Estudio por espectroscopía Brillouin de las ondas acústicas de superficie en láminas delgadas depositadas por sputtering: influencia de la nanoestructura y la morfología

Tesis doctoral inédita. Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Aplicada. Fecha de lectura: 16-12-201

Local and medium range order influence on the magnetic behavior of sputtered Ga-Rich FeGa thin films

We have investigated the influence of the growth power on the structural properties of Fe100-xGax (x ca. 29) films sputtered in the ballistic regime in the oblique incidence. By means of different structural characterizations, mainly X-ray diffraction and X-ray absorption spectroscopy, we have reached a deeper understanding about the influence of the local and medium range order on the magnetic behavior of Ga-rich FeGa thin films. On the one hand, the increase of the growth power reduces the crystallite size (medium order) that promotes the decrease of the coercive field of the layers. On the other hand, the growth power also determines the local order as it controls the formation of the A2, B2, and D03 structural phases. The increase of the uniaxial in-plane magnetic anisotropy with growth power has been correlated with the enhancement of both Ga pairs and a tetragonal distortion. The results presented in this work give more evidence about the magnetic anisotropy sources in Ga-rich FeGa alloys, and therefore, it helps to understand how to achieve a better control of the magnetic properties in this family of alloys.This work has been financially supported through project MAT2015-66888-C3-3-R (MINECO/FEDER) and RTI2018-097895-B-C43 of the Spanish Ministry of Economy and Competitiveness and through PR26/16-3B-2 of Santander and Universidad Complutense de Madrid. We thank "CAI Difracción de rayos-X" of UCM for the X-ray diffractometry measurements and the "Instituto de Sistemas Optoelectrónicos y Microtecnología" (ISOM) for using its facilities. We also want to thank BM25-Spline, the Spanish CRG at ESRF for providing beamtime

Correlation between local structure and magnetic behavior in co-sputtered TbxFe73Ga27−x (7 ≤ x ≤ 11) thin films

We report on the evolution of the microstructure of Tb-Fe-Ga films deposited by co-sputtering from Tb33Fe67 and Fe72Ga28 targets. The sputtering power was fixed (90 W) in the Fe72Ga28 whereas it was increased from 50 to 90 W in the Tb33Fe67 target resulting on TbxFe73Ga27−x layers with 7 ≤ x ≤ 11. The local structure was determined by means of x-ray absorption fine structure spectroscopy at Fe-K, Ga-K and Tb-L3 edges. The increase of Tb in the alloy promotes the phase segregation that produces a larger amount of the TbFe2 structural phase. The structural results have been correlated with the magnetic characterization that shows the enhancement of the out-of-plane component of the magnetization.This work has been financially supported through projects MAT2015-66888-C3-3-R and PIE-2010-OE-013-200014 of the Spanish Ministry of Economy and Competitiveness (MINECO/ FEDER) and through the project PR26/16-3B-2 of Santander and Universidad Complutense de Madrid. We thank “Instituto de Sistemas Optoelectronicos y Microtecnología ” (ISOM) for using its facilities. We also want to thank ESRF and BM25-Spline, the Spanish CRG at ESRF, for providing beamtime

Nanostructured Au(111)/Oxide epitaxial heterostructures with tailoring plasmonic response by a one-step strategy

In this work, we present a strategy for developing epitaxial incommensurate nanostructured Au/oxide heterostructures with tunable plasmonic response. Previously, high-quality single-phase and single-oriented alfa-Fe2O3(0001) thin films were achieved, which have been used as a template for noble-metal epitaxial deposition. The complex systems have been grown by pulsed laser deposition on two different types of oxide substrates: alfa-Al2O3(0001) and SrTiO3(111). A one-step procedure has been achieved tailoring the isolated character and the morphological features of Au nanostructures through the substrate temperature during Au growth, without altering the structural characteristics of the hematite layer that is identified as a single iron oxide phase. The epitaxial character and the lattice coupling of Au/oxide bilayers are mediated through the sort of oxide substrate. Single-oriented Au(111) islands are disposed with a rotation of 30° between their crystallographic axes and those of alfa-Fe2O3(0001). The Au(111) and SrTiO3(111) lattices are collinear, while a rotation of 30° happens with respect to the alfa-Al2O3(0001) lattice. The crystallographic domain size and crystalline order of the hematite structure and the Au nanostructured layer are dependent on the substrate type and the Au growth temperature, respectively. Besides, the functional character of the complex systems has been tested. The localized surface plasmons related to Au nanostructures are excited and controlled through the fabrication parameters, tuning the optical resonance with the degree of Au nanostructuring.This work was supported by the Ministerio Español de Ciencia, Innovación y Universidades (MCIU), and the Consejo Superior de Investigaciones Cientificas (CSIC) through the project PIE-2010 6 OE 013. J.L.-S. and E.E. acknowledge the FPI fellowship and the Torres Quevedo contract (ref: PTQ-14-07289). The ESRF, MCIU, and CSIC are acknowledged for the provision of synchrotron radiation facilities. The authors thank Dr M.A. García for the useful discussion about the plasmonic results and Carlos Beltrán for technical support during the experiments at the BM25 beamline at The ESRF

Selenium speciation in waste materials from an exhausted Iberian Pyrite Belt mine

Selenium is an essential nutrient for humans, animals and microorganisms, but it becomes toxic at concentrations slightly above the nutritional levels. This naturally occurring trace element can be released to the environment from various anthropogenic sources such as mining, agricultural, petrochemical and industrial processes and its toxicity is developed along a complex cycle involving adsorption by soil components and subsequent accumulation by plants. In the environment, selenium can occur in several oxidation states ranging from selenide (Se=) to elemental selenium (Se0), selenite (Se4+) and selenate (Se6+). Selenium contents above 900 ppm were recently assigned in mine wastes from the sulphur factory at the São Domingos exhausted pyrite mine exploited in the Iberian Pyrite Belt (south Portugal) since Roman times until 1966. Aiming at a sustainable remediation of this mining site, an X-ray absorption spectroscopy study using synchrotron radiation, combined with X-ray diffraction, was undertaken to clarify the speciation state of selenium and the nature of Se-carrier phase(s). The results show that selenium does not significantly replace sulphur under the form of selenate in the dominant sulphate phases and occasionally remains as a substituting selenide anion in debris of the original sulphides present in the mine waste materials

Mineralogy and chemistry of incrustations resulting from the 2014-15 eruption of Fogo volcano, Cape Verde

The last eruption of the Fogo volcano, in the Cape Verde Archipelago, occurred in 2014-2015. A mineralogical and chemical study was undertaken on fumarole incrustations resulting from this event and compared with results obtained from the previous 1995 eruption. The mineralogical constitution of the fumarole deposits was assessed by X-ray diffraction and the chemical characterization was performed through X-ray fluorescence spectrometry with a wavelength dispersive system and by energy dispersive X-ray fluorescence at the European Synchrotron Radiation Facility. The most common compounds/minerals in solid deposits were sulfur, sodium chloride, and calcium sulphates with variable degrees of hydration, sodium sulphate, hydrated sulphates of sodium aluminum, potassium magnesium, or sodium magnesium and a fluorine-bearing mineral. Thenardite (Na2SO4) and its polymorph (phase III) were found simultaneously for the first time in incrustations, to the best of our knowledge. A large span of minor and trace elements present in incrustations (Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Fe, Ni, Cu, Zn, As, Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Ba, Ce, Tl, Pb) were also identified, some of them potentially hazardous to animal and human health. This study reveals that low temperature incrustations, allied to the atmospheric conditions of Fogo volcano, constitute a natural laboratory to observe the process of mineral formation-namely the Na2SO4 phase III considered metastable.This research received financial support from FCT (Fundação para a Ciência e Tecnologia) through project FIRE (PTDC/GEO-GEO/1123/2014). We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities and in particular in using beamline BM 25A. Special thanks are due to the guides Manuel Montrond Fernandes (Izaquiel) and Edimar Montrond that helped us in the Fogo volcano. J.P. Veiga acknowledges funding by FEDER funds through the COMPETE 2020 Programme and National Funds through FCT-Portuguese Foundation for Science and Technology under the project UID/CTM/50025/2013 and the funding from the European Union Horizon 2020 research and innovation programme H2020-DRS-2015 GA nr. 700395 (HERACLES project). Carla Candeias would also like to acknowledge FCT by the grant SFRH/BPD/99636/2014 and UID/GEO/04035/2013. Special thanks are also due to Dr. Tonci Balic-Žunic, an anonymous reviewer and to the editors for their comments on the manuscript. We are also grateful to Dr. Lídia Quental for the satellite image of Fogo Island

Improving the co and ch4 gas sensor response at room temperature of alpha-fe2o3(0001) epitaxial thin films grown on srtio3(111) incorporating au(111) islands

In this work, the functional character of complex -Fe2O3(0001)/SrTiO3(111) and Au(111) islands/ -Fe2O3(0001)/SrTiO3(111) heterostructures has been proven as gas sensors at room temperature. Epitaxial Au islands and -Fe2O3 thin film are grown by pulsed laser deposition on SrTiO3(111) substrates. Intrinsic parameters such as the composition, particle size and epitaxial character are investigated for their influence on the gas sensing response. Both Au and -Fe2O3 layer show an island-type growth with an average particle size of 40 and 62 nm, respectively. The epitaxial and incommensurate growth is evidenced, confirming a rotation of 30 between the in-plane crystallographic axes of -Fe2O3(0001) structure and those of SrTiO3(111) substrate and between the in-plane crystallographic axes of Au(111) and those of -Fe2O3(0001) structure. -Fe2O3 is the only phase of iron oxide identified before and after its functionalization with Au nanoparticles. In addition, its structural characteristics are also preserved after Au deposition, with minor changes at short-range order. Conductance measurements of Au(111)/ -Fe2O3(0001)/SrTiO3(111) system show that the incorporation of epitaxial Au islands on top of the -Fe2O3(0001) layer induces an enhancement of the gas-sensing activity of around 25% under CO and 35% under CH4 gas exposure, in comparison to a bare -Fe2O3(0001) layer grown on SrTiO3(111) substrates. In addition, the response of the heterostructures to CO gas exposure is around 5–10% higher than to CH4 gas in each case.This work has been supported by the Ministerio Español de Ciencia e Innovación (MICINN) and the Consejo Superior de Investigaciones Cientificas (CSIC) through the projects PIE-2010-OE-013- 200014, PIE 2021-60-E-030 and RTI2018-095303-A-C52. The ESRF, MICINN and CSIC are acknowledged for the provision of synchrotron radiation facilities. A.S. acknowledges financial support from Comunidad de Madrid for an “Atracción de Talento Investigador” Contract (2017-t2/IND5395)

Cation distribution of cobalt ferrite electrosynthesized nanoparticles. A methodological comparison

The present work seeks to analyse the structural and magnetic properties of cobalt ferrite nanoparticles obtained by electrochemical synthesis by high-resolution transmission electronic microscopy (HRTEM), X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy (MS), neutron diffraction (ND) and SQUID magnetometer. The cationic distribution is analyzed by different techniques. The inversion degree determined by the most accurate measurements was 0.73(1), and the formula for the nanoparticles therefore was (↑Co0.27Fe0.73)[↓Co0.73Fe1.27]O4. The magnetic moment found from DC and Mössbauer spectroscopy measurements was 3.8(3) μB, and the coercivity was 7870 Oe at 100 K.This work is supported by the MINECO/FEDER Project MAT2015- 67557-C2-2-P. The authors are grateful to the Institut Laue Langevin and the Spanish CRG D1B for the neutron beam-time allocated (experiment codes 5-31-2259 and CRG-1940; https://doi.org/10. 5291/ILL-DATA.5-31-2259) and to the SpLine CRG beamline staff at ESRF for assistance during XAS experiments

Novel perovskite materials for thermal water splitting at moderate temperature

Materials with the formula Sr_2CoNb_1-xTi_xO_(6-delta) (x=1.00, 0.70; delta=number of oxygen vacancies) present a cubic perovskite-like structure. They are easily and reversibly reduced in N_2 or Ar and re-oxidized in air upon heating. Oxidation by water (wet N_2), involving splitting of water at a temperature as low as 700 ºC, produces hydrogen. Both compounds displayed outstanding H_2 production in the first thermochemical cycle, the Sr_2CoNb_(0.30)Ti_(0.70)O_(6-delta) material retaining its outstanding performance upon cycling, whereas the hydrogen yield of the x=1 oxide showed a continuous decay. The retention of the materials' ability to promote water splitting correlated with their structural, chemical, and redox reversibility upon cycling. On reduction/oxidation, Co ions reversibly changed their oxidation state to compensate the release/recovery of oxygen in both compounds. However, in Sr_2CoTiO_(6-delta), two phases with different oxygen contents segregated, whereas in Sr_2CoNb_(0.30)Ti_(0.70)O_(6-delta) this effect was not evident. Therefore, this latter material displayed a hydrogen production as high as 410 mu molH_2/g_(perovskite) after eight thermochemical cycles at 700 ºC, which is among the highest ever reported, making this perovskite a promising candidate for thermosolar water splitting in real devices

Anomalous local lattice disorder and distortion in A2Mo2O7 pyrochlores

We present an extended X-ray absorption fine structure study of the pyrochlores A2Mo2O7 (A ¼ Gd, Dy, Ho, Er), as a function of temperature. While in the three spin-glass compositions Dy2Mo2O7, Ho2Mo2O7 and Er2Mo2O7 the Debye temperatures are in accordance with other pyrochlores and the static disorder contributions are compatible with a lattice frustration, in the low-temperature-ferromagnetic Gd2Mo2O7 system we point out an anomalous enhancement of the local structure disorder below about 225 K down to low temperatures. Moreover, considering the general pyrochlore predisposition towards structural disorder, we prove the presence on a local scale of at least a bimodal distribution of the Mo-O(1) octahedral interatomic distances for all the studied compounds, consisting of two shorter and four longer bond lengths. Our results suggest that the local structure order parameter plays an important role in the ferromagnetic or spin-glass phase stabilization.We gratefully acknowledge the Spanish CRG at the ESRF for providing beamtime under experiment HC-2414. F.R.-M. is also indebted to MINECO for a ‘Ramon y Cajal’ contract (ref: RyC-2015- 18626), which is co-financed by the European Social Fund 2014-20. G.L. thanks M. Lucaccini and G. Tavilla for their valuable technical support and C. Robustelli for the supply of cryogenic liquids during the magnetization measurements