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This dissertation is focused on ferromagnetic, multiferroics, and two-dimensional (2D) perovskites, exploring different unique collective magnetic and ferroic characters: (1) ferromagnetic thin film Co/Pd multilayers, (2) BaTiO3-BiFeO3 (BTO-BFO) a magneto-electric materials system, and (3) CuCl4 halide organic-inorganic perovskites. Low-power all-optical memory offers a unique opportunity to achieve ultra-fast magnetic switching in which the switching dynamics are not thermally mediated and occur on the order of the laser pulse. However, it is challenging to achieve a low-power optically excited magnetization precession angle above 90 degrees, which is required for magnetic switching. Co/Pd thin film multilayers were investigated for their potentially large perpendicular magnetic anisotropy (PMA) with three differing regimes of magnetic anisotropy: in-plane, weakly out-of-plane, and out-of-plane. Utilizing the time-resolved magneto-optical Kerr effect (TR-MOKE), we observed clear magnetic precession (on the order of a few GHz) with magnetic precession angle increasing (up to 4.5 degrees) for thinner Co samples which demonstrated stronger PMA. We observed a clear connection between PMA strength and precession amplitude as well as a large efficiency of energy transfer between spin and orbital subsystems for our strongest PMA sample. BTO-BFO is a strong room-temperature multiferroic with enhanced magneto-electric properties compared to BFO. We utilized time-resolved differential reflectivity (TR-DR) and TR-MOKE to observe strong coherent acoustic phonons in thin films as well as nanorods. Our nanorods showed additional modes (a new 20 GHz and 6 GHz mode) not observed in thin films including the fast 33 GHz mode which showed some weak tunability with high magnetic fields (up to 10 T). The observed tunability of these modes in an external magnetic field shows interesting coupling between magnetic moment and phononic modes which may be caused by the breaking of the spin-cycloid at the interface of the nanorods and the surface of the nanorods. We also observed second harmonic generation (SHG) emission which demonstrated a large enhancement in our nanorod structures with further observation of wavelength dependence. Finally, ferromagnetic resonance on our nanorod and thin film BTO-BFO structures indicated very weak Gilbert damping (on the order of 10−3), demonstrating the practicality of our structure for low-spin loss applications. Lastly, this dissertation focuses on a project around CuCl4 and CuCl2Br2 perovskites in which we observed time-dependent SHG. An increase in SHG as a function of infrared laser exposure is shown to coincide with changes in the crystal structure of the Cu perovskite materials. This increase in SHG was shown to last for a few days after hours of laser exposure indicating a slow hysteretic change to the crystal structure of the perovskites.

This dissertation presents three projects that investigate the complex phenomena of ferroelectricity under different conditions in BaTiO3, LiNbO3, and LiZnSb using first-principles density functional calculations. Extended defects in ferroelectric solids play a crucial role in reducing the lifetime and performance of ferroelectric devices by causing fatigue, domain pinning, and aging. Thus, understanding their impact is of critical importance for the development of reliable and high-performance ferroelectric devices. In addition, hyperferroelectricity is an intriguing phenomenon that has attracted much attention in recent years. Despite the existence of depolarization field, spontaneous polarization persists under an open-circuit boundary condition (OCBC), making hyperferroelectric materials promising candidates for various electronic applications. In the first project, we investigate the energetic, structural, and electric properties caused by extended planar oxygen vacancies in ferroelectric BaTiO3. Oxygen vacancies of different charge states, namely V_O^(2+), V_O^(1+), and V_O^0, are studied. We find that, while planar V_O^q vacancies of all three charge states may take place under the oxygen-poor condition, planar V_O^(2+) vacancies can still occur even under the oxygen-rich condition. Furthermore, our calculations show that BaTiO3 with planar V_O^0 or V_O^(1+) vacancies is metallic, while BaTiO3 with planar V_O^(2+) vacancies is an insulator. This finding reveals an important conclusion that, for BaTiO3 with planar V_O^(2+) vacancies, no mobile charges are available to screen the polarization, electric polarization is thus well defined and can be rigorously computed. Moreover, we discover that extended planar V_O^(2+) vacancies produce a devastating effect on ferroelectricity by drastically reducing the electric polarization, which is markedly different from isolated V_O^(2+) vacancies (which are benign to ferroelectricity). The polarization dead layer caused by planar oxygen vacancies is shown to be around 72Å. In the second project, we investigate the hyperferroelectric properties of LiNbO3. We find that (i) the longitudinal-optic A2u(LO1) phonon is soft with imaginary frequency 96i cm−1 when centrosymmetric LiNbO3 is under OCBC. (ii) The A2u(LO1) phonon can yield, under OCBC, a free-energy minimum with well depth of -9 meV at a nonzero polarization of 0.023 C/m2, thereby capable of producing hyperferroelectricity. (iii) The origin that A2u(LO1) can induce HyFE stems from the extraordinarily small mode effective charge of this phonon. (iv) Despite that A2u(LO1) can induce HyFE, we find the ground state of LiNbO3 under OCBC is not polar, revealing that the existence of a soft LO phonon does not guarantee HyFE. (v) We further show that LiNbO3 under OCBC may exhibit unusual tri-stable polarization states, with two potential wells of depth -23 meV and -1.9 meV. However, small polarization and shallow potential well depth are found so far in most hyperferroelectric materials, including LiNbO3, which severely limits their applications. In the third project, we report the discovery of a giant hyperferroelectricity in LiZbSb. The HyFE polarization is shown to be remarkably large with P=0.282 C/m2 under OCBC, which is one order of magnitude greater than the HyFE polarization in LiNbO3. Furthermore, HyFE in LiZnSb is found to be exceptionally stable with a well depth of electric free energy ∆F=-332 meV, which makes LiZnSb a possible hyperferroelectric solid at room temperature. The origin of the giant hyperferroelectricity in LiZnSb is attributed to the large mode effective charge of the soft longitudinal-optic phonon and the large high-frequency dielectric constant. The HyFE strain dependence in LiZnSb is also examined.

In recent years, two-dimensional (2D) van der Waals (vdW) materials have aroused much interest for their unique structural, thermal, optical, and electronic properties and have become a hot topic in condensed matter physics and material science. Many research methods, including scanning tunneling microscopy (STM), transmission electron microscopy (TEM), optical and transport measurements, have been used to investigate these unique properties. Among them, STM stands out as a powerful characterization tool with atomic resolution and is capable of simultaneously revealing both atomic structures and local electronic properties. This dissertation focuses on scanning tunneling microscopy and spectroscopy (STM/S) investigation of 2D metal chalcogenides and heterostructures. The first part of the dissertation focuses on the continuous interface in WS2/MoS2 heterostructures grown by the chemical vapor deposition (CVD) method. We observed a closed interface between the MoS2 monolayer and the heterobilayer with atomic resolution. Furthermore, our scanning tunneling spectroscopy (STS) results and density functional theory (DFT) calculations revealed band gaps of the heterobilayer and the MoS2 monolayer agree with previously reported values for MoS2 monolayer and MoS2/WS2 heterobilayer on SiO2 fabricated through the mechanical exfoliation method. The results could deepen our understanding of the growth mechanism, interlayer interactions and electronic structures of 2D transition metal dichalcogenides (TMD) heterostructures synthesized via CVD. The second part of the dissertation focuses on phase transformation in 2D In2Se3. We observed that 2D In2Se3 layers with thickness ranging from single to ~20 layers stabilized at the beta phase with a superstructure at room temperature. After cooling down to around 180 K, the beta phase converted to a more stable beta' phase that was distinct from previously reported phases in 2D In2Se3. The kinetics of the reversible thermally driven beta-to-beta' phase transformation was investigated by temperature dependent transmission electron microscopy and Raman spectroscopy, combined with the expected minimum-energy pathways obtained from our first-principles calculations. Furthermore, DFT calculations reveal in-plane ferroelectricity in the beta' phase. STS measurements show that the indirect bandgap of monolayer beta' In2Se3 is 2.50 eV, which is larger than that of the multilayer form with a measured value of 2.05 eV. Our results on the reversible thermally driven phase transformation in 2D In2Se3 will provide insights to tune the functionalities of 2D In2Se3 and other emerging 2D ferroelectric materials and shed light on their numerous potential applications like non-volatile memory devices. The third part of the dissertation focuses on domain boundaries in 2D ferroelectric In2Se3. The atomic structure of domain boundaries in two-dimensional (2D) ferroelectric beta' In2Se3 is visualized with scanning tunneling microscopy and spectroscopy (STM/S) combined with DFT calculations. A double-barrier energy potential across the 60° tail to tail domain boundaries in monolayer beta' In2Se3 is also revealed. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials.

Quantum paraelectric materials like SrTiO3 and KTaO3 exhibit many unusual material properties arising from their quantum fluctuations, including a high dielectric constant and unconventional uperconductivity. In combination with other unusual properties like strong spin orbit coupling and charged ionic layers in KTaO3, this leads to the observation of various intriguing phenomena. The primary goal of this thesis is to investigate these ferroelectric and quantum paraelectric states, with a focus on the evolution of polarization, quantum fluctuations and electron-phonon coupling with chemistry and doping, using computational first-principle methods and SrTiO3, KTaO3 and BaTiO3 as model systems. To achieve this goal, I first investigate ferroelectric polarization in BaTiO3 and how it can persist despite the presence of free charge carriers, two principally contraindicated phenomena. Next, I create a model, based on quantities easily extracted from DFT and DFPT calculations, to better understand the quantum paraelectric state and use this model to study the so-called isotope effect in SrTiO3. Finally, I calculate the electron-phonon coupling strength in KTaO3, motivated by its highly anisotropic surface superconductivity, to gain insight into which phonon modes may be relevant for the electron-phonon pairing mechanism. From the study of BaTiO3, we learn both that polarity and metallicity can coexist in an originally ferroelectric material, up to relatively high doping levels, and how doping ions affect the geometry and polarization of the resulting structure, ranging from complete suppression to an unexpected increase of polarization. Next, we show that the simple model for quantum paraelectricity works well and see that the actual isotope effect in SrTiO3, namely the substitution of O16 with its heavier isotope O18, probably has a smaller effect than expected and geometry effects may be more important than initially thought. The electron-phonon coupling calculations show that while there is no direct correlation between the orientation dependence of the coupling strength λ and the experimentally measured superconducting critical temperatures, confirming that classical BCS theory is not directly applicable to KTaO3, a strong localization of λ around Γ indicates a pairing mechanism involving the polar soft mode. Overall, this thesis reveals new insights into the relationship between chemistry, conductivity, ferroelectricity, quantum paraelectricity and electron-phonon interactions in this important class of materials.

Over the last decades, ferroelectric materials have become essential components in a range of applications, from sensors and actuators to non-volatile memories. This was made possible by unprecedented progress in the synthesis and characterization of ferroelectric materials, allowing for precise control of important properties, such as the spontaneous electrical polarization or the electro-mechanical coupling. An essential step in this development was the advancement in oxide thin-film engineering, which enabled the study of ferroelectric materials beyond bulk crystals. However, under the spatial confinement of a thin-film geometry, the functional properties of ferroelectric materials can differ greatly from their bulk-crystal counterpart. It is therefore imperative to study the influence that structural defects and interfaces have on the functionality of thin-film ferroelectrics. In this thesis, we use scanning transmission electron microscopy to study the crystal structure and electronic properties of a variety of ferroelectric materials. In lead titanate, we map the ferroelectric polarization at the atomic scale under competitive and cooperative interface configurations, which, together with complementary in-situ second harmonic generation microscopy, allowed the stabilization of a robust, single-domain configuration in a ferroelectric capacitor. We then developed a focus-ion beam workflow that allows us to prepare ferroelectric thin-films in a capacitor-like geometry for in-situ electrical biasing experiments in the transmission electron microscope. Applying this technique to lead zirconate titanate thin films, we investigate the response of the ferroelectric polarization to an applied electric field in the presence of structural defects and domain boundaries. Combining atomically resolved high-angle annular dark-field and medium resolution differential phase contrast, we observed a reduction of the average domain-wall mobility in highly defective regions, with inhomogeneous domain wall motion at the local scale. Finally, we employed electron energy loss spectroscopy to study the electronic structure and chemical composition of our materials. In lead zirconate titanate, we find the accumulation of oxygen vacancies stabilizing energetically unfavorable domain configurations in the presence of the substrate interface. In hexagonal yttrium manganite, we quantify the Mn valence state to correlate it with changes in the structural distortion of the material, observed at its interfaces. We further combine this analysis with the in-situ heating capabilities of our microscope to investigate the changes in the electronic structure of yttrium manganite during its ferroelectric phase transition. The results presented in this work demonstrate the utility of investigating the properties of ferroelectric materials down to the atomic scale. Such characterizations will enable the design of energy-efficient oxide electronics of the future. Furthermore, our work also highlights the synergies with complementary characterization techniques, such as piezo-response force microscopy, in-situ harmonic generation and theoretical modelling, which, together with transmission electron microscopy, allow for a more complete understanding of the material properties across all length scales.

Oxide electronics have emerged as an alternative to replace the current silicon-based technology. Owing to a rich elemental composition compared to that of doped silicon, transition metal oxides can host a wide range of physical phenomena. This is especially true when oxides are integrated into ultrathin epitaxial heterostructures, in which additional properties arise from the created interfaces. Their crystal structure is, furthermore, compatible with long-range order. In particular, ferromagnetic and ferroelectric systems have gathered considerable attention due to their characteristic non-volatile response to applied external fields. While ferroic oxides are indisputable candidates for low-energy-consuming applications, there are still a few setbacks left to overcome in order to integrate them into competitive device schemes. With the work performed during the course of this thesis, we strive to provide solutions for existing limitations to the implementation of ferroic states in nanoscale devices. We place emphasis on interfacial effects in epitaxial heterostructures and their influence on ferroic order. Making use of a non-conventional approach of epitaxially combining layers with different ferroic anisotropies, we uncover novel fundamental concepts likely to benefit the ever-evolving field of oxide electronics. We identify the in-plane-polarized Aurivillius compounds as promising candidates for tuning interfacial electrostatics and achieving interfacial polar continuity in epitaxial hybrid heterostructures with ferroelectric perovskite oxides. The stabilized coalescent layer-by-layer growth mode ensures the single-crystallinity of these layered ferroelectrics, while the sub-unit-cell thickness control of the films enables detailed investigations of their polar state. For instance, the thickness-dependent in-plane-polarized domain configuration in the resulting epitaxial Aurivillius films prompts us to propose new means for ferroic domain and functional domain-wall engineering via structural defect ordering. Moving ahead with the integration of Aurivillius films into perovskite-based heterostructures, we show that the Aurivillius compounds utilized as in-plane-polarized buffer layers can overcome the notorious limitation associated with the critical thickness for ferroelectricity in canonical out-of-plane-polarized perovskite ferroelectrics. We additionally demonstrate that buffers of the Aurivillius phase can be instrumental for domain and domain-wall engineering in the room-temperature multiferroic BiFeO3. In particular, we observe a uniform chirality stabilized in Néel-like domain walls in BiFeO3 grown on our in-plane-polarized Bi5FeTi3O15 Aurivillius layer. This likely constitutes one of the first experimental signatures of the electric counterpart to the Dzyaloshinskii-Moriya interaction in magnetically ordered compounds. Lastly, we explore magnetoelectric phase control in heterostructures combining both ferroelectric and ferromagnetic order. In a proof-of-concept multiferroic heterostructure, we mimic magnetoelectric domain walls by inserting ultrathin ferromagnetic La1-xSrxMnO3 in between two ferroelectric layers. We show that its magnetization and conductivity can be controlled by changing polarization directions in the adjacent ferroelectric layers only. This opens up new possibilities for voltage-based tuning of magnetization and conductivity at the nanoscale.

The transmission electron microscope (TEM) provides multiple modes to study structure-property relationships in materials. In particular, quantum materials, whose properties may be affected by subtle alterations to atomic configuration, require such a valuable instrument for illuminating these relationships. Scanning transmission electron microscopy (STEM) and convergent beam electron diffraction (CBED) are two techniques within the TEM that may be utilized to examine the atomic structure and crystal symmetries of materials. Cadmium arsenide (Cd3As2) is classified as a topological semimetal due to its linear band crossings in the bulk electronic band structure and surface states. Different publications in the past have proposed competing centrosymmetric and non-centrosymmetric crystal structures for Cd3As2, an important distinction that has major implications on the expected band structure and underlying physics. Due to the complicated crystal structure of Cd3As2, as well as the limitations in x-ray diffraction techniques in identifying an inversion center, particularly in thin films, we used CBED to determine the exact point group symmetries of epitaxially grown Cd3As2, finding it to possess an inversion center. Because Cd3As2 possesses fourfold rotational symmetry and an inversion center, it can be classified as a Dirac semimetal. We sought to topologically tune Cd3As2 by introducing biaxial epitaxial stress, producing tensile and compressively stressed films. Using CBED, we found that fourfold rotational symmetry was lost in compressively strained, (112)-oriented Cd3As2, promoting the opening of a band gap and a topological insulator state. We also found that inversion symmetry was broken in tensile strained, (001)-oriented Cd3As2, which may lead to band splitting away from the node and suggest previous studies proposing a non-centrosymmetric crystal structure may have been influenced by residual strain. Strontium titanate (SrTiO3) is an incipient ferroelectric material in which a ferroelectric state may be realized through strain. Of interest in this thesis is realization of the ferroelectric state by inducing compressive epitaxial stress. SrTiO3 also undergoes a superconducting transition at low temperatures, though the mechanism by which this occurs is not completely understood, nor is the relationship between superconductivity and ferroelectricity in this material. Past studies have intimated a relationship between nano-sized polar domains that form at room temperature and these phenomena. Namely, the size of the polar nanodomains may be considered a predictor of certain superconducting parameters. In this study, the SrTiO3 film thickness was varied, and STEM was used to identify Ti-site displacements and to map out the polar domains. We found that polar nanodomains existed in only the thickest film (70 nm), but were screened in all others. The two thickest films (70 nm and 40 nm) underwent ferroelectric and superconducting transitions, while thinner films did not. A critical thickness is predicted to exist between 25 nm and 40 nm for these phenomena in compressively strained SrTiO3 films.

Although superconductivity in strontium titanate (SrTiO3) was discovered in the 1960s, the origin of superconductivity in SrTiO3 has not been completely understood. Moreover, its unusual superconducting characteristics have made SrTiO3 a fascinating playground to potentially study multiple aspects of unconventional superconductivity. For example, SrTiO3 is one of the most dilute superconductors where standard Bardeen-Cooper-Schrieffer (BCS theory) is believed to be no longer valid. Furthermore, the presence of superconductivity and ferroelectricity and their interplay possibly leads to an unconventional pairing mechanism in SrTiO3. In this dissertation, high-quality, doped, ferroelectric, compressively strained SrTiO3 thin films were grown using hybrid molecular beam epitaxy (MBE) to unveil unique superconducting properties. First, we present unconventional characteristics of relatively thick (160-180 nm) strained SrTiO3 films. In some strained films, we show signatures of unusual superconducting states such as in-plane critical fields far above the Pauli limiting field and second harmonic resistances. The study of superconductivity in ultra-thin SrTiO3 films is hindered by strong surface carrier depletion, which makes thin films insulating. We solve this issue by introducing thin (~10 nm) EuTiO3 capping layers. We show that the EuTiO3 capping layer effectively prevents strong carrier depletion in SrTiO3 films and that a capped 40 nm-thick SrTiO3 film becomes superconducting. We utilize the EuTiO3 capping layer to elucidate the connection between superconductivity and ferroelectricity in SrTiO3 films. Systematic film thickness study shows that superconductivity and ferroelectricity are suppressed simultaneously in the SrTiO3 films of thicknesses less than 40 nm. We speculate that this result implies that broken inversion symmetry (spin-orbit coupling) plays an important role in the superconductivity of strained SrTiO3 films.

In this work, we present a comprehensive computational and theoretical study of the structural phase space of multiferroic Bi-Fe-O and the influence of the structure on functional properties. Complex oxides are known for their many different functionalities, including ferroelectricity, ferromagnetism, or piezoelectricity, which are relevant for technological applications. Multiferroicity, in which at least two different ferroic orders (ferromagnetism, ferroelectricity, and ferroelasticity) coexist and couple in a single phase is of particular interest and can enable, for example, control of magnetic properties using an applied electric field. BiFeO3 (BFO) is one of the most studied multiferroic materials because of the coexistence of magnetic and polar orders at room temperature promising for applications. The substantial intrinsic functionalities of complex oxides can be further enhanced by growing them as thin films in superlattices or heterostructures. In most cases identified to date, this is a result of the change in the material’s lattice constants caused by epitaxial strain imposed by the substrate. This change in lattice constants can modify the functionalities, for example causing a phase change to a higher polar state in BFO. In heterostructures of polar and non-polar materials, the polar discontinuity at the interface results in an accumulation of surface charges, which in turn create an electric field, known as the depolarising field. This de- polarising field can cause the formation of domains in the polar material and even change the orientation of the polarisation, in order to reduce the energy penalty of the interfacial charge. We start our study by considering the case in which a phase transition to a non-polar phase with low relative energy is preferred over polar domains formation and present a new phase with a larger unit cell size and surprising properties matching recent experimental results. We then explore the phase space of BFO using density functional theory (DFT) calculations and reveal several low-energy phases with large unit cells which could be stabilised by a polar dis- continuity at the interface. Finally, we consider a heterostructure in which the polar discontinuity is partially reduced by the presence of differently charged ionic layers and show that this causes an isosymmetric phase transition from two phases with the same symmetry but different polar states in highly-strained BFO. Next, we study a new candidate multiferroic material, bismuth hexaferrite, which exhibits net magnetic and possible ferroelectric moments at room temperature, experimentally demonstrated. We show that the net magnetic moment is likely related to the presence of Fe vacancies and compute the energy barrier between opposite orientations of the polarisation in order to evaluate the likelihood for it to be ferroelectric. This result paves the way for the exploration of new materials in the Bi-Fe-O compositional phase space with technologically relevant properties. Finally, we conclude our work by proposing an accelerated method combining DFT, irreducible phonon modes, and machine learning to explore the potential energy surface of BFO and we predict several low-energy structures, suggesting that the phase space of BFO is still far from being completely known. This thesis highlights the richness of the phase space of Bi-Fe-O and the importance of the boundary conditions provided by the heterostructure, in particular, the electrostatics at the interface in determining the properties of functional oxides. Finally, the methods that we developed provide a new accelerated approach to structure discovery. They are broadly applicable and can certainly lead to discoveries in other materials.

The central material of interest for this thesis is strontium titanate (SrTiO3). Doped SrTiO3 exhibits a wide range of remarkable properties. The focus of this work is to investigate its ferroelectric and superconducting behavior. We utilize scanning transmission electron microscopy (STEM) to understand the role of the nanoscale SrTiO3 structure in these properties. SrTiO3 is an incipient ferroelectric in unstrained, pure form, but easily becomes ferroelectric when subjected to small perturbations. Understanding the nature of the ferroelectric transition in SrTiO3 is essential as it can play an important role in other properties. A hallmark of order-disorder transitions is the formation of polar domains above the Curie temperature. These polar regions percolate below the Curie temperature to form a long-range ordered ferroelectric state. Using high-angle annular dark-field STEM and analyzing off-centering of Ti columns, we show local polar regions at room temperature in compressively strained SrTiO3 films, highlighting the order-disorder nature of the ferroelectric transition in this material. Next, we focus on understanding the competition between mobile carriers and polar crystal distortions. Elucidating the nature of this competition is of great interest for polar superconductors, which have attracted significant interest for their potential to host unconventional superconducting states. We observe a systematic suppression of the nanodomains and ferroelectricity with increasing amount of Sm dopant atoms. The itinerant electrons screen polar distortions and disrupt the nanodomains above the Curie temperature. The results provide direct evidence that long-range Coulomb interactions, already present in the paraelectric phase, are driving the ferroelectric transition and are becoming increasingly short-ranged with doping. Moreover, we show that the disorder caused by the dopant atoms themselves presents a second contribution to the destabilization of the ferroelectric state. This disorder destroys the nanodomains at high dopant concentrations. The focus of the second part of this thesis concerns the superconductivity of SrTiO3. Doped SrTiO3 is a superconductor whose pairing mechanism is still not fully understood. Superconductivity in SrTiO3 occurs in the vicinity of a ferroelectric instability, and recent theories suggest a link between two orders. Here, by using atomic structure imaging, we find that the superconducting critical temperature correlates with the length scale of polar order. The superconducting transition temperature is enhanced when polar nanodomains are sufficiently large. In these cases, the Cooper pairs reside in a non-centrosymmetric environment with strong spin-orbit coupling. The findings point to the length scale of polar nanodomains and spin-orbit coupling as important parameters controlling the superconductivity of SrTiO3. We investigate the robustness of superconductivity to magnetic and nonmagnetic disorder and discuss the importance of findings in explaining the unconventional nature of superconductivity in this material.

Dogan et al.[1], investigated the causes of ferroelectricity in doped hafnia using ab initio methods. Similarly, we investigated the stability of Al doped hafnia using quantum mechanical methods. There are many different phases of Hafnia: monoclinic, tetragonal, cubic and orthorhombic. Starting with the monoclinic phase of Hafnia, Hafnia undergoes phase transitions which result in different space groups. The temperature at which the tetragonal phase is induced is 2000 K and cubic phase is induced at 2900 K[1]. Different dielectric constants vary from phase to phase. The average dielectric constants are highest for the cubic and tetragonal phases. In order to force a high temperature phase to be stable in Hafnia, one would need to introduce cation dopants. The polar orthorhombic phase is well known to exhibit ferroelectricity. Since inducing this phase with about 40 GPa it is more effective to induce a ferroelectric phase in Hafnia via doping methods. Doping methods for Hafnia are well established in [1] and demonstrated for Si, Ge, La and other elements. The rhombohedral phase is of interest and investigated first in [35]. Since ferroelectricity in the rhombohedral phase is difficult to stabilize, dopants are introduced analogously to the orthorhombic case. In this work, we doped the monoclinic, tetragonal, orthorhombic and rhombohedral phase of Hafnia with Al to investigate the effect of dopants on ferroelectricity and relevant EXAFS data was also produced. Using plane wave density functional theory, the tetragonal, orthorhombic, and trigonal phases of aluminum doped hafnia were geometrically optimized. The resulting 3 percent, 6 percent, and 7 percent Al doped hafnia structures were used for generating expected EXAFS spectra for experimental data analysis and validation. Specifically, these DFT calculated structures are going to be used in the non-linear least square EXAFS fits of the thin films HfAlO2.

Replacing the traditional SiO2 gate oxide in a MOSFET with ferroelectric HfO2 creates a 1T memory device referred to as a FeFET. The bi-stable polarization states cause a retained threshold voltage shift known as the memory window. Ferroelectric HfO2 offers a number of material and electrical advantages over perovskite based ferroelectrics such as PZT or SBT. Due to its use as a high-k dielectric, the ALD capability and etch characteristics of hafnium oxide are well documented. Ferroelectric HfO2 has been shown to be thermally stable up to 1000 C, making gate first FeFET processes feasible. Electrically, HfO2 is capable of achieving much larger memory windows due to a high coercive field, on the order of 1-2 MV/cm. This property also allows for much thinner films (nm) without degradation of the memory window, and the potential for finFET applications. This work focuses on the integration of aluminum doped HfO2 into a standard RIT FET process. Previous work at RIT has led to the development of an ALD recipe and subsequent anneal to induce the ferroelectric crystal phase in Al:HfO2. In this work, n-channel MOSFETs with aluminum gate/20nm Al:HfO2/p-Si have been de- signed and fabricated. Etching of Al:HfO2 has been investigated using chlorine based plasma etching. The devices show a subthreshold slope of 75 mV/dec. Pulse testing reveals significant threshold voltage shift due to electron charge trapping commonly observed in Hf based dielectrics. I-V characteristics show mobility degradation, which is caused by Coulomb scattering as a result of trapped charges. For the devices to exhibit ferroelectric behavior with high on-state current, measurement and mitigation of charge trapping need to be further investigated.

Ferroelectric (FE) nanostructures have attracted considerable attention as our abilities improve to synthesize them and to predict their properties by theoretical means. Depolarizing field effects at interfaces of FE heterostructures are particularly notable for causing topological defects such as FE vortices and negative dielectric responses in superlattices. In this thesis, I employ two large-scale atomistic techniques, the first-principles-based effective Hamiltonian (HEff) method and the linear-scaling three-dimensional fragment (LS3DF) method. I use these methods to explore optical rotation in FE vortices, electro-optic effects in FE vortices and skyrmions, and voltage amplification via negative capacitance in ferroelectric-paraelectric superlattices. We employ HEff in Monte Carlo and molecular dynamics schemes to maximize spontaneous optical rotation in a BaTiO$_3$/SrTiO$_3$ nanocomposite. For a small bias field, maximal optical rotation was realized at room temperature. The result has acquired greater relevance since Ramesh and coworkers observed ``emergent chirality" in FE vortex arrays in PbTiO$_3$/SrTiO$_3$ superlattices. In a similar nanocomposite as above, we use the combined HEff and LS3DF method to study how band gap and band alignment evolves along the path from a polar-toroidal to an electrical skyrmion state. Temperature control of the vortex provides substantially larger range of control of bandgap and band alignment than field control of the skyrmion. Using temperature and electric fields to manipulate polarization and bond angle distortion in both constituent materials provides an additional handle for bandgap engineering in such nanostructures. We then use HEff to study BaTiO$_3$/SrTiO$_3$ superlattices as a platform for negative differential capacitance. We implement an atomistic framework amenable to simulation of negative capacitance in strained superlattices. In these systems, we predict misfit epitaxial strain control allows for broadly extending the operable temperature range for negative. By manipulating this strain, we observed switching of negative capacitance behavior between both constituent materials of the superlattice at low temperature.

There is intense fundamental and technological interest in perovskite oxides, a large class of ceramic materials that exhibit an unprecedented diversity of physical properties. Some perovskites exhibit ferroelectricity, the existence of a field-switchable electric polarization. Other perovskites exhibit rotations of the oxygen octahedra, which buckles bonds and leads to rich phase diagrams of magnetic and electronic order. This thesis investigates perovskites in which ferroelectricity and octahedral rotations appear together and are strongly coupled. We use first-principles methods and simple physical models to elucidate trends across multiple materials. The first part of this thesis introduces a new model for the interplay of ferroelectricity and octahedral rotations based on ferri-electricity. The second part of this thesis demonstrates for the first time perovskites that exhibit coupled ferroelectricity and octahedral rotations have electrical properties that are not possible in perovskites that exhibit ferroelectricity alone. The final part of this thesis is the first theoretical study into ferroelectric switching that takes into account the variety of structural domains that arise due to the octahedral rotations. These findings contribute to understanding how novel properties can emerge in materials with strongly coupled degrees of freedom.

The discovery of emergent phenomena arising from the interplay between magnetism, ferroelectricity and structure in multiferroic materials provides a myriad of potential capabilities in future solid state technologies. Despite extensive investigations in this field, many unsolved issues surround magnetoelectric coupling in several multiferroic systems, and in many cases there are competing approaches that attempt to describe their underlying mechanisms. Collectively, the projects in this thesis represent a two-pronged approach that utilises the complementary techniques of Raman light scattering and neutron diffraction as a means to investigate the contentious role of crystal structure in magnetoelectric coupling for the type-II multiferroic RMnO3 (R = Tb, Dy) and RMn2O5 systems (R = Tb, Ho, Y), and furthermore in potentially-multiferroic ErFeO3. The isotopic substitution of oxygen-18 significantly modifies the lattice dynamics of transition-metal oxide systems. In many materials, this can result in isotope-induced shifts in magnetic, ferroelectric, or superconducting phase transition temperatures that provide new understanding into the underlying physical processes that define their properties. Remarkably, oxygen-isotope substitution in RMnO3 (R = Tb, Dy) systems does not alter the onset of the multiferroic regime, indicating that magnetoelectric coupling is primarily a magnetically-driven phenomenon within these compounds. The combination of Raman scattering and neutron diffraction techniques allowed several key insights to be obtained. For both RMnO3 and RMn2O5 systems, Raman spectroscopy revealed that magnetically-driven ferroelectricity imposes significant effects that alter mode energies and lifetimes which indicate displacement-induced ferroelectricity. Furthermore, we observe remarkable structural behaviours in the form of altered bond lengths or faint Bragg reflections that indicate the existence of symmetry breaking. Our Raman study on ErFeO3 presents evidence of broken symmetry hitherto unobserved, as well as the interplay between magnetism and structure from strong interactions between different magnetic sublattices. Our combined approach provides crucial answers in understanding the emergent phenomena derived from the complex interplay between magnetic, ferroelectric and structural properties within these materials.

This dissertation is devoted to understanding the La0.7Sr0.3MnO3 (LSMO)/ PbZr0.2Ti0.8O3 (PZT) magnetoelectric interface through synchrotron x-ray absorption and other techniques. The word magnetoelectric (ME) describes the coupling effects in certain materials that exhibit a change of the magnetic (electric) order with the change of the electric (magnetic) field. Materials that are ME could potentially advance current technology. Faster, more sensitive, and more energy efficient devices can be built with ME materials as compared to the present systems. Practical ME materials are essential for the realization of this advancement. Single phase ME materials are rare and the known few do not have strong coupling effects at ambient temperatures. Bilayer (or multilayer) systems, however, provide a feasible alternative because they sometimes exhibit ME coupling effects at the interface(s). As an example of multilayer systems, ME coupling effects were previously reported by Vaz et. al. in between ferromagnetic LSMO and ferroelectric PZT, where the Mn valence changed with the varying external electric field. Through the use of a programmable shutter, small thickness gradients were created in both LSMO and PZT layers grown by pulsed laser deposition. A few flat samples were also grown for comparison. These samples were characterized by synchrotron methods including fluorescence mapping, micro x-ray diffraction (µXRD), x-ray absorption near edge spectroscopy (XANES), x-ray magnetic circular dichroism (XMCD), and photoemission electron microscopy (PEEM) as well as non-synchrotron based lab techniques such as atomic force microscopy (AFM) and scanning transmission electron microscopy (STEM). Wedge samples were locally smooth over experimental spot sizes and had uniform thickness gradients. Interfaces were sharp, exhibiting epitaxial growth. The magnetization increased with LSMO thickness. Mn valence in LSMO is associated with the LSMO magnetization states and was extensively studied in this dissertation. With the increase of LSMO thickness, Mn valence increased. A depth dependent valence model was developed to fit the LSMO thickness dependent valence results. The Mn charges were found to rearrange at the LSMO/PZT interface, and the effects of the ferroelectric polarization and polar interface on this interfacial valence were theoretically treated. In order to understand the effects of PZT on LSMO, Mn valence was measured at varied PZT thickness. Mn valence was found to be smaller when the PZT thickness was under 65 nm and decreased with decreasing PZT thickness in the PZT thickness region of 0 to 65 nm. Piezoresponse force microscopy (PFM) showed a transformation in PZT from polydomain to monodomain with decreasing thickness. Charges within the LSMO layer drawn to the interface to screen the PZT surface charge should theoretically vary with PZT domain structures, and would lead to a PZT thickness dependent Mn valence. This ferroelectric modulation of Mn valence charge was confirmed by measuring Mn valence in locally poled PZT. These discoveries agree with a charge modulated interfacial ME coupling mechanism. This thickness dependence study additionally indicates that thin PZT is preferred to avoid in-plane ferroelectric domain orientations and to maximize the coupling effect with LSMO. In order to understand the relation of the magnetic domains to the ferroelectric domains on the microscopic scale appropriate for devices, linear and circular dichroic images were taken by PEEM at both the Mn and Ti absorption L-edges. At these interfaces, uncompensated spins were first seen in images taken with circularly polarized x-rays at the Ti absorption L-edge. These spins preferred to orient perpendicular to the LSMO ferromagnetic direction, consistent with magnetic biquadratic coupling.

Transition metal oxides form a large class of compounds that exhibit a very rich and diverse physics. This thesis involves a first-principles computational study of some transition metal oxides and their different physical properties. We study the emergence of ferroelectricity in Sr-Ti-O layered perovskites with strain, the microscopic mechanism of magnetodielectric coupling in EuTiO3 , and the structural trends of early transition metal dioxides with the Rutile structure. We explain and support the results of our first-principles calculations with physical models for each system.

First principles calculations provide a powerful tool for sorting out the interplay of chemical composition and structure with the physical properties of materials. In this dissertation, I discuss the physical properties and their microscopic basis within this framework for following illustrative examples. (i) The Zintl phase hydrides, where I find H is anionic and the formation of covalent sp2 bonds in the Al/Ga/Al-Si planes, which is a highly unusual bonding configuration for these elements. (ii) PbTe, which shows strong coupling between the longitudinal acoustic and transverse optic modes that may explain its low thermal conductivity. (iii) The double perovskites BiPbZnNbO6 and BiSrZnNbO6, where introducing size disorder at A-site prevents the BO6 octahedra from tiling and enhances the polar behavior. (iv) FeSe, which shares the salient electronic and magnetic features of other Fe superconductors and cannot be described as a conventional electron phonon superconductor. (v) NbFe2, which is near a magnetic quantum critical point and shows strong competition between various magnetic orderings that may explain its unusual non-Fermi liquid behavior at very low temperatures. (vi) The nickel analogues of Fe superconductors LaNiPO and BaNi2As2, where I show that superconductivity is of conventional electron-phonon type in contrast to the Fe-based superconductors. (vii) Noncentrosymmetric LaNiC2, which I find is a conventional electron-phonon superconductor with intermediate coupling.

Ferroelectric and piezoelectric ceramic reinforced metal matrix composites are new materials being explored for vibration damping purposes. The high damping ability of ferroelectric and piezoelectric ceramics such as barium titanate (BaTiO3) and lead zirconate titanate (PZT) is due to the anelastic response of ferroelastic domain walls to applied external stress. In piezoelectric ceramics, vibration energy can also be dissipated through the direct piezoelectric effect if the appropriate electric circuit is connected across the ceramic. In this work we have examined the vibration damping behavior of BaTiO3, nickel-barium titanate (Ni-BaTiO3) composites and nickel-lead zirconate titanate (Ni-PZT) composites. BaTiO3 ceramics were fabricated by a combination of uniaxial pressing and cold isostatic pressing followed by sintering in air. Low frequency (0.1Hz-10Hz) damping capacity of BaTiO3, tanδ has been measured in three-point bend configuration on a dynamic mechanical analyzer. Tanδ has been found to increase with temperature up to the Curie temperature (Tc) of BaTiO3, after which there was a drop in damping capacity values due to the disappearance of ferroelectric domains above Tc. Furthermore within the frequency range tested, tanδ has been found to decrease with increasing vibration frequency. We also observed that tanδ decays with the number of vibration cycles (N). The decrease in tanδ with N, however, is fully recovered if BaTiO3 is heated above the Tc. Ni-BaTiO3 composite composed of a layer of BaTiO3 ceramic sandwiched between two layers of Ni were fabricated using a combination of electroless plating and electroforming. The damping behavior of the composite was analyzed in terms of the damping mechanisms below Tc and the damping mechanisms above Tc of BaTiO3. Below Tc, vibration damping ability of the composite was highly influenced by ferroelastic damping in the BaTiO3 component. Above the Curie temperature, the damping capacity was influence more by the inherent damping mechanisms in the nickel matrix. The damping mechanisms in Ni-PZT composites were evaluated at a low vibration frequency of 1Hz. In these composites we identified ferroelastic domain wall motion as the main damping mechanism active below the Tc of PZT. Using a poled PZT ceramic enhanced the damping capacity of the composite because of favorable ferroelastic domain orientation in the direction of applied stress. Based on our experimental results, we found no evidence of a direct piezoelectric damping mechanism in the Ni-PZT composites.

The integration of sensors, actuators, and control systems is an ongoing process in a wide range of applications covering automotive, medical, military, and consumer electronic markets. Four major families of ceramic and metallic actuators are under development: piezoelectrics, electrostrictors, magnetostrictors, and shape-memory alloys. All of these materials undergo at least two phase transformations with coupled thermodynamic order parameters. These transformations lead to complex domain wall behaviors, which are driven by electric fields (ferroelectrics), magnetic fields (ferromagnetics), or mechanical stress (ferroelastics) as they transform from nonferroic to ferroic states, contributing to the sensing and actuating capabilities. This research focuses on two multiferroic crystals, Pb(Mg1/3Nb2/3)O3-PbTiO3 and Fe-Ga, which are characterized by the co-existence and coupling of ferroelectric polarization and ferroelastic strain, or ferro-magnetization and ferroelastic strain. These materials break the conventional boundary between piezoelectric and electrostrictors, or magnetostrictors and shape-memory alloys. Upon applying field or in a poled condition, they yield not only a large strain but also a large strain over field ratio, which is desired and much benefits for advanced actuator and sensor applications. In this thesis, particular attention has been given to understand the structure-property relationships of these two types of materials from atomic to the nano/macro scale. X-ray and neutron diffraction were used to obtain the lattice structure and phase transformation characteristics. Piezoresponse and magnetic force microscopy were performed to establish the dependence of domain configurations on composition, thermal history and applied fields. It has been found that polar nano regions (PNRs) make significant contributions to the enhanced electromechanical properties of PMN-x%PT crystals via assisting intermediate phase transformation. With increasing PT concentration, an evolution of PNRï  PND (polar nano domains)-> micron-domains-> macro-domains was found. In addition, a domain hierarchy was observed for the compositions near a morphotropic phase boundary (MPB) on various length scales ranging from nanometer to millimeter. The existence of a domain hierarchy down to the nm scale fulfills the requirement of low domain wall energy, which is necessary for polarization rotation. Thus, upon applying an E-field along direction(s) in a composition near the MPB, low symmetry phase transitions (monoclinic or orthorhombic) can easily be induced. For PMN-30%PT, a complete E-T (electric field vs temperature) diagram has been established. As for Fe-x at.% Ga alloys, short-range Ga-pairs serve as both magnetic and magnetoelastic defects, coupling magnetic domains with bulk elastic strain, and contributing to enhanced magnetostriction. Such short-range ordering was evidenced by a clear 2theta peak broadening on neutron scattering profiles near A2-DO3 phase boundary. In addition, a strong degree of preferred [100] orientation was found in the magnetic domains of Fe-12 at.%Ga and Fe-20 at.%Ga alloys with the A2 or A2+DO3 structures, which clearly indicates a deviation from cubic symmetry; however, no domain alignment was found in Fe-25 at.%Ga with the DO3 structure. Furthermore, an increasing degree of domain fluctuations was found during magnetization rotation, which may be related to short-range Ga-pairs cluster with a large local anisotropy constant, due to a lower-symmetry structure.

Ferroelectric-reinforced metal matrix composites (FR-MMCs) show promise as high damping materials for structural applications. Most structural materials are valued based on their stiffness and strength; however, stiff materials typically have limited inherent ability to dampen mechanical or acoustic vibrations. The addition of ferroelectric ceramic particles may also augment the strength of the matrix, creating a multifunctional composite. In this work, the damping behavior of FR-MMCs created by the addition of barium titanate (BaTiO3) discontinuous reinforcement in a bearing bronze (Cu-10w%Sn) matrix has been studied. It has been shown that even when combined with other traditional composite mechanisms, added damping ability has been achieved due to the ferroelectric nature of the reinforcement. FR-MMCs currently represent a material system capable of exhibiting increased damping ability, as compared to the structural metal matrix alone.

Metal matrix composites offer unique opportunities for achieving multi-functionality in materials. In an attempt to investigate the possibility of enhancing damping characteristics of structural metals, copper was reinforced with tetragonal ferroelectric BaTiO3 particulates (Cu-BaTiO3 composites) using powder metallurgy techniques. The effect of particulate size and three processing conditions, sintering atmosphere, cooling rate and, uniaxial compaction pressure on the tetragonality and hence the ferroelectric properties of barium titanate powder were investigated using differential scanning calorimetry (DSC) and x-ray diffraction (XRD). The results show that sintering atmosphere and cooling rates have little effect on the tetragonality of barium titanate powder. Tetragonality of barium titanate powder decreased gradually with decreasing particle size. The decrease in tetragonality with decreasing particle size, however, was only severe in the very fine powders. Although no direct relationship was found between uniaxial compaction pressure and tetragonality, uniaxial pressure may also decrease the tetragonality of barium titanate. Three Cu-BaTiO3 composites, D1, D2 and D3 reinforced with 40vol% barium titanate particles of average sizes 209μm, 66μm and 2μm were respectively fabricated. The retention of the ferroelectric tetragonal phase of barium titanate after composite processing was confirmed by DSC. Composite microstructures observed using optical and scanning electron microscopy revealed uniform dispersions of barium titanate particles in D1 and D2. In D3, the barium titanate formed a chain-like structure because of extensive agglomeration of the fine reinforcement particles. Damping characteristics of the composites were evaluated between 25oC and 165oC at a frequency of 1Hz using dynamic mechanical analysis (DMA). The relative damping capacities (tanδ) in the composites were higher than the unreinforced metal. The damping capacity of composites D1 and D2 was also found to be dependent on temperature. Damping capacity was high from room temperature up to the Curie point of barium titanate, after which there was a slight drop in damping values probably due to a loss in ferroelectric properties. The small drop in damping values recorded in excess of the Curie temperature is an indication that ferroelectricity contributes little to the overall damping capacity of the Cu-BaTiO3 composites. This results from either a reduced ferroelectric damping in barium titanate particles or, poor stress transfer from matrix to reinforcement because of the weak and porous copper-barium titanate interface.