Oxygen vacancy driven interfacial phenomena in oxide heterostructures

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de Materiales, leída el 28-06-2017Los óxidos complejos son una familia de compuestos de óxidos de metales de transición con un amplio espectro de propiedades para su aplicación en electrónica de óxidos, como son: ferromagnetismo, antiferromagnetismo, superconductividad, ferroelectricidad, etc. En particular, son buenos candidatos para dispositivos reales, ya que las interfases de estos materiales generan un escenario prometedor gracias a la interacción entre sus efectos eléctricos y magnéticos [1]. Las distintas interacciones entre los distintos grados de libertad como son la carga, el espín, la red cristalina y los orbitales dan lugar a una amplia variedad de fases electrónicas. Por tanto, la combinación de estas propiedades en heteroestructuras de alta calidad cristalina pueden dar lugar a diferentes estados electrónicos [2]. El comportamiento de los electrones en estos sistemas está sujeto a las interacciones culombianas con primeros vecinos y, por tanto, los materiales no se pueden describir con el modelo clásico de bandas. Un ejemplo de este tipo de comportamiento es el que se da en los aislantes de Mott, en los que la repulsión culombiana es suficientemente intensa para inducir un estado aislante a pesar de que la teoría de bandas prediga exactamente lo opuesto para una ocupación parcial de las bandas [3]. Es bien sabido que este tipo de materiales crecen con deficiencias de oxígeno, las cuales pueden dar lugar a cambios en sus las propiedades físicas, ya sea por el efecto electrónico de dopado o por los cambios estructurales que puedan generar en la red cristalina. Por ejemplo, el LaMnO3 estequiometrico es un aislante antiferromagnético que presenta ferromagnetismo cuando la red es deficiente en oxígeno [4]. Otros estudios demuestran que la familia de cobaltitas (La,Sr)CoO3 en película delgada sobre sustratos de NdGaO3 y (La,Sr)(Al,Ta)O3 tiene diferentes imanaciones de saturación [5]. Es estos experimentos se ha demostrado que, la diferencia en el contenido de vacantes de oxígeno en las películas delgadas para diferentes sustratos pueden ser responsables de la mejora observada en las propiedades magnéticas. Un caso particularmente importante es el del LaCoO3, en el que se ha demostrado que cuando se crece en película delgada hay que tener en cuenta la combinación de la tensión epitaxial y las vacantes de oxígeno, pues ambos fenómenos son interdependientes. En este caso, la estabilización de espín del Co2+ y Co3+ está directamente relacionado con una superestructura ordenada de vacantes en la red que da lugar a la interacción de canje ferromagnética...The quest for novel functionalities in strongly correlated oxide systems has often been determined by reducing dimensionality and designing nanostructured systems that exploit new physical effects. Any physical phenomena occurring within nanometric length scales are often ruled by the properties of small active regions, such as interfaces, point or extended defects, etc. In the growth process of oxide based heterostructures, the oxygen content constitutes a degree of freedom that can strongly affect the physical properties. Thus, the study of interfacial effects driven by oxygen vacancies is of great interest in order to pursue new functionalities aimed at designing future devices. Under these premises, in this thesis we have grown and characterized a number of systems comprised of complex oxide heterostructures where the O stoichiometry is tuned to produce physical behaviors not present in bulk. The local atomic structure and chemical properties of our samples have been probed with one of the most powerful techniques available currently to explore materials in real space with atomic resolution: aberration corrected scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy (EELS). By means of spatially resolved analyses of slight fluctuations of the local crystalline structure, composition or electronic environment imposed by the presence of O point defects defects, we have been able to explain a number of physical effects in oxide interfaces of interest in hot fields such as spintronics or energy materials, establishing the link between the atomic local structure and the macroscopic world...Depto. de Física de MaterialesFac. de Ciencias FísicasTRUEunpu

Unraveling dzyaloshinskii-moriya interaction and chiral nature of graphene/cobalt interface

A major challenge for future spintronics is to develop suitable spin transport channels with long spin lifetime and propagation length. Graphene can meet these requirements, even at room temperature. On the other side, taking advantage of the fast motion of chiral textures, that is, Neel-type domain walls and magnetic skyrmions, can satisfy the demands for high- density data storage, low power consumption, and high processing speed. We have engineered epitaxial structures where an epitaxial ferromagnetic Co layer is sandwiched between an epitaxial Pt(111) buffer grown in turn onto MgO(111) substrates and a grapheme layer. We provide evidence of a graphene-induced enhancement of the perpendicular magnetic anisotropy up to 4 nm thick Co films and of the existence of chiral left-handed Neel-type domain walls stabilized by the effective Dzyaloshinskii-Moriya interaction (DMI) in the stack. The experiments show evidence of a sizable DMI at the gr/Co interface, which is described in terms of a conduction electron mediated Rashba-DMI mechanism and points opposite to the spin orbit coupling-induced DMI at the Co/Pt interface. In addition, the presence of graphene results in (i) a surfactant action for the Co growth, producing an intercalated, flat, highly perfect face-centered cubic film, pseudomorphic with Pt and (ii) an efficient protection from oxidation. The magnetic chiral texture is stable at room temperature and grown on insulating substrate. Our findings open new routes to control chiral spin structures using interfacial engineering in graphene-based systems for future spin- orbitronics devices fully integrated on oxide substrates

Yttria-stabilized zirconia/SrTiO_(3) oxide heteroepitaxial interface with symmetry discontinuity

We show that yttria-stabilized zirconia (YSZ) films deposited on structurally dissimilar SrTiO_(3)(110) substrates exhibit two-dimensional layer-by-layer growth. We observed that, up to a thickness of about 15 nm, the square (001) basal plane of the cubic YSZ grows epitaxially on the rectangular (110) crystallographic plane of SrTiO3 substrates, with [110]YSZ(001)//[001]SrTiO_(3)(110) epitaxial relationship. Thus, the heterointerface presents symmetry discontinuity between the YSZ(001) film and the lower surface symmetry SrTiO_(3)(110) substrate. Beyond this specific case, we envisage similar approaches to develop other innovative oxide interfaces showing similar crystal symmetry discontinuities

Dimensionality-driven metal-insulator transition in spin-orbit-coupled IrO_2

A metal-insulator transition is observed in spin-orbit-coupled IrO_2 thin films upon reduction of the film thickness. In the epitaxially grown samples, the critical thickness (t similar to 1.5-2.2 nm) is found to depend on growth orientation (001), (100) or (110). Interestingly from the applied point of view, the insulating behavior is found even in polycrystalline ultrathin films. By analyzing the experimental electrical response with various theoretical models, we find good fits to the Efros-Shklovskii-VRH and the Arrhenius-type behaviors, which suggests an important role of electron correlations in determining the electrical properties of IrO_2. Our magnetic measurements also point to a significant role of magnetic order. Altogether, our results would point to a mixed Slater- and Mott-type of insulator

Graphite to diamond transition induced by photoelectric absorption of ultraviolet photons

The phase transition from graphite to diamond is an appealing object of study because of many fundamental and also, practical reasons. The out-of-plane distortions required for the transition are a good tool to understand the collective behaviour of layered materials (graphene, graphite) and the van der Waals forces. As today, two basic processes have been successfully tested to drive this transition: strong shocks and high energy femtolaser excitation. They induce it by increasing either pressure or temperature on graphite. In this work, we report a third method consisting in the irradiation of graphite with ultraviolet photons of energies above 4.4 eV. We show high resolution electron microscopy images of pyrolytic carbon evidencing the dislocation of the superficial graphitic layers after irradiation and the formation of crystallite islands within them. Electron energy loss spectroscopy of the islands show that the sp(2) to sp(3) hybridation transition is a surface effect. High sensitivity X-ray diffraction experiments and Raman spectroscopy confirm the formation of diamond within the islands

Modified magnetic anisotropy at LaCoO_(3)/La_(0.7)Sr_(0.3)MnO_(3) interfaces

Controlling magnetic anisotropy is an important objective towards engineering novel magnetic device concepts in oxide electronics. In thin film manganites, magnetic anisotropy is weak and it is primarily determined by the substrate, through induced structural distortions resulting from epitaxial mismatch strain. On the other hand, in cobaltites, with a stronger spin orbit interaction, magnetic anisotropy is typically much stronger. In this paper, we show that interfacing La0.7Sr0.3MnO3 (LSMO) with an ultrathin LaCoO3 (LCO) layer drastically modifies the magnetic anisotropy of the manganite, making it independent of the substrate and closer to the magnetic isotropy characterizing its rhombohedral structure. Ferromagnetic resonance measurements evidence a tendency of manganite magnetic moments to point out-of-plane suggesting non collinear magnetic interactions at the interface. These results may be of interest for the design of oxide interfaces with tailored magnetic structures for new oxide devices

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

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

Caracterización estructural y de transporte de películas ultradelgadas de conductores iónicos (Structural and transport characterization of ultrathin films of ionic conductors).

Ionic conductor complex oxides poise a most interesting promise as electrolytes for solid oxide fuel cells (SOFC). In particular, high quality thin films, superlattices and interfaces may exhibit a dramatic enhancement of the ionic conductivity, since such heterostructures may accommodate large amounts of strain. Coherently strained interfaces provide a very interesting playground towards the search of enhanced diffusivities near room temperature, aimed at improving devices for energy generation and storage. In this work, thin films of ZrO_2:Y_2O_3 (YSZ) and ZrO_2:Y_2O_3/YAlO_3 (YSZ/YAP) superlattices have been grown on different YAP substrates in order to tune the degree of epitaxial strain in the system. Different characterization techniques, as X-ray diffraction or scanning transmission electron microscopy have been used to study the role of the growth orientation and the interface strain in transport properties [RESUMEN] Los óxidos de metales de transición son una amplia familia de materiales, entre los que destacan los sólidos iónicos por su aplicaciones como materiales para la generación y almacenamiento de energía. En concreto, en películas delgadas de alta calidad en su estructura aparecen nuevas propiedades como una gran mejora en la conductividad iónica, fundamental para su utilización en celdas de combustible de electrolito sólido (SOFC: solid oxide fuel cells). Se ha demostrado que estos materiales se pueden combinar en heteroestructuras soportando grandes tensiones sin romper en islas o dominios estructurales. En estos sistemas, las interfases coherentemente tensionadas presentan un campo de estudio muy prometedor en la búsqueda de conductores iónicos con difusividades mayores cerca de temperatura ambiente. En este trabajo se han crecido láminas delgadas de ZrO_2:Y_2O_3 (YSZ) y superredes de ZrO_2:Y_2O_3/ YAlO_3 (YSZ/YAP) sobre distintos sustratos de YAlO3 (YAP) para su posterior caracterización mediante técnicas como difracción de rayos X y microscopía electrónica de transmisión con barrido ,con el objetivo principal de intentar dilucidar el rol de la orientación de crecimiento y las tensiones epitaxiales en las propiedades de transporte