Estudio de interfases en óxidos complejos por técnicas avanzadas de microscopía electrónica

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, leída el 07-05-2015Pequeños cambios a nivel atómico de la estructura, composición o estado electrónico de un material pueden producir sorprendentes efectos macroscópicos. En particular, en óxidos complejos basados en metales de transición, un gran número de fenómenos físicos como transiciones metal-aislante, magnetorresistencia colosal o multiferroicidad son extremadamente sensibles a estas variaciones. Por tanto, para abordar el estudio de sistemas con tales características, técnicas experimentales con capacidad de analizar materiales a escala atómica y en el espacio real se hacen indispensables. La microscopía electrónica de transmisión con barrido combinada con la espectroscopia de pérdida de energía de electrones (EELS) forman una pareja con posibilidades únicas para estos estudios. Estas técnicas han crecido enormemente desde el desarrollo del corrector de aberración esférica en la última década y su alta resolución espacial nos permite ahora estudiar átomos individuales. El uso de estos equipos supone una herramienta única para el estudio de sistemas complejos, más aún cuando la dimensionalidad se reduce a pocos nanómetros como en películas delgadas o interfaces. En estos casos, técnicas de difracción promediadas macroscópicamente pueden no ser suficientemente sensibles a los parámetros que rigen la física relevante y por tanto, la gran sensibilidad espacial de la microscopía electrónica supone una gran ventaja. El objetivo principal de este trabajo será precisamente establecer la conexión entre los mecanismos a nivel atómico y las propiedades físicas de una serie de sistemas basados en óxidos complejos cuidadosamente escogidos. Analizaremos en el espacio real fluctuaciones mínimas, casi por debajo del umbral de detectabilidad, responsables últimas del comportamiento macroscópico.En primer lugar, se ha estudiado como pequeñas concentraciones de vacantes de oxígeno, tanto inducidas mediante irradiación como intrínsecas al material, pueden determinar las propiedades físicas macroscópicas del sistema. Se ha observado cómo procesos de irradiación dan lugar a la formación de una capa de TiO con alto grado cristalino en la superficie de monocristales de TiO2 y como además pueden producir estados metálicos superficiales en un aislante de bandas como es el SrTiO3. Se ha analizado además como la reestructuración electrónica debida a la presencia de vacantes de oxígeno estructurales explica por primera vez el origen electroestático del bloqueo iónico en fronteras de grano de materiales con importantes aplicaciones energéticas. Se ha abordado también el estudio de pequeñas variaciones estructurales, en particular, distorsiones colectivas de la red de oxígeno en heteroestructuras de óxidos complejos y su relación con la aparición de estados físicos inexistentes en los materiales masivos. Se ha encontrado una correlación entre rotaciones del octaedro de oxígenos producidas por tensiones epitaxiales y la estabilización de una fase interfacial ferromagnética y conductora en superredes formadas por óxidos aislantes. Además, se ha extendido este análisis a sistemas más complejos como uniones túnel multiferroicas donde se ha obtenido la configuración de dominios ferroeléctricos midiendo las distorsiones en la red de oxígenos para cada celda unidad. Este estudio muestra una de las primeras observaciones experimentales de una configuración de dominios ferroeléctricos tipo head-to-head en capas ultra-delgadas. Se ha encontrado además la presencia de una carga de apantallamiento confinada a la pared de dominio que genera estados electrónicos accesibles en el interior de la barrera ferroeléctrica, proporcionando los mecanismos para estabilizar un tuneleamiento cuántico resonante.El continuo desarrollo de estas técnicas experimentales hace vislumbrar un futuro prometedor tanto para la ciencia de materiales como para la microscopía electrónica. La exploración a escala atómica de fenómenos físicos aún por desvelar está ahora, más que nunca a nuestro alcance.Fac. de Ciencias FísicasTRUEunpu

In-Plane Anisotropic Optical and Mechanical Properties of Two-Dimensional MoO3

Molybdenum trioxide (MoO3) in-plane anisotropy has increasingly attracted the attention of the scientific community in the last few years. Many of the observed in-plane anisotropic properties stem from the anisotropic refractive index and elastic constants of the material but a comprehensive analysis of these fundamental properties is still lacking. Here we employ Raman and micro-reflectance measurements, using polarized light, to determine the angular dependence of the refractive index of thin MoO3 flakes and we study the directional dependence of the MoO3 Young's modulus using the buckling metrology method. We found that MoO3 displays one of the largest in-plane anisotropic mechanical properties reported for 2D materials so far.This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement n degrees 755655, ERC-StG 2017 project 2D-TOPSENSE), the European Commission, under the Graphene Flagship (Core 3, grant number 881603), the Spanish Ministry of Economy, Industry and Competitiveness through the grant MAT201787134-C2-2-R. R.F. acknowledges the support from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO) through a Juan de la Cierva-formacion fellowship 2017 FJCI-2017-32919. S.P. acknowledges the fellowship PRE2018-084818. R. D'A. acknowledges financial support from the grant Grupos Consolidados UPV/EHU del Gobierno Vasco (Grant No. IT1249-19), the support of the MICINN through the grant "SelectDFT" (Grant No. FIS2016-79464-P) and travel support from the MINECO grant "TowTherm" (Grant No. MINECOG17/A01). G.S.-S. acknowledges financial support from Spanish MICIU RTI2018-099054-J-I00 and MICINN IJC2018-038164-I. Electron microscopy observations were carried out at the Centro Nacional de Microscopia Electronica, CNME-UC

Magnetic phase diagram, magnetotransport and inverse magnetocaloric effect in the noncollinear antiferromagnet Mn5Si3

This Accepted Manuscript will be available for reuse under a CC BY-NC-ND licence after 24 months of embargo periodThe antiferromagnet Mn5Si3 has recently attracted attention because a noncollinear spin arrangement has been shown to produce a topological anomalous Hall effect and an inverse magnetocaloric effect. Here we synthesize single crystals of Mn5Si3 using flux growth. We determine the phase diagram through magnetization and measure the magnetoresistance and the Hall effect. We find the collinear and noncollinear antiferromagnetic phases at low temperatures and, in addition, a third magnetic phase, in between the two antiferromagnetic phases. The latter magnetic phase might be caused by strain produced by Cu inclusions. This suggests that fluctuations of the mixed character magnetic ordering in this compound can be easily quenched by stressThis work was supported by the Spanish MINECO (Consolider Ingenio Molecular Nanoscience CSD2007-00010 program, FIS2017-84330-R, MDM-2014-0377, MAT2014-52405-C2-2-R, FJCI-2015-25427 and CSD2009-00013), by the Comunidad de Madrid through program NANOMAGCOST-CM (S2018 NMT-4321) and MAD2D-CM (S2013/MIT-3007) and by EU (Graphene Core1 contract No. 696656, Nanopyme FP7-NMP-2012 SMALL-6 NMP3-SL-2012 310516 and COST CA16218

Formation of titanium monoxide (001) single-crystalline thin film induced by ion bombardment of titanium dioxide (110)

© 2015 Macmillan Publishers Limited. All rights reserved. A plethora of technological applications justify why titanium dioxide is probably the most studied oxide, and an optimal exploitation of its properties quite frequently requires a controlled modification of the surface. Low-energy ion bombardment is one of the most extended techniques for this purpose and has been recently used in titanium oxides, among other applications, to favour resistive switching mechanisms or to form transparent conductive layers. Surfaces modified in this way are frequently described as reduced and defective, with a high density of oxygen vacancies. Here we show, at variance with this view, that high ion doses on rutile titanium dioxide (110) induce its transformation into a nanometric and single-crystalline titanium monoxide (001) thin film with rocksalt structure. The discovery of this ability may pave the way to new technical applications of ion bombardment not previously reported, which can be used to fabricate heterostructures and interfaces.Peer Reviewe

Hexagonal Hybrid Bismuthene by Molecular Interface Engineering

High-quality devices based on layered heterostructures are typically built from materials obtained by complex solid-state physical approaches or laborious mechanical exfoliation and transfer. Meanwhile, wet-chemically synthesized materials commonly suffer from surface residuals and intrinsic defects. Here, we synthesize using an unprecedented colloidal photocatalyzed, one-pot redox reaction a few-layers bismuth hybrid of “electronic grade” structural quality. Intriguingly, the material presents a sulfur-alkyl-functionalized reconstructed surface that prevents it from oxidation and leads to a tuned electronic structure that results from the altered arrangement of the surface. The metallic behavior of the hybrid is supported by ab initio predictions and room temperature transport measurements of individual nanoflakes. Our findings indicate how surface reconstructions in two-dimensional (2D) systems can promote unexpected properties that can pave the way to new functionalities and devices. Moreover, this scalable synthetic process opens new avenues for applications in plasmonics or electronic (and spintronic) device fabrication. Beyond electronics, this 2D hybrid material may be of interest in organic catalysis, biomedicine, or energy storage and conversion

Ferroionic inversion of spin polarization in a spin-memristor

Magnetoelectric coupling in artificial multiferroic interfaces can be drastically affected by the switching of oxygen vacancies and by the inversion of the ferroelectric polarization. Disentangling both effects is of major importance toward exploiting these effects in practical spintronic or spinorbitronic devices. We report on the independent control of ferroelectric and oxygen vacancy switching in multiferroic tunnel junctions with a La_(0.7)Sr_(0.3)MnO_3 bottom electrode, a BaTiO_3 ferroelectric barrier, and a Ni top electrode. We show that the concurrence of interface oxidation and ferroelectric switching allows for the controlled inversion of the interface spin polarization. Moreover, we show the possibility of a spin-memristor where the controlled oxidation of the interface allows for a continuum of memresistance states in the tunneling magnetoresistance. These results signal interesting new avenues toward neuromorphic devices where, as in practical neurons, the electronic response is controlled by electrochemical degrees of freedom

Enhancement of vortex liquid phase and reentrant behavior in NiBi_(3) single crystals

We investigate the vortex phase diagram of needle shaped high quality NiBi3 single crystals by transport measurements. The current is applied along the crystalline b-axis of this intermetallic quasi-1D BCS superconductor. The single crystals show a Ginzburg-Levanyuk (G (i)) parameter of about 10(-7), larger by two orders of magnitude than G _(i) in elemental low T_(c) BCS superconductors. Vortex phase diagram, critical currents and pinning forces have been extracted from the experimental data. We observe (i) an enhancement of the vortex liquid phase, (ii) a reentrance of the liquid phase at low fields and (iii) an unusual magnetic field dependence of the pinning force. We suggest that these phenomena result from the interplay between pinning due to quenched disorder and the quasi-1D character of the material which could lead, for instance, to more complex pinning mechanisms at play

Controlled sign reversal of electroresistance in oxide tunnel junctions by electrochemical-ferroelectric coupling

The persistence of ferroelectricity in ultrathin layers relies critically on screening or compensation of polarization charges which otherwise destabilize the ferroelectric state. At surfaces, charged defects play a crucial role in the screening mechanism triggering novel mixed electrochemical-ferroelectric states. At interfaces, however, the coupling between ferroelectric and electrochemical states has remained unexplored. Here, we make use of the dynamic formation of the oxygen vacancy profile in the nanometerthick barrier of a ferroelectric tunnel junction to demonstrate the interplay between electrochemical and ferroelectric degrees of freedom at an oxide interface. We fabricate ferroelectric tunnel junctions with a La_0.7Sr_0.3MnO_3 bottom electrode and BaTiO_3 ferroelectric barrier. We use poling strategies to promote the generation and transport of oxygen vacancies at the metallic top electrode. Generated oxygen vacancies control the stability of the ferroelectric polarization and modify its coercive fields. The ferroelectric polarization, in turn, controls the ionization of oxygen vacancies well above the limits of thermodynamic equilibrium, triggering the build up of a Schottky barrier at the interface which can be turned on and off with ferroelectric switching. This interplay between electronic and electrochemical degrees of freedom yields very large values of the electroresistance (more than 10^6% at low temperatures) and enables a controlled switching between clockwise and counterclockwise switching modes in the same junction (and consequently, a change of the sign of the electroresistance). The strong coupling found between electrochemical and electronic degrees of freedom sheds light on the growing debate between resistive and ferroelectric switching in ferroelectric tunnel junctions, and moreover, can be the source of novel concepts in memory devices and neuromorphie computing