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Se reporta el proceso de síntesis del material complejo de tipo perovskita Bi0.5R0.5FeO3 (R=Eu, Sm, Dy) mediante el método cerámico, así como su caracterización estructural, óptica, magnética y eléctrica. Los datos refinados de difracción de rayos X revelaron que este material cristaliza en una estructura ortorrómbica (grupo espacial Pnma número 62). La respuesta óptica en la curva de espectroscopia de reflectancia difusa proporcionó un valor de brecha energética típico de los materiales semiconductores. La magnetización presentó un comportamiento histerético de campo coercitivo muy bajo característico del ferromagnetismo débil, para todas las temperaturas examinadas por debajo de 300 K y la aplicación de varios campos magnéticos. Las curvas de permitividad eléctrica real y compleja evidenciaron la presencia de procesos de relajación dieléctrica a 113 K, lo que concuerda con los reportes sobre corrientes piroeléctricas y termoestimuladas en función de la temperatura y revela la aparición de polarización ferroeléctrica por debajo de los 113 K, con un posible acoplamiento magnetoeléctrico. Por otro lado, se hizo un estudio teórico de la estructura electrónica con y sin la fase distorsiva de Berry. Se hicieron cálculos ab initio siguiendo el formalismo de la teoría funcional de la densidad y el método de ondas planas pseudopotenciales, formalismo en el cual los mecanismos de intercambio y correlación se describen mediante el enfoque de gradiente generalizado (GGA + U) considerando la polarización de espín. El análisis con la fase de Berry sugirió la presencia de ferroelectricidad a menos de 113 K, lo que concuerda con el análisis experimental que sugiere un comportamiento biferroico a bajas temperaturas debido a que la fase distorsiva introduce hibridaciones entre los estados 3d-Fe y 2p-O que favorecen la aparición de interacciones Dzyaloshinskii-Moriya, las cuales, a su vez, facilitan la aparición de ferroelectricidad en condiciones de ferromagnetismo débil. Las propiedades termodinámicas en presencia y ausencia de la fase de Berry en el modelo cuasi-armónico de Debyerevelaron la aparición de una transición ferroeléctrica a los 113 K, lo que corrobora la naturaleza magnetoeléctrica a bajas temperaturas. El carácter ferromagnético semiconductor a temperatura ambiente potencia este material para aplicaciones en tecnología espintrónica. We report here the synthesis process of the perovskite-like complex material Bi0.5R0.5FeO3 (R=Eu, Sm, Dy) using the ceramic method, as well as its structural, optical, magnetic, and electrical characterizations. Refined X-ray diffraction data revealed that this material crystallizes in an orthorhombic structure (space group Pnma number 62). The band gap value in the optical response shown in the diffuse reflectance spectroscopy curve was typical of semiconductor materials. The magnetization exhibited a very low coercive field hysteretic behavior, which is characteristic of weak ferromagnetism, for all temperatures examined below 300 K and the various magnetic fields applied. The real and complex electric permittivity curves showed the occurrence of dielectric relaxation processes at 113 K in agreement with reports of pyroelectric and thermo-stimulated currents as a function of temperature revealing the appearance of ferroelectric polarization below 113 K with possible magnetoelectric coupling. On the other hand, we made a theoretical study of the electronic structure with and without the inclusion of a Berry distortional phase and ab-initio calculations following the density functional theory formalism and the pseudopotential plane wave method. In this formalism, the exchange and correlation mechanisms are described by the generalized gradient approach (GGA + U) considering spin polarization. The Berry phase analysis suggested the occurrence of ferroelectricity at temperatures below 113 K consistent with the experimental analysis evidencing a biferroic behavior at low temperatures given that the distortional phase introduces hybridizations between the 3d-Fe and 2p-O states favoring the appearance of Dzyaloshinskii-Moriya interactions, which, in turn, facilitate the appearance of ferroelectricity coexisting with a weak ferromagnetism. The thermodynamic properties in the presence or absence of the Berry phase by means of the Debye quasiharmonic model revealed the appearance of a ferroelectric transition at 113 K, which corroborates their magnetoelectric nature at low temperatures. The ferromagnetic semiconducting character found at room temperature enhances this material for applications in spintronics technology.

The appropriate choice of the transition metal complex and metal surface electronic structure opens the possibility to control the spin of the charge carriers through the resulting hybrid molecule/metal spinterface in a single-molecule electrical contact at room temperature. The single-molecule conductance of a Au/molecule/Ni junction can be switched by flipping the magnetization direction of the ferromagnetic electrode. The requirements of the molecule include not just the presence of unpaired electrons: the electronic configuration of the metal center has to provide occupied or empty orbitals that strongly interact with the junction metal electrodes and that are close in energy to their Fermi levels for one of the electronic spins only. The key ingredient for the metal surface is to provide an efficient spin texture induced by the spin-orbit coupling in the topological surface states that results in an efficient spin-dependent interaction with the orbitals of the molecule. The strong magnetoresistance effect found in this kind of single-molecule wire opens a new approach for the design of room-temperature nanoscale devices based on spin-polarized currents controlled at molecular level.

[EN] Local perturbations in the relative orientation of the magnetic moments in a continuous magnetic system can propagate in the form of waves. These so-called spin waves represent a promising candidate as an information carrier for spin-based low-power applications. A localized, energy-efficient excitation of coherent and short-wavelength spin waves is a crucial technological requirement, and alternatives to excitation via the Oersted field of an alternating current must be explored. Here, we show how a domain wall pinned at a geometrical constriction in a perpendicularly magnetized thin nanowire emits spin waves when forced to rotate by the application of a low direct current flowing along the wire. Spin waves are excited by the in-plane stray field of the rotating domain wall and propagate at an odd harmonic of the domain wall rotation frequency in the direction of the electron's flow. The application of an external field, opposing domain wall depinning induced by the current, breaks the symmetry for spin wave propagation in the two domains, allowing emission in both directions but at different frequencies. The results presented define a new approach to manufacture tuneable high-frequency spin wave emitters of easy fabrication and low power consumption., Comisión Europea (P7-PEOPLE-2013-ITN 608031) Gobierno de España (MAT2014-52477-C5-4-P) Junta de Castilla y Leon (SA282U14, SA090U16)

[EN]The study of magnetism at the nanoscale has important applications in everyday life. As an example, the vast majority of all data is currently stored in magnetic hard drives, while magnetic sensors are ubiquitous in automotive applications and in the internet of things (IoT) technology. The interaction between the spins of conducting electrons and those of the localized magnetic moments of a ferromagnet is at the base of a new field of studies called Spintronics, whose technological perspectives are to overcome the existing semiconductor technology in terms of power saving, endurance and reliability. Domain wall propagation is the mechanism through which a magnetic system changes its state when its equilibrium is perturbed via an external action and its dynamics is well described using the micromagnetic formalism. Micromagnetic nu- merical simulations are a proficient tool that links experimental observations and theoretical predictions, leading the way in the theoretical understanding of magne- tization dynamics and domain wall motion. In chapter 1 we lay down the fundamental physical concepts of the micromag- netic description of magnetism and present the principal analytical tools used through- out the the rest of the work. Chapter 2 is dedicated to the description of the nu- merical solver used in this work: a custom micromagnetic code based on C++ and CUDA programming languages, developed within the group. Subsequently, we fo- cus on two different problems, making use of the descriptive and predictive power of micromagnetic simulations respectively. In chapter 3, we investigate the effect of disorder on field driven domain wall dynamics in CoFeB thin films. Such structures are the building blocks of MRAM memories and the understanding of field driven domain wall dynamics is a key step towards the optimization of device functionality. Exploiting the ability of mi- cromagnetic simulations to reproduce certain disorder features realistically, we get insight into magnetization dynamics taking place at a scale below instruments reso- lution, uncovering important connections between domain wall dynamics and ma- terial disorder features. Disorder triggers internal domain wall dynamics that gen- erates a faster energy dissipation and a faster domain wall propagation in the high- field regime. In chapter 4, we propose a new spintronic device based on the emission of spin waves by means of the controlled rotation of a domain wall in a ferromagnetic wire. The transmission of information via the periodic oscillatory perturbation of mag- netization, called spin wave, offers new perspectives in the design of low power sensors and emitters. We design a system with realistic material characteristics and investigate how the self oscillatory state of a domain wall, induced by the injection of a charge current, can emit a spin wave signal at frequency of tens of GHz that directly depends on the injected current intensity.

The finding of high-spin molecules that could behave as conventional magnets has been one of the main challenges in Molecular Magnetism. Here, the exchange interactions, present in the highest-spin molecule published in the literature, Fe42, have been analysed using theoretical methods based on Density Functional Theory. The system with a total spin value S = 45 is formed by 42 iron centres containing 18 high-spin FeIII ferromagnetically coupled and 24 diamagnetic low-spin FeII ions. The bridging ligands between the two paramagnetic centres are two cyanide ligands coordinated to the diamagnetic FeII cations. Calculations were performed using either small Fe4 or Fe3 models or the whole Fe42 complex, showing the presence of two different ferromagnetic couplings between the paramagnetic FeIII centres. Finally, Quantum Monte Carlo simulations for the whole system were carried out in order to compare the experimental and simulated magnetic susceptibility curves from the calculated exchange coupling constants with the experimental one. This comparison allows for the evaluation of the accuracy of different exchange-correlation functionals to reproduce such magnetic properties.

Luminescence spectroscopy has been used to characterize MgO films prepared by rf-sputtering. A clear correlation is found between the appearance of an emission peak centered at approximately 460 nm and the detection of ferromagnetic ordering in the samples. We suggest that cationic vacancies are responsible for the blue-light emission by introducing p states into the electronic band-gap. In accordance with this, our results strongly indicate that cationic vacancies are at the heart of the appearance of long-range magnetic ordering in MgO films.

We show both theoretical and experimental evidences of the appearance of ferromagnetism in MgO thin films. First-principles calculations allow predicting the possibility of the formation of a local moment in MgO, provided the existence of Mg vacancies which create holes on acceptor levels near the O 2p-dominated valence band. Magnetic measurements evidence of the existence of room-temperature ferromagnetism in MgO thin films. High-resolution transmission electron microscopy demonstrates the existence of cation vacancies in our samples. Finally, by applying the element specificity of the x-ray magnetic circular dichroism technique, we also demonstrate that the magnetic moments of the system arise from the spin polarization of the 2p electrons of oxygen atoms surrounding Mg vacancies.

Electron-energy-loss spectroscopy is used to map composition and electronic states in epitaxial La2/3Ca1/3MnO3 (LCMO) films of various thicknesses grown on SrTiO3 (001) and (110) substrates. For relatively thick films (≥20 nm), epitaxial tensile strain in (001) films promotes a compositional La/Ca gradient across the film thickness, being the interface La rich, while the relaxed (110) films are chemically homogeneous. In contrast, much thinner (001) and (110) LCMO films display a different La/Ca distribution, being La rich at the free surface. The observed distinct thickness-dependent composition gradient behavior reflects a balance between strain-induced elastic energy minimization and kinetic effects during growth.

Orthorhombic YMnO3 (YMO) epitaxial thin films were deposited on SrTiO3 (STO) single-crystal substrates. We show that the out-of-plane texture of the YMO films can be tailored using STO substrates having (001), (110), or (111) orientations. We report on the magnetic properties of the YMO(010) films grown on STO(001) substrates. The dependence of the susceptibility on the temperature indicates that the films are antiferromagnetic below the Néel temperature (around 35 K). Orthorhombic YMO(010) films were also deposited on an epitaxial buffer layer of ferromagnetic and metallic SrRuO3 (SRO). The magnetic hysteresis loops of SRO show exchange bias at temperatures below the Néel temperature of YMO. These results confirm that the YMO films are antiferromagnetic and demonstrate that magnetoelectric YMO can be integrated in functional epitaxial architectures.

We have studied the abrupt and hysteretic changes of resistance in MgO-based capacitor devices. The switching behavior is discussed in terms of the formation and rupture of conduction filaments due to the migration of structural defects in the electric field, together with the redox events which affects the mobile carriers. The results presented in this paper suggest that MgO transparent films combining ferromagnetism and multilevel switching characteristics might pave the way for a new method for spintronic multibit data storage.