Effect of a built-in electric field in asymmetric ferroelectric tunnel junctions

The contribution of a built-in electric field to ferroelectric phase transition in asymmetric ferroelectric tunnel junctions is studied using a multiscale thermodynamic model. It is demonstrated in details that there exists a critical thickness at which an unusual ferroelectric-\'\' polar non-ferroelectric\rq\rq phase transition occurs in asymmetric ferroelectric tunnel junctions. In the \'\' polar non-ferroelectric\rq\rq phase, there is only one non-switchable polarization which is caused by the competition between the depolarizing field and the built-in field, and closure-like domains are proposed to form to minimize the system energy. The transition temperature is found to decrease monotonically as the ferroelectric barrier thickness is decreased and the reduction becomes more significant for the thinner ferroelectric layers. As a matter of fact, the built-in electric field does not only result in smearing of phase transition but also forces the transition to take place at a reduced temperature. Such findings may impose a fundamental limit on the work temperature and thus should be further taken into account in the future ferroelectric tunnel junction-type or ferroelectric capacitor-type devices.Comment: 9 pages, 8 figures, submitted to PR

Origin of the orbital and spin orderings in rare-earth titanates

Rare-earth titanates RTiO$_3$ are Mott insulators displaying a rich physical behavior, featuring most notably orbital and spin orders in their ground state. The origin of their ferromagnetic to antiferromagnetic transition as a function of the size of the rare-earth however remains debated. Here we show on the basis of symmetry analysis and first-principles calculations that although rare-earth titanates are nominally Jahn-Teller active, the Jahn-Teller distortion is negligible and irrelevant for the description of the ground state properties. At the same time, we demonstrate that the combination of two antipolar motions produces an effective Jahn-Teller-like motion which is the key of the varying spin-orbital orders appearing in titanates. Thus, titanates are prototypical examples illustrating how a subtle interplay between several lattice distortions commonly appearing in perovskites can produce orbital orderings and insulating phases irrespective of proper Jahn-Teller motions.Comment: Accepted in Physical Review

Creixement i estudi de capes primes de manganites de valència mixta

Consultable des del TDXA portada consta: Realitzada en règim de cotutela amb l'Institut National des Sciences Appliquées de ToulouseTítol obtingut de la portada digitalitzadaLos materiales objeto de estudio en esta Tesis son óxidos de Mn del tipo La2/3A1/3MnO3 (A=Ca, Sr), que presentan una transición ferro-paramagnética acompañada de una transición metal-aislante a temperaturas del orden de TC=300K. La aplicación de un campo magnético favorece la aparición del orden ferromagnético produciendo un desplazamiento de la transición hacia temperaturas más elevadas. Esto genera una fuerte disminución de la resistividad, dando lugar a un importante efecto magnetoresistivo (magnetoresistencia colosal). Dado el complicado entramado de los diferentes fenómenos físicos concurrentes en las manganitas, estos óxidos tienen un gran interés tanto desde el punto de vista básico como del desarrollo de nuevos dispositivos de electrónica de espín. Dentro de este marco, esta tesis está organizada en tres apartados : 1. Una gran parte del trabajo experimental ha consistido en el desarrollo de un sistema de crecimiento de láminas delgadas mediante pulverización catódica de radiofrecuencia. Los distintos elementos del sistema (calefactores, portasustratos, magnetrones, etc) se controlan por ordenador, lo cual permite crecer muestras de forma automática. 2. La segunda parte del trabajo consiste en controlar el crecimiento de láminas del compuesto La2/3Ca1/3MnO3 a nivel nanoscópico. La influencia de las condiciones de depósito sobre los distintos parámetros claves del material (parámetros de celda, temperaturas de transición, resistividad, rugosidad, etc) ha sido estudiada de exhaustivamente y se ha conseguido la fabricación de muestras de alta calidad de forma reproducible. 3. La parte más importante de la investigación se ha dedicado a determinar el papel de las intercaras sobre las propiedades de capas de manganita. Como es bien sabido, la interacción responsable del ferromagnetismo en este material es muy sensible a las distorsiones estructurales, lo cual, combinado con la alta polarización de los portadores de carga, lleva a una magnetoresistencia de bajo campo muy fuerte en cerámicas nanométricas. Esta alta polarización también da lugar a una fuerte respuesta a campo débil en uniones túnel. Nuestra contribución consiste en la fabricación de intercaras artificiales de varios tipos y en la correlación de la magnetoresistencia obtenida con el análisis estructural. Las propiedades magnetoelectrónicas de las intercaras entre una manganita y un aislante (SrTiO3, LaAlO3 o NdGaO3) también han sido estudiadas. En estas regiones la temperatura de Curie TC y la conductividad eléctrica son más bajas que en el bulk, lo cual se atribuye a la coexistencia de fases con distintas propiedades electrónicas. Por otra parte, fenómenos más fundamentales como la magnetoresistencia anisotrópica y el efecto Hall anómalo también han sido objeto de estudio y se han intentado correlacionar con el acoplamiento espín-órbita y el estado magnético cerca de TC.This thesis reports on properties of Mn oxides of the type La2/3A1/3MnO3 (A=Ca, Sr) which exhibit a ferromagnetic-to-paramagnetic transition concomitant of a metallic-to-insulator transition around TC=300K. The application of an external magnetic field favours the ferromagnetic ordering of Mn moments which leads to a shift of TC to higher temperatures, which in turn produces a decrease of the resistivity. This gives rise to a strong magnetoresistive effect called colossal magnetoresistance. In these compounds, many interactions have similar energy scales and compete to determine the ground state, making manganites very interesting materials from the basic point of view as well as for the development of new devices for spin electronics. Within this frame, this thesis is focused on the three following points : 1. The biggest effort in the experimental work has been dedicated to the set-up of a radiofrequency sputtering system for the fabrication of thin films of oxides. The different elements of the system (substrate heaters and holders, magnetrons, etc) are computer-controlled, which allows the growth of films and heterostructures in a fully automated fashion. 2. The second part of the work consisted in controlling the growth of thin films of mixed valence manganites, mainly La2/3Ca1/3MnO3, at the nanoscopic scale. The influence of several growth parameters on the key characteristics of the material (cell parameters, transition temperatures, resisitivity, roughness, etc) has been extensively studied and a reproducible fabrication of high-quality films has been achieved. 3. The main part of the work is focused on the role of interfaces on the properties of manganite films. It is well know that the interaction responsible for ferromagnetism in these compounds is very sensitive to structural distortions which, combined with a high polarisation of the charge carriers, leads to a strong low-field magnetoresistance in nanometric ceramic powders. This high polarisation also gives rise to a large low-field response in tunnel junctions. To probe these effects, we have fabricated artificial interfaces of several types and correlated their magnetotransport response with data from structural analysis. We have also studied the magnetoelectronic properties of the interface between a manganite film and an insulator (SrTiO3, LaAlO3, NdGaO3). A marked decrease of TC and of the electrical conductivity compared to bulk material is observed. This correlates with the detection of multiphase separation between regions with different electronic properties at these interfaces. Besides, fundamental transport phenomena such as anisotropic magnetoresistance and anomalous Hall have been investigated. Their magnitude and temperature dependence have been analysed and a semi-quantitative explanation involving the spin-orbit interaction and the electronic texture close to TC has been proposed.Cette Thèse a été consacrée à l'étude d'oxydes de Mn La2/3A1/3MnO3 (A=Ca, Sr) présentant une transition ferromagnétique-paramagnétique concomitante d'une transition métal-isolant aux environs de TC=300K. L'application d'un champ magnétique externe favorise un ordre ferromagnétique du moment des ions Mn, ce qui déplace TC vers les hautes températures et induit une forte diminution de la résistivité. Ce fort effet magnétorésistif est appelé magnétorésistance colossale. Dans ces composés, de nombreux interactions sont en compétition ce qui les rend très intéressants du point de vue fondamental comme pour le développement de nouveaux dispositifs en électronique de spin. Dans ce cadre, cette thèse est structurée autour des trois points suivants : 1. Un grand effort expérimental a été fourni pour développer un bâti de pulvérisation cathodique radiofréquence destiné à l'élaboration de couches minces d'oxydes. Les différents éléments du système (chaufferettes, porte-substrats, magnétrons) sont pilotés par ordinateur ce qui permet de fabriquer des couches minces de façon totalement automatique. 2. La seconde partie du travail a consisté à contrôler la croissance de couches minces de La2/3Ca1/3MnO3 à l'échelle nanoscopique. L'influence des paramètres de croissance sur certaines caractéristiques du matériau (paramètres de maille, températures de transition, résistivité, rugosité de surface, etc) a été étudiée de façon extensive de sorte que des couches de très bonne qualité ont pu être obtenues de façon reproductible. 3. Le rôle des interfaces sur les propriétés physiques de couches de manganite a fait l'objet de la majeure partie de cette thèse. En effet, l'interaction responsable du ferromagnétisme de ces composés est très sensible aux distorsions structurales, ce qui, du fait de la forte polarisation de spin des porteurs, donne lieu à une forte magnétorésistance à faible champ dans des céramiques nanométriques. Cette forte polarisation conduit également à une forte réponse magnétorésistive dans des jonctions tunnel. Nous nous sommes donc intéressés aux propriétés d'interfaces et avons fabriqué des interfaces artificielles de plusieurs types, et relié leur propriétés de magnétotransport aux résultats des analyses structurales. Par ailleurs, nous avons étudié les interfaces entre une manganite et un isolant (SrTiO3, LaAlO3, NdGaO3). Celles-ci possèdent une Tc et une conductivité beaucoup plus faibles que le matériau massif, ce qui est lié à la coexistence de régions présentant des propriétés électroniques différentes (séparation de phase). Par ailleurs, nous nous sommes également intéressés à des phénomènes de transport intrinsèques comme la magnétorésistance anisotrope et l'effet Hall extraordinaire dont nous avons essayé de comprendre la dépendance en température en la reliant au couplage spin-orbite et à la texture électronique près de TC

Growth and magnetic properties of multiferroic LaxBi1-xMnO3 thin films

A comparative study of LaxBi1-xMnO3 thin films grown on SrTiO3 substrates is reported. It is shown that these films grow epitaxially in a narrow pressure-temperature range. A detailed structural and compositional characterization of the films is performed within the growth window. The structure and the magnetization of this system are investigated. We find a clear correlation between the magnetization and the unit-cell volume that we ascribe to Bi deficiency and the resultant introduction of a mixed valence on the Mn ions. On these grounds, we show that the reduced magnetization of LaxBi1-xMnO3 thin films compared to the bulk can be explained quantitatively by a simple model, taking into account the deviation from nominal composition and the Goodenough-Kanamori-Anderson rules of magnetic interactions

Origin of band gaps in 3d perovskite oxides

With their broad range of magnetic, electronic and structural properties, transition metal perovskite oxides ABO3 have long served as a platform for testing condensed matter theories. In particular, their insulating character - found in most compounds - is often ascribed to dynamical electronic correlations through the celebrated Mott-Hubbard mechanism where gaping arises from a uniform, symmetry-preserving electron repulsion mechanism. However, structural distortions are ubiquitous in perovskites and their relevance with respect to dynamical correlations in producing this rich array of properties remains an open question. Here, we address the origin of band gap opening in the whole family of 3d perovskite oxides. We show that a single-determinant mean-field approach such as density functional theory (DFT) successfully describes the structural, magnetic and electronic properties of the whole series, at low and high temperatures. We find that insulation occurs via energy-lowering crystal symmetry reduction (octahedral rotations, Jahn-Teller and bond disproportionation effects), as well as intrinsic electronic instabilities, all lifting orbital degeneracies. Our work therefore suggests that whereas ABO3 oxides may be complicated, they are not necessarily strongly correlated. It also opens the way towards systematic investigations of doping and defect physics in perovskites, essential for the full realization of oxide-based electronics

Mott gapping in 3d ABO3 perovskites without Mott-Hubbard interelectronic U

The existence of band gaps in Mott insulators such as perovskite oxides with partially filled 3d shells has been traditionally explained in terms of strong, dynamic inter-electronic repulsion codified by the on-site repulsion energy U in the Hubbard Hamiltonian. The success of the "DFT+U approach" where an empirical on-site potential term U is added to the exchange-and correlation Density Functional Theory (DFT) raised questions on whether U in DFT+U represents interelectronic correlation in the same way as it does in the Hubbard Hamiltonian, and if empiricism in selecting U can be avoided. Here we illustrate that ab-initio DFT without any U is able to predict gapping trends and structural symmetry breaking (octahedra rotations, Jahn-Teller modes, bond disproportionation) for all ABO3 3d perovskites from titanates to nickelates in both spin-ordered and spin disordered paramagnetic phases. We describe the paramagnetic phases as a supercell where individual sites can have different local environments thereby allowing DFT to develop finite moments on different sites as long as the total cell has zero moment. We use a recently developed exchange and correlation functional ("SCAN") that is sanctioned by the usual single-determinant, mean-field DFT paradigm with static correlations, but has a more precise rendering of self-interaction cancelation. Our results suggest that strong dynamic electronic correlations are not playing a universal role in gapping of 3d ABO3 Mott insulators, and opens the way for future applications of DFT for studying a plethora of complexity effects that depend on the existence of gaps, such as doping, defects, and band alignment in ABO3 oxides

First-principles study of electron and hole doping in perovskite nickelates

Rare-earth nickelates R$^{3+}$Ni$^{3+}$O$_3$ (R=Lu-Pr, Y) show a striking metal-insulator transition in their bulk phase whose temperature can be tuned by the rare-earth radius. These compounds are also the parent phases of the newly identified infinite layer RNiO2 superconductors. Although intensive theoretical works have been devoted to understand the origin of the metal-insulator transition in the bulk, there have only been a few studies on the role of hole and electron doping by rare-earth substitutions in RNiO$_3$ materials. Using first-principles calculations based on density functional theory (DFT) we study the effect of hole and electron doping in a prototypical nickelate SmNiO3. We perform calculations without Hubbard-like U potential on Ni 3d levels but with a meta-GGA better amending self-interaction errors. We find that at low doping, polarons form with intermediate localized states in the band gap resulting in a semiconducting behavior. At larger doping, the intermediate states spread more and more in the band gap until they merge either with the valence (hole doping) or the conduction (electron doping) band, ultimately resulting in a metallic state at 25% of R cation substitution. These results are reminiscent of experimental data available in the literature and demonstrate that DFT simulations without any empirical parameter are qualified for studying doping effects in correlated oxides and to explore the mechanisms underlying the superconducting phase of rare-earth nickelates