Correlated Functional Oxides

Iron oxides, which are also called ferrites, have been known to humans since ancient times and have been the subject of intensive research activity from fundamental as well as practical perspectives for a long time. One of the most advantageous properties of iron oxides is that they are chemically stable and nontoxic. In addition, Fe and O are earth-abundant elements [high Clarke numbers for Fe (4.7) and O (49.5)]. These features mean that the ferrites are suitable for applications in low cost, environmentally-friendly electronics. In this chapter, we examine some of the functional iron oxides and their heterostructures. First, we focus on the growth of FeO (wüstite) epitaxial thin films, and their properties as p-type transparent semiconductors will be discussed. Next, we will consider α-Fe₂O₃ (hematite), which is well known as the main component of red rust. It is demonstrated that band engineering and control of the crystal growth direction of α-Fe₂O₃ are useful to enhance its photoelectrochemical properties for high efficiency water splitting using sunlight. The third topic is Fe₃O₄ (magnetite), which is known to be a ferromagnetic oxide semiconductor. The control of the carrier type in Fe₃O₄ and its possible application to spintronic devices will be discussed. Finally, we will focus on the spin-fluctuation system in iron oxides. The long-term potentiation with the photomemory effect is observed in a Si-substituted garnet ferrite with high temperature spin-glass-like properties, which mimics the pre- and post-synaptic potentials of biological systems.

The properties of a ferroelectric, (001)-oriented, thin film clamped to a substrate are investigated analytically and numerically. The emphasis is on the tetragonal, polydomain, ferroelectric phase, using a three domain structure, as is observed experimentally, instead of the two-domain structure used in earlier literature. The previously used, very restrictive set of boundary conditions, arising from the domain walls, is relaxed, creating more modes for energy relaxation. It is argued that this approach gives a more realistic description of the clamped ferroelectric film. It is shown that for the ferroelectric oxides PbZr_1−x Ti _x O_3, the tetragonal, polydomain phase is present over a wide range of substrate induced strains for \( x \ge 0.5 \), corresponding to the tetragonal side of the bulk phase diagram. A polydomain, rhombohedral phase is present for \( x < 0.5 \), at the bulk rhombohedral side. Phase-temperature diagrams, and ferroelectric, dielectric, and piezoelectric properties as well as lattice parameters are calculated as function of substrate induced strain and applied field. The analytical formulation allows the decomposition of the numerically obtained values of these properties into three different causes: domain wall motion, field induced elastic effects, and piezoelectric effects. It is found that domain wall motion and polarization rotation of the in-plane oriented domains under an applied field contribute most to the properties, while the out-of-plane oriented domains hardly contribute.

Advances in atomically controlled synthesis of complex oxide heterostructures made possible to utilize the so-called strain engineering which enabled stabilization of new and unique structures at the oxide–oxide interfaces and in ultrathin films. Strain accommodation across the heteroepitaxial oxide interface is driven by different lattice parameters as well as by distinct crystallographic symmetry of the film and the substrate. Using advanced high-resolution X-ray diffraction (HR-XRD), we identified structural distortions at the heteroepitaxial interfaces and in ultrathin epitaxial perovskite oxide films that are induced by two distinct phenomena related to strain-octahedral coupling: lattice mismatch and interface symmetry mismatch. We show that unit cell distortions at the heteroepitaxial oxide interfaces significantly differ from structural distortions away from the interface region observed in thick coherent epitaxial oxide films. HR-XRD results reveal a formation of the novel structures at the interfaces between crystallographically dissimilar perovskite oxides. These structures are stabilized due to a unique coupling between octahedral rotations/deformations induced by symmetry mismatch and octahedral rotations due to lattice mismatch. The combination of crystallographic symmetry mismatch at the interface with the lattice mismatch offers new routes for strain engineering in functional complex oxide heterostructures that enables emergent physical phenomena and offer potential for future devices.

Superconducting samples of MgB₂ prepared by ex situ spark plasma sintering were characterized by magnetic measurements emphasizing functional characteristics such as the critical current density J _c, the irreversibility field H _irr or the product J _c(0) · µ₀ · H _irr, and the pinning-force-related parameters extracted within the universal scaling law and the percolation-based theory. Additions introduced into MgB₂ were classified as following: approximately chemically inert (type 1: h-BN, c-BN, and graphene), reactive with formation of M_yB_z (type 2: RE₂O₃ with RE being a rare earth element such as Ho, La, or Eu) or Mg_uM_v (type 3: Sb, Sb₂O₃, Bi, Bi₂O₃, Te, TeO₂, Ge, and GeO₂), and additives which are source of carbon substituting for boron in the crystal lattice of MgB₂ (type 4: fullerene (F), F + c-BN, SiC + Te, Ge₂H₁₀C₆O₇, and B₄C). Each group of additives show specific features, but within each group there are differences. When considering the influence of the additive of types 1–3, one has to pay attention also to substitutional x-carbon level which shows a strong influence on the functional and on the pinning-force-related parameters. A general trend is that at low x and high temperatures (>~15 K), samples are in the point pinning region and contribution of the grain boundary pinning is increasing when the additive amount is higher and the temperature is lower. There are also exceptions and within the general trend there are notable differences among the samples. From a practical point of view, additives such as c-BN, Te, Ge₂H₁₀C₆O₇, or B₄C are shown to increase high magnetic field functional characteristics such as J _c and H _irr, while suppression of J _c at low magnetic fields is minimized.

This chapter provides IR spectral range. The chapter provides IR theoretical importance and its application to analyze the oxide materials. In the theory, energy levels in molecules, different vibrational modes in molecules, and overtones and harmonics are discussed. Different phase sample preparation procedure is described in brief. Various forms of IR spectra and where they employed to analyze the compounds are given. The applications are mainly concerned to nanoferrites, and molecules and minerals. These applications are used to identify the structure, hydroxyls, carbonate ions, silicates, oxides and hydroxides, sulfides, and sulfates. Special attention is given in thin film metrology how IR spectra will be useful to analyze the structure of the film. Effort is made in the usage of IR in environmental studies.

Three-dimensional (3D) periodic nanopillar/nanodot structures embedded in a matrix of another material have considerable potential in devices exploiting spin/electronic couplings generated at new constituted lateral interfaces between two different phases. To fabricate periodic nanocomposite oxide films, a self-assembling growth technique from a composition-adjusted single target using pulsed laser deposition is promising. This chapter describes the fundamental growth mechanism of self-assembly synthesis and demonstrates the fabrication technique in preparing nanocomposite thin films composed of a spinel-type magnetic semiconductor (Fe,Zn)₃O₄ and a perovskite-type ferroelectric BiFeO₃. As an advanced fabrication technique to obtain precise periodic nanocomposite structures, we furthermore introduce a 3D nano-seeding assembly technique. This technique resolves longstanding issues of precise positioning, size alignment, and configuration inversion of materials.

The role of insulating layer is critical in electronic devices and hence preparing a reliable insulating substrate and insulating thin films is a starting point to fabricate any types of devices: Electron distribution engineering is entirely thanks to “insulating” character of insulators. In this chapter, we show that various properties other than the insulating character can be tuned in insulating oxide thin films, giving a significant controllability in designing surface and interface properties. In the first part of this chapter we show that the work function of oxide heterostructures is largely modulated using various aspects of insulating thin films. One of the properties characteristic of insulating films is “polarity,” which has increasingly been attracting attention. In the last part of this chapter we address growth and characterization of MgO(111) film, which is a representative polar oxide system.

Here we describe our recent results as to nanoscale resistive switching phenomena by utilizing single-crystalline metal oxide nanowires. The nanowires are grown via vapor–liquid–solid method. A single nanowire device was fabricated by integrating with top-down lithography techniques. It was found that the use of planer-type nanowire ReRAM devices allows us to examine not only the intrinsic nanoscale resistive switching properties, which have been buried in conventional capacitor devices, but also for designing nanoscale resistive memory devices.

Pulsed laser deposition (PLD) is one of the most powerful techniques to deposit thin films of multielemental materials such as electronic functional oxides. The PLD is a quite simple and convenient technique, so it is easy to prepare a variety of thin films. However, the physics beyond process has not been fully understood. The comprehension of PLD physics is expected to be fundamental to lead the improvement of thin films quality. This chapter investigates the physical processes of PLD, describing unique photochemical reaction during laser ablation, then discusses how conditions of the laser affect the ablation process for the very simple case of the ablation of alkaline earth metals. In this part, a unique photochemical process is shown. In the latter part of the chapter, it is introduced a technique based on in situ reflection of high-energy electron diffraction, to monitor the PLD process in real time. We consider that the studies reported in this chapter will be a first step enabling the PLD fabricating “tailored” oxide heterostructures and playing a key role in new physics.