Synchrotron infrared spectroscopy of domain walls and high pressure phases of multiferroics

Synchrotron light sources provide high throughput, broadband infrared light enabling the development of novel techniques, inaccessible using traditional sources. High pressure techniques benefit greatly, as significant signal loss occurs from focusing the light through diamonds into a small area. Intense infrared light offers an avenue to perform spatially-resolved spectroscopy in areas smaller than the diffraction limit by focusing the light within the near-field limit. We take advantage of the synchrotron light source to perform infrared studies at high pressures and on spots smaller than 20×20 nm2. We investigate nanoscale heterogeneity with spatially resolved techniques and reveal pressure-induced phase transitions via high pressure spectroscopy. We then unravel these complicated findings by incorporating group theoretical symmetry analysis and lattice dynamics calculations. The utilization of a synchrotron light source offers the broadband, high throughput infrared light that unifies these projects and enables the understanding of how vibrational modes contribute to unexplored phenomena. Because multiferroic materials exhibit heterogeneity in the form of domains and domain walls, Ca3Ti2O7 and h-Lu0.6Sc0.4FeO3 provide platforms to reveal the infrared response of different domain walls. These nanoscale objects have eluded study due to their size, but near-field infrared spectroscopy provides an opportunity to investigate domain walls, by performing a line scan over a wall of interest. We reveal that the domain wall widths in Ca3Ti2O7 and h-Lu0.6Sc0.4FeO3 are 60-100 nm wide and remain insulating. We perform high pressure infrared spectroscopy to reveal pressure-induced structural phase transitions. Combined with symmetry analysis and complimentary lattice dynamics calculations, we assign high pressure phases by comparing experimentally observed changes in the vibrational response with predicted mode patterns for a series of candidate space groups. For the case of hybrid improper ferroelectric Sr3Sn2O7, we discover that the set of structural phase transitions as a function of pressure mirror the reported sequence as a function of temperature. A similar analysis is performed on multiferroic h-Lu0.6Sc0.4FeO3 . We reveal a structural transition from a polar → antipolar space group at 15 GPa. We relate this distortion to changes in the bipyramidal tilting modes and competing structural trends in this linear magnetoelectric ferrite

Inelastic light scattering signatures of magnetic ordering and topological properties in strongly-correlated electron systems

We investigate strongly-correlated electron systems where magnetically frustrated and Kondo insulating states show significant sensitivity to disorder and defects. Inelastic light scattering provides an effective tool for probing magnetic and electronic structures of novel states that arise from correlations through their excitation spectra along with characterizing weak distortions and disorder within crystal lattices. We explore triangular-lattice Heisenberg antiferromagnets where ground-state properties are altered from an isotropic triangular-lattice model by weak disorder present in the compounds. In the anisotropic triangular-lattice Heisenberg antiferromagnet SrCr2O4, a coupling of the lattice to magnetic degrees of freedom is observed through a redistribution of phonon intensities that onsets with the formation of peaks within the magnetic spectrum. The triangular-lattice compound NiGa2S4 with competing interactions shows spin freezing where theoretical modeling suggests a magnetically ordered ground state. We demonstrate evidence for structural disorder within this compound that would modify the Heisenberg Hamiltonian through the loss of superexchange pathways and a breaking of crystal inversion symmetry. Heavy fermion systems combine localized magnetism with itinerant electron bands that can drive Kondo insulating behavior in certain materials. Renewed interest in Kondo insulator SmB6 has arisen as a result of the proposal of a strongly correlated topological insulating state in this material which is dependent on the formation of a hybridization gap at low temperatures. Inelastic scattering shows a 10 meV symmetry forbidden mode appearing in the spectrum in samples known to have off-stoichiometries assigned to scattering from acoustic modes away from the Brillouin zone center that results from crystal defects. The intensity of this band is used as a measure of Sm vacancies to identify their effect on the low temperature electronic properties of the system. A suppression of the bulk hybridization gap is found in samples with as little as 1% Sm vacancies demonstrating the sensitivity of the topological insulating state to weak structural disorder

Electromagnetic Materials Design Using Machine Learning and Materials Informatics

The limitations in materials remain the fundamental reason for suboptimal performance of many electronic devices. Discovering novel materials with desirable electronic, magnetic and optical properties and exploring the uncharted facets of existing materials are vital in driving the technology forward. Traditionally, materials discovery involved human intuition and trial-and-error experimental loops which may take years for successful development of a new material. While computational methods were later employed to speed up this process, such approaches are difficult to be scaled up for complex material systems. Recently, machine learning (ML) has emerged as an exciting tool for accurate prediction of materials properties orders of magnitudes faster than computational or experimental characterizations. In this thesis, I discuss how state-of-the-art (SOTA) ML techniques can be applied to design materials that potentially display desirable electromagnetic properties. The core of this thesis is structured around three major projects that involve both ML algorithmic advancements and applications of such models in discovering materials for frequency reconfigurable antennas. There are two material property prediction schemes - one that uses details of the crystal structure of materials and the other that only uses their chemical composition. I first develop a novel graph neural network (GNN) architecture that can be applied in both domains simultaneously and show that the proposed model achieves SOTA results in many material property prediction test cases. The model is then employed to predict the frequency-dependent dielectric constant of materials and nominate new compositions that potentially exhibit epsilon-near-zero (ENZ) properties. In the second topic, I specifically focus on the study of doped barium strontium titanate (BST), a microwave ferroelectric material widely used in tunable antennas. I will discuss how an existing semi-empirical model developed for pure ferrorlectrics can be extended to model ferroelectric-dielectric composites over a wide bandwidth by incorporating deep transfer learning and experimental data. Dielectric constant predictions using the improved model show good agreement with our experimental measurements performed under various sintering conditions. The third research theme of my thesis extends the search space to a larger family of materials, namely, disordered perovskite-oxides. Such materials exhibit intriguing electromagnetic phenomena, including electrically tunable dielectric constant that facilitates frequency reconfigurability of antennas. In this work, I have developed supervised and unsupervised ML models underpinned by experimental data to screen new compositions analogous to existing high-performing perovskites and validate my results with density functional theory (DFT) simulations

Resistive switching in ferroelectric polycrystalline Yttrium Manganese Oxide thin films

A memristor is a two-terminal device which exhibits a hysteresis loop in the current-voltage characteristics. Resistive switching refers to reversible non-volatile change in state of the resistance. There exists a wide range of materials which show resistive switching i.e, phase change materials are used in today’s technology which are a main component of the resistive random access memory. In actual research, mostly metal oxides are investigated regarding their resistive switching which is based on migration of anions and cations. Additionally, in hexagonal manganites, h-RMnO3 (R = Y, In, Sc, Ho,...,Lu), the multiferroic properties and nano-sized conducting domain walls introduce further interesting aspects in this material class which may contribute to additional features in resistive switching. This dissertation investigates the resistive switching in yttrium manganite thin film (Y1Mn1O3, Y0.95Mn1.05O3, Y1Mn0.99Ti0.01O3 and Y0.94Mn1.05Ti0.01O3) based metal-insulator-metal structures with different top electrodes (Au or Al) and bottom electrodes (Pt or Pt/Ti or Pt/Cr) in 2-point DC probe measurements. Yttrium manganite thin films have been deposited by pulsed laser deposition on metal coated SiO2/Si substrates. Electrical characterization of yttrium manganite thin films in a metal-insulator-metal structure exhibit electroforming-free unipolar resistive switching. High voltages and currents are required for SET (V_ ) and RESET (I_ ), respectively. The observed resistive switching is assigned to the formation (low resistance state) and rupture (high resistance state) of conductive, metallic-like filaments induced by a thermo-chemical phenomena. Observed unipolar RS is classified as the thermo-chemical memory (TCM) resistive switching phenomena related to the locally increased temperature. The stability of conductive filaments leads to good retention of the programmed states with large memory window (OFF to ON resistance in the order of 10^4 - 10^6, depends on electrodes, electrode size and composition of yttrium manganite thin films). The endurance or number of loading cycles of the resistive switching devices are improved and is in the order of 10^3 for Y1Mn1O3 and Y0.95Mn1.05O3 composition with Al-top electrodes and Pt-bottom electrode. The maximum number of loading cycles is observed for an applied negative bias, a preferential negative polarity for switching the yttrium manganite thin film devices with Au or Al top electrodes and Pt or Pt/Ti bottom electrodes. Whereas, yttrium manganite thin film devices with Pt/Cr-bottom electrode and Al-top electrodes a preferential positive bias is required for switching the devices. Temperature-dependent measurements of yttrium manganite thin films deposited on Pt/SiO2/Si show semiconducting and metallic-like conduction in high resistance state and low resistance state, respectively. The activation energy () extracted in the ohmic region for hopping of holes localized at Mn4+ is in the range of 0.36 eV - 0.43 eV. Scanning electron microscopy in secondary electron emission mode with an in-lens detector and a small acceleration voltage of 1.0 kV is used to characterize the ferroelectric charged domain network formation in polycrystalline hexagonal yttrium manganite thin film. The observed bright regions correspond to local polarization vector with upward polarization components (+P ) and dark regions to local polarization vector with downward polarization components (-P ). A dense domain network is observed for Mn-rich samples (Y0.95Mn1.05O3 and Y0.94Mn1.05Ti0.01O3) in comparison to Y1Mn1O3 and Y1Mn0.99Ti0.01O3 with smaller grains show isolated charged domains. The observed dependency of different compositions to the charged domain density network in yttrium manganite thin films may influenced by different factors: stoichiometry gradient, oxygen, dopant concentration and the resulting grain structure.Ein Memristor ist ein Bauelement, welches eine Hysterese beim Vermessen seiner IU-Kennlinie aufweist. Dieses als „Widerstandsschalten“ bezeichnete Phänomen beruht auf der nichtflüchtigen Veränderung des Widerstandes. Es existiert eine breite Auswahl an Materialien, welche Widerstandsschalten zeigen, z.B. sind Phasenwechselmaterialien die Hauptkomponenten in aktuellen RRAMs. Aktuelle werden hauptsächlich Metalloxide untersucht, welche durch Migration von Anionen und Kationen Widerstandsschalten hervorrufen. Weitere Materialien wie hexagonale Manganoxidverbindungen RMnO3 (R = Y, In, Sc, Ho,...,Lu), besitzen zusätzliche multiferroische Eigenschaften, bei denen geladene Domänengrenzen weitere interessante Aspekte in dieser Materialklasse einführen und das Widerstandsschalten beeinflussen können. Die vorliegende Dissertation untersucht das Widerstandsschalten in Yttriummanganoxid-Dünnfilmen mit unterschiedlichen Kompositionen und unterschiedlichen Elektrodenmaterialien. Y1Mn1O3, Y0.95Mn1.05O3, Y1Mn0.99Ti0.01O3 und Y0.94Mn1.05Ti0.01O3, wurden mittels gepulster Laserdeposition auf metallisierte Si/SiO2 Substrate abgeschieden. Die elektrische Charakterisierung von Yttriummanganoxid-Dünnfilmen in einer Metall-Isolator-Metall Sandwichstruktur weist auf elektroformierungsfreies, unipolares Widerstandsschalten hin. Das beobachtete Widerstandsschalten wird auf die Formierung (niederohmiger Zustand) und Zerstörung (hochohmiger Zustand) des leitfähigen, metallischen Filaments (geladenen Domänengrenzen oder auch Vortices), verursacht durch thermisch-chemische Vorgänge, zurückgeführt. Die geladenen Domänengrenzen und/oder Vortices in Yttriummanganoxid-Dünnfilmen beeinflussen unter Umständen als nanoskalige Objekte die Formierung der leitfähigen Filamente. Die Stabilität der leitfähigen Filamente führt zu einer guten Langzeitspeicherung der programmierten Zustände, welche auch ein sehr großes Speicherfenster (Widerstandsverhältnis zwischen Aus/An-Zustand von 10^5) aufweisen. Die großen Widerstandsverhältnisse sind z.B. für die Herstellung von Auswahlschaltern (selektoren) in Crossbar-Strukturen notwendig, um die möglicherweise auftretenden Kriechströme in Crossbar-Strukturen zu unterdrücken, welche sonst Lesefehler der adressierten Zellen hervorrufen würden. Die Wiederbeschreibbarkeit ist in der Größenordnung von ca. 10^3, abhängig von der chemischen Zusammensetzung des Yttriummanganoxide-Dünnfilmes und vom verwendeten Elektrodenmaterial. Resultate der Charakterisierung mittels Rasterelektronenmikroskopie im Sekundärelektronenmodus mit einer kleinen Beschleunigungsspannung von 1.0 kV weisen auf geladene ferroelektrische Domänen in polykristallinem hexagonalen YMnO3 Dünnfilmen hin. Deswegen muß der Einfluss von geladenen Domänengrenzen und multiferroischen Vortices auf das beobachtete Widerstandsschalten in hexagonalem YMnO3 berücksichtigt werden

Matériaux multiferroïques : structure, ordres et couplages. Une étude par spectroscopie Raman

Multiferroics are materials in which magnetic, electric and elastic orders can coexist in the same phase. These orders can be coupled to each other and their study of high interest to understand the mecanisms at stake in the multiferroic materials. This PhD thesis has been focused on the study of several multiferroic compounds by the mean of Raman spectroscopy. In bismuth ferrite (BiFeO₃), the effect of strain on the magnetic order, both on thin films (epitaxial strain) and single crystals (hydrostatic pressure), has been thoroughly investigated. This thesis also focuses on the study of hybrid magneto-electric excitations (electromagnons) in type II multiferroic compounds with strong ferroelectric polarizations such as CaMn₇O₁₂ and TbMnO₃. Furthermore, phonons modes and of low energy excitations have been measured and studied (especially under magnetic field) in compounds with frustrated magnetic orders such as h-YMnO₃, h-YbMnO₃ and in the niobium iron langasite (Ba₃NbFe₃Si₂O₁₄).Les matériaux multiferroïques sont des matériaux dans lesquels des ordres magnétiques, électriques et élastiques peuvent coexister dans une même phase. Ces ordres peuvent être couplés entre eux et l’étude de ces couplages permet de mieux comprendre les mécanismes à l’œuvre dans ces matériaux. Cette thèse porte sur l’étude de différents composés multiferroïques par spectroscopie Raman. Dans la ferrite de bismuth (BiFeO₃), l'effet de la contrainte sur le magnétisme, aussi bien sur les films minces (par contrainte épitaxiale) que le bulk (par pression hydrostatique) est étudié en détail. Cette thèse présente également une étude des excitations hybrides magnéto-électriques (électromagnons) dans les composés de type II à forte polarisation ferroélectrique comme CaMn₇O₁₂ et TbMnO₃. En outre, les modes de phonons ainsi que les excitations de basses énergies ont été étudiés (notamment sous champ magnétique) dans des composés au magnétisme frustré comme h-YMnO₃, h-YbMnO₃ et dans le langasite de fer au niobium