Metallic diluted dimerization in VO2 tweeds

Altres ajuts: ICN2 was also supported by the CERCA programme (Generalitat de Catalunya).The observation of electronic phase separation textures in vanadium dioxide, a prototypical electron-correlated oxide, has recently added new perspectives on the long standing debate about its metal-insulator transition and its applications. Yet, the lack of atomically resolved information on phases accompanying such complex patterns still hinders a comprehensive understanding of the transition and its implementation in practical devices. In this work, atomic resolution imaging and spectroscopy unveils the existence of ferroelastic tweed structures on ≈5 nm length scales, well below the resolution limit of currently used spectroscopic imaging techniques. Moreover, density functional theory calculations show that this pretransitional fine-scale tweed, which on average looks and behaves like the standard metallic rutile phase, is in fact weaved by semi-dimerized chains of vanadium in a new monoclinic phase that represents a structural bridge to the monoclinic insulating ground state. These observations provide a multiscale perspective for the interpretation of existing data, whereby phase coexistence and structural intermixing can occur all the way down to the atomic scale

Structure and magnetic properties of Co-doped ZnO nanoparticles

In this work we carefully analyze the role of the microstructure on the magnetic properties of Co-doped ZnO nanoparticles prepared by the vaporization-condensation method in a solar reactor. We show that a close correlation exists between microstructural features and the appearance of ferromagnetism. Both shape and size of the particles, as well as the microstructure, can be controlled by changing the pressure inside the evaporation chamber, as evidenced by transmission electron microscopy micrographs and high resolution electron microscopy HREM. X-ray diffraction patterns and HREM make evident the absence of any significant Co segregation or any other phase different from würtzite type ZnO. On the other hand, electron energy loss spectroscopy analyses performed on several particles of würtzite type ZnO yielded an average Co concentration in good agreement with the nominal composition. Samples prepared in low pressure 10 Torr exhibit a very homogeneous microstructure and are ferromagnetic at low temperature but they have a very small saturation moment, well below that expected for a Co2+ ion. Conversely, samples prepared at higher pressure conditions 70–100 Torr show a defective microstructure and are paramagnetic and increasing the Co content does not induce ferromagnetism

Role of the microstructure on the magnetic properties of Co-doped ZnO nanoparticles

We report on the magnetic and structural properties of Co-doped ZnO nanoparticles prepared by the vaporization-condensation method in a solar reactor. X-ray diffraction data and high-resolution electron microscopy (HREM) confirm the total absence of metallic Co clusters or any other phase different from würtzite-type ZnO.Electron energy loss spectroscopy analyses performed on several particles indicate that the oxidation state of Co is +2 and yield an average Co concentration of 4.5at.%, in good agreement with the nominal composition. Transmission electron microscopy micrographs show that shape and size of the particles are strongly dependent on the preparation conditions, as well as the microstructure as evidenced by HREM. Ferromagnetism is only found in samples prepared in vacuum revealing a close correlation between microstructure and magnetic properties

Antisite Defects and Chemical Expansion in Low-damping, High-magnetization Yttrium Iron Garnet Films

Yttrium iron garnet is widely investigated for its suitability in applications ranging from magneto-optical and microwave devices to magnonics. However, in the few-nanometer thickness range, epitaxial films exhibit a strong variability in magnetic behavior that hinders their implementation in technological devices. Here, direct visualization and spectroscopy of the atomic structure of a nominally stoichiometric thin film, exhibiting a small damping factor of 3.0 ⋅ 10, reveals the occurrence of Y-excess octahedral antisite defects. The two-magnon strength is very small, Γ≈10 Oe, indicating a very low occurrence of scattering centers. Notably, the saturation magnetization, 4πM=2.10 (±0.01) kOe, is higher than the bulk value, in consistency with the suppression of magnetic moment in the minority octahedral sublattice by the observed antisite defects. Analysis of elemental concentration profiles across the substrate-film interface suggests that the Y-excess is originated from unbalanced cationic interdiffusion during the early growth stages

Estudio de cerámicas nanoestructuradas mediante difracción de rayos X y microscopía electrónica de alta resolución

Este proyecto trata sobre la determinación de las relaciones epitaxiales que se dan entre una capa de NGO (Oxido de Neodimio-Galio) y una capa depositada de CGO (Óxido de Cerio dopado con Gadolinio). Con ello buscamos estudiar indirectamente como podemos producir las dislocaciones antes citadas mediante la tensión superficial que se crea al dar lugar un crecimiento heteroepitaxial auto-ensamblado de nanohilos sobre un substrato. Para utilizar en el futuro esta cerámica nanoestructurada como plantillas de superconductores. Abordaremos este objetivo mediante dos vertientes distintas. Por un lado, mediante el estudio de una muestra mediante difracción de rayos X en dos dimensiones (DRX2). Y paralelamente mediante su visualización usando Microscopía Electrónica de Transmisión (MET)

Tunneling anisotropic magnetoresistance in La2/3Sr1/3MnO3/LaAlO3/Pt tunnel junctions

The magnetotransport properties of La2/3 Sr1/3 MnO3 (LSMO)/ LaAlO3 (LAO)/Pt tunneling junctions have been analyzed as a function of temperature and magnetic field. The junctions exhibit magnetoresistance (MR) values of about 37%, at H=90 kOe at low temperature. However, the temperature dependence of MR indicates a clear distinct origin than that of conventional colossal MR. In addition, tunneling anisotropic MR (TAMR) values around 4% are found at low temperature and its angular dependence reflects the expected uniaxial anisotropy. The use of TAMR response could be an alternative of much easier technological implementation than conventional MTJs since only one magnetic electrode is required, thus opening the door to the implementation of more versatile devices. However, further studies are required in order to improve the strong temperature dependence at the present stage

Influence of porosity on the critical currents of trifluoroacetate-MOD YBa2Cu3O7 films

The influence of porosity on the superconducting properties have been investigated on YBa/sub 2/Cu/sub 3/O/sub 7/ thin films deposited on LaAlO/sub 3/ [100] substrates by the so-called Trifluoroacetate (TFA) route. Micro-Raman spectroscopy have been used to determine the concentration of c-axis grains /spl delta/ in different samples and their influence on the final film porosity as observed from SEM imaging. This has been compared with measurements of resistivity and critical currents in the same samples. We prove that this /spl delta/ fraction is the main parameter controlling the porosity and hence the normal-state resistivity of the thin films. The optimization of the microstructure of these YBa/sub 2/Cu/sub 3/O/sub 7/ TFA films allow to have high critical currents : J/sub c/ = 3 /spl times/ 10/sup 6/ A/cm/sup 2/ at 77 K

Self-Arranged Misfit Dislocation Network Formation upon Strain Release in La0.7Sr0.3MnO3/LaAlO3(100) Epitaxial Films under Compressive Strain

Lattice-mismatched epitaxial films of LaSrMnO (LSMO) on LaAlO (001) substrates develop a crossed pattern of misfit dislocations above a critical thickness of 2.5 nm. Upon film thickness increases, the dislocation density progressively increases, and the dislocation spacing distribution becomes narrower. At a film thickness of 7.0 nm, the misfit dislocation density is close to the saturation for full relaxation. The misfit dislocation arrangement produces a 2D lateral periodic structure modulation (Λ≈ 16 nm) alternating two differentiated phases: one phase fully coherent with the substrate and a fully relaxed phase. This modulation is confined to the interface region between film and substrate. This phase separation is clearly identified by X-ray diffraction and further proven in the macroscopic resistivity measurements as a combination of two transition temperatures (with low and high T). Films thicker than 7.0 nm show progressive relaxation, and their macroscopic resistivity becomes similar than that of the bulk material. Therefore, this study identifies the growth conditions and thickness ranges that facilitate the formation of laterally modulated nanocomposites with functional properties notably different from those of fully coherent or fully relaxed material

Estudio termodinámico sobre la mineralogénesis de la Galena en la mina de Cierco (Alta Ribagorza)

Se efectúa un estudio de la mineralogénesis de la galena en la mina de Cierco (Alta Riborgaza). Se determina a partir de cálculos termodinamicos la solubilidad de la galena en una solución hidrotermal a 150ºC y 3 molal de NaCl y se estudian 10s efectos de los diferentes posibles agentes precipitantes. A partir de las mineralizaciones que acompañan a la galena, del conocimiento que se dispone de las inclusiones presentes en las mineralizaciones se efectúan posibles hipótesis sobre la formación del yacimiento

Estudio termodinámico sobre la mineralogénesis de la Galena en la mina de Cierco (Alta Ribagorza)

Se efectúa un estudio de la mineralogénesis de la galena en la mina de Cierco (Alta Riborgaza). Se determina a partir de cálculos termodinamicos la solubilidad de la galena en una solución hidrotermal a 150ºC y 3 molal de NaCl y se estudian 10s efectos de los diferentes posibles agentes precipitantes. A partir de las mineralizaciones que acompañan a la galena, del conocimiento que se dispone de las inclusiones presentes en las mineralizaciones se efectúan posibles hipótesis sobre la formación del yacimiento