, respectively, and lead to a cation distribution for NiCo2O4 of {Ni3+0.1Co2+0.9}tet[Ni3.5+0.9Co2.5+1.1]octO4. This electronic configuration is consistent with magnetisation measurements if applied magnetic fields cause a charge redistribution on the octahedral sites to favour Co3+ and Ni3+. The surface of NiCo2O4 was examined by X-ray photoelectron spectroscopy (XPS) and found to have a different composition containing Co2+, Co3+, Ni2+, Ni3+ and, probably, Ni4+.

Electrooxidation of 2-chlorophenol on polyNiTSPc-modified glassy carbon electrodes

Abstract The electrooxidation of 2-chlorophenol (2-CP) on poly-nickel tetrasulfophthalocyanine (polyNiTSPc) modified glassy carbon (GC) and indium tin oxide (ITO) electrodes was studied by cyclic voltammetry and in situ UV–Vis transmission spectroscopy. The polyNiTSPc films were characterized by XPS and AFM, both before and after reaction with 2-CP. The results indicate that NiTSPc polymerizes at both GC and ITO electrodes, apparently retaining its phthalocyanine structure in which the Ni atoms play a role similar to that in Ni(OH) 2 films. In the oxidation of 2-CP at polyNiTSPc-modified electrodes the electrode fouling by a layer of polymerized 2-CP oxidation products (polyCP) is weaker than on the bare GC electrode, probably because the polyCP layer formed on polyNiTSPc–ITO is very rough, as indicated by AFM, thus allowing the passage of CP molecules to the polyNiTSPc film.

Ultrasmall Iron Oxide Nanoparticles for Biomedical Applications: Improving the Colloidal and Magnetic Properties

A considerable increase in the saturation magnetization, M(s) (40%), and initial susceptibility of ultrasmall (<5 nm) iron oxide nanoparticles prepared by laser pyrolysis was obtained through an optimized acid treatment. Moreover, a significant enhancement in the colloidal properties, such as smaller aggregate sizes in aqueous media and increased surface charge densities, was found after this chemical protocol. The results are consistent with a reduction in nanoparticle surface disorder induced by a dissolution-recrystallization mechanism.

Magnetism in nanometer-thick magnetite

The oldest known magnetic material, magnetite, is of current interest for use in spintronics as a thin film. An open question is how thin can magnetite films be and still retain the robust ferrimagnetism required for many applications. We have grown 1-nm-thick magnetite crystals and characterized them in situ by electron and photoelectron microscopies including selected-area x-ray circular dichroism. Well-defined magnetic patterns are observed in individual nanocrystals up to at least 520 K, establishing the retention of ferrimagnetism in magnetite two unit cells thick.

Investigation into the effect of Si doping on the performance of SrFeO3−δ SOFC electrode materials

In this paper we report the successful incorporation of silicon into SrFeO3−δ perovskite materials for potential applications as electrode materials for solid oxide fuel cells. It is observed that Si doping leads to a change from a tetragonal cell (with partial ordering of oxygen vacancies) to a cubic one (with the oxygen vacancies disordered). Annealing experiments in 5% H2/95% N2 (up to 800 °C) also showed the stabilization of the cubic form for the Si-doped samples under reducing conditions, suggesting that they may be suitable for both cathode and anode applications. In contrast to the cubic cell of the reduced Si doped system, reduction of undoped SrFeO3−δ leads to the formation of a brownmillerite structure with ordered oxide ion vacancies. SrFe0.90Si0.10O3−δ and SrFe0.85Si0.15O3−δ were analysed by neutron powder diffraction, and the data confirmed the cubic cell, with no long range oxygen vacancy ordering. Mossbauer spectroscopy data were also recorded for SrFe0.90Si0.10O3−δ, and indicated the presence of only Fe3+ and Fe5+ (i.e. disproportionation of Fe4+ to Fe3+ and Fe5+) for such doped samples. Conductivity measurements showed an improvement in the conductivity on Si doping. Composite electrodes with 50% Ce0.9Gd0.1O1.95 were therefore examined on dense Ce0.9Gd0.1O1.95 pellets in two different atmospheres: air and 5% H2/95% N2. In both atmospheres an improvement in the area specific resistance (ASR) values is observed for the Si-doped samples. Thus the results show that silicon can be incorporated into SrFeO3−δ-based materials and can have a beneficial effect on the performance, making them potentially suitable for use as cathode and anode materials in symmetrical SOFCs.

Magnetic and hysteretic properties of Fe-filled nanotubes

The hysteretic properties of Fe-filled carbon nanotubes samples are presented and related to their magnetic phase distribution obtained by Mossbauer spectroscopy. The most relevant result corresponds to the observation of a high anisotropy /spl gamma/-Fe phase getting magnetically ordered at 100 K. Below this temperature, irreversibility in the high field zero field cooling (ZFC)-field cooling (FC) curves, as well as shifted hysteresis loops are observed. These results are explained in terms of the blocking of the high anisotropy phase and the development of exchange anisotropy.

Preparation and characterisation of tin-doped α-FeOOH (goethite)

Tin-doped α-FeOOH (goethite) has been prepared directly by the hydrothermal processing of a precipitate. The morphology of the crystals, which is reflected in the 57Fe Mossbauer spectra, depends on the pH of the precipitating media. The 119Sn Mossbauer spectra are consistent with a complex microstructure around the octahedrally coordinated tin ions in the goethite structure and are sensitive to the changes in the magnetic structure of α-FeOOH resulting from tin doping. 57Fe Mossbauer spectroscopy and electron microscopy show acicular tin-doped α-FeOOH to be converted to tin-doped α-Fe2O3 with identical morphology by mild heating in air.

Magnetic properties of the FexMn0.70−xAl0.30 (0.40⩽x⩽0.58) alloy series

The magnetic properties of the FexMn0.7−xAl0.30 (0.40⩽x⩽0.58) alloy series have been investigated by means of 57Fe Mossbauer spectroscopy and ac magnetic susceptibility measurements. From our data, we propose a magnetic phase diagram for the system which is adequately reproduced by a diluted and random-bond Ising model which was analyzed on the basis of the mean-field renormalization-group method.

On the origin of remanence enhancement in exchange-uncoupled CoFe2O4-based composites

Abundant efforts are being devoted to the production of nanostructured composites based on ferrites where the exchange-spring mechanism yields larger energy products. However, experimental results occasionally contradict the predictions, and the origin of the phenomenology is not always thoroughly understood. Magnetic properties may eventually evolve in trends similar to that of exchange-spring magnets due to structural changes unrelated to exchange-coupling mechanisms at hard/soft interfaces. Thus, the evaluation of the possible sources of magnetic improvement is pivotal in determining and optimizing the extent of exchange-coupling in spring magnets. Here, we report the observation of remanence improvement and single-phase reversal behavior at room-temperature in CoFe2O4/FeCo composites, where the hard and soft phases are predominantly uncoupled. The results are explained in terms of the temperature evolution of the individual properties of CoFe2O4. Causes for the observation of single-phase magnetizatio...

Wavelength and pulse duration effects on laser induced changes on raw pigments used in paintings

In this study, the reaction of widely used artists pigments in raw form to pulsed laser radiation of different wavelengths and pulse duration was investigated. Vermilion, lead chromate and malachite (in the form of pellets) were irradiated using laser pulses of 500 fs at 248 nm, and pulses of 150 ps and 15 ns at 1064 and 213 nm. Optical microscopy, colorimetry, spectrofluorimetry, micro-Raman spectroscopy and X-ray photoelectron spectroscopy were employed to characterize the physicochemical changes induced to the pigments. Change of crystalline phase was identified for vermilion while reduction processes take place for lead chromate and malachite. It was found that these effects were minimized by application of ultraviolet ultrashort pulses (of femtosecond and picosecond duration) as compared with changes occurring by pulsed infrared irradiation (of both picosecond and nanosecond duration). The results presented are discussed in relation to previous research on painted mock-ups in order to elucidate the role and significance of the binding media in the laser induced discoloration of painted surfaces and thus to propose optimum laser cleaning practices.