Session V32: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides - Interfaces, Layered Materials, and Growth

Electric-field control of magnetization in Co$_{40}$Fe$_{40}$B$_{20 }$/ Pb(Mg$_{1/3}$Nb$_{2/3})_{0.7}$Ti$_{0.3}$O$_{3}$ structure at room temperature

Electric-field control of magnetization is important for new generation information storage technology with high integration density and low power consumption. A lot of work has been carried out on electric-field control of magnetization in artificial ferromagnetic-ferroelectric (FM-FE) two-phase systems via the piezo-strain effect. Beside strain, electric/elastic domains and phase structure also play important roles in the electric-field control of magnetization, especially in the case of amorphous FM film without magnetocrystalline anisotropy. We report a large magnetoelectric effect in a Co$_{40}$Fe$_{40}$B$_{20}$/Pb(Mg$_{1/3}$Nb$_{2/3})_{0.7}$Ti$_{0.3}$O$_{3}$ structure at room temperature. Investigations on the ferroelectric domains, phase structures, magnetic domains and strain with \textit{in situ} electric fields reveal a new mechanism for electric-field control of magnetization. This work provides a new way to realize large magnetoelectric coupling and is significant for applications, especially in the field of CoFeB-based spin valves to achieve electric-controlled magnetic random access memories.   

Coherently Coupled ZnO and VO$_{2}$ Interface studied by Photoluminescence and electrical transport across a phase transition

In this work we report a study of a coherently coupled interface consisting of a ZnO layer grown on top of an oriented VO$_{2}$ layer on sapphire by photoluminescence and electrical transport measurements across the VO$_{2}$ metal insulator phase transition (MIT). The photoluminescence of the ZnO layer showed a broad hysteresis induced by the phase transition of VO$_{2}$ while the width of the electrical hysteresis was narrow and unaffected by the over layer. The enhanced width of the PL hysteresis was due to the formation of defects during the MIT as evidenced by a broad hysteresis in the opposite direction to that of the band edge PL in the defect luminescense. Unlike VO$_{2}$ the defects in ZnO did not fully recover across the phase transition. From the defect luminescence data, oxygen interstitials were found to be the predominant defects in ZnO mediated by the strain from the VO$_{2}$ phase transition. Such coherently coupled interfaces could be of use in characterizing the stability of a variety of interfaces and also for novel device application.   

Ferroelectricity and compositional inversion symmetry breaking in PbTiO$_{3}$/SrRuO$_{3}$ superlattices

Most work to date on artificially layered ferroelectric superlattices has utilized the insulating titanium perovskite oxides (e.g. PbTiO$_{3}$, BaTiO$_{3}$, CaTiO$_{3}$ and SrTiO$_{3}$) as ``building blocks,'' from which a layered structure is assembled by sequential deposition. However, the need for new functionalities, particularly related to magnetism, demands that we expand this set. The much-studied compound SrRuO$_{3}$ provides the proof of concept that metallic magnetic oxides can transform into thin-film dielectric components in certain heterostructures, in this case PbTiO$_{3}$/SrRuO$_{3}$ superlattices. Our high quality epitaxial PbTiO$_{3}$/SrRuO$_{3}$ superlattices, grown by RF magnetron sputtering on SrTiO$_{3}$ substrates (with SrRuO$_{3}$ bottom electrodes) show both ferroelectricity, and, as they have both A and B site variation, compositional breaking of inversion symmetry. We will present experimental measurements made on this system using x-ray diffraction, transmission electron microscopy, and electrical characterization, which together with first principles density functional theory simulations, demonstrate how, as the constituent layer thicknesses are varied, the metallicity of the superlattice changes and polarization evolves.   

Interface control of emergent ferroic order in Ruddlesden-Popper Sr$_{n+1}$Ti$_n$O$_{3n+1}$

We have discovered from first-principles an unusual polar state in the low n Sr$_{n+1}$Ti$_n$O$_{3n+1}$ Ruddlesden-Popper (RP) layered perovskites in which ferroelectricity is nearly degenerate with antiferroelectricity, a relatively rare form of ferroic order. We show that epitaxial strain plays a key role in tuning the ``perpendicular coherence length'' of the ferroelectric mode, and does not induce ferroelectricity in these low dimensional RP materials as is well known to occur in SrTiO$_3$. These systems present an opportunity to manipulate the coherence length of a ferroic distortion in a controlled way, without disorder or a free surface. [T. Birol, N. A. Benedek, C. J. Fennie, Physical Review Letters, in press]   

Use of dimensionality to enhance tunable microwave dielectrics

The miniaturization and integration of frequency-agile microwave circuits---\textit{tunable} filters, resonators, phase shifters and more---with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at GHz frequencies can be tuned by applying a quasi-static electric field. Appropriate systems, e.g., Ba$_{x}$Sr$_{1-x}$TiO$_{3}$, have a paraelectric-to-ferroelectric transition just below ambient temperature, providing high tunability. Unfortunately such films suffer significant losses arising from defects. Recognizing that progress is stymied by dielectric loss, we start with a system with exceptionally low loss---Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$ phases---where in-plane crystallographic shear (SrO)$_{2}$ faults provide an alternative to point defects for accommodating non-stoichiometry. In this talk we will establish both experimentally and theoretically the emergence of a ferroelectric and highly tunable ground state in biaxially strained Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$ phases with $n\ge $3 at frequencies up to 40~GHz. With increasing $n$ the (SrO)$_{2}$ faults are separated further than the ferroelectric coherence length perpendicular to the in-plane polarization, enabling tunability with a figure of merit at room temperature that rivals all known tunable microwave dielectrics.   

Ferroelectric-Antiferroelectric Phase Control: Interplay Between Octahedral Tilt, Polarization, and Chemistry at BFO-LSMO Interfaces

Atomically-defined interfaces between complex oxides offer a paradigm of novel electronic and coupled functionalities. Here, we report the stabilization of the ferroelectric and antiferroelectric phases at the BFO-LSMO interface though termination control and reveal associated atomic-scale mechanisms with the help of aberration-corrected scanning transmission electron microscopy combined with Electron Energy Loss Spectroscopy. The BFO thin film grown on MnO$_{2}$-terminated surface of LSMO exhibits stabilized ferroelectric phase at the interface. The interfacial and bulk polarization are antiparallel, giving rise to head to head ferroelectric domain wall mostly parallel to the interface. In comparison, the film grown on (Sr,La)O-terminated surface of LSMO exhibits antiferroelectric phase in the vicinity of the interface, with associated ferroelectric-antiferroelectric domain wall in the bulk. Details of tilt and polarization behavior, as well as electronic properties at the interfaces including charged domain walls and FE-AFE walls, will be presented. These results imply that the structural parameters such as octahedral tilt and ferroelectricity in BFO can be directly controlled by modifying the interface structure.   

Interplay of couplings between antiferrodistortive, ferroelectric, and strain degrees of freedom in monodomain PbTiO$_{3}$/SrTiO$_{3}$ superlattices

We report first-principles calculations, within the density functional theory, on the coupling between epitaxial strain, polarization, and oxygen octahedra rotations in monodomain (PbTiO$_{3}$)$_{n}$/(SrTiO$_{3}$)$_{n}$ superlattices. We show how the interplay between (i) the epitaxial strain and (ii) the electrostatic conditions at the interfaces can be used to control the orientation of the main axis of the system -- defined by the direction of the polarization or the rotation axis of the oxygen octahedra. The electrostatic constrains at the interface facilitate the rotation of the polarization and, as a consequence, we predict large piezoelectric responses at epitaxial strains smaller than those that would be required considering only strain effects. In addition, ferroelectric (FE) and antiferrodistortive (AFD) modes are strongly coupled, with different rotation angles in the TiO$_{6}$ octahedra as a function of the polarization direction. The magnitude of the rotations cannot be explained by the usual steric arguments alone, and a covalent model is proposed to account for the large polarization-tilting coupling. The energy gain due to the FE-AFD coupling decreases with the periodicity of the superlattice, becoming negligible for $n \ge 3$.   

Engineered polarization rotation in PbTiO$_{3}$/CaTiO$_{3}$ superlattices

Large piezoelectric responses, such as those seen in PbZr$_{x}$Ti$_{1-x}$O$_{3}$ in the vicinity of the compositional morphotropic phase boundary, can occur when the direction of the polarization in a ferroelectric material can rotate. Here we show experimentally that a similar enhancement of the piezoelectric response can be achieved in artificially layered epitaxial superlattices composed of alternating layers of PbTiO$_{3}$ and CaTiO$_{3}$ deposited on SrTiO$_{3}$ substrates by RF magnetron sputtering. The exceptional quality of our samples is demonstrated by x-ray diffraction and transmission electron microscopy. The structural and functional properties of the materials have been measured as a function of relative layer thickness. Electrically measured ferroelectric polarization and dielectric constants corroborate the enhancement of d$_{33}$ we have measured experimentally using piezoforce microscopy. The structural changes, which allow polarization rotation, have been directly measured using grazing incidence in-plane x-ray diffraction (at NSLS X21 and X22C). Finally, the as-grown domain structure has been imaged with piezoforce microscopy, further confirming polarization rotation and explaining the unusual switching dynamics observed in our electrical characterization.   

In situ studies on ferroelectric BaTiO$_{3}$ interface

Ferroelectric phase stability in ferroelectric films is critically dependent on the surface and interface phenomena, especially governed by electrostatic depolarization energy. Predictions for the minimum critical film thickness for ferroelectricity have continuously decreased down to few unit cells. We have examined surface/interface atomic structures of ultrathin BaTiO$_{3}$ (BTO) films grown on conductive SrRuO$_{3}$ (SRO) and Nb-doped SrTiO$_{3}$. The surface structure of BTO/SRO was refined using in-situ Low Energy Electron Diffraction (LEED) I-V, resulting to observation of polar distortion in ultrathin ($\ge $ 4 ML) BTO films. The in-situ Scanning Tunneling Microscopy (STM) has been performed prior and after BTO deposition on SRO. However, the unusual 2x2 reconstruction is observed for 1-2 ML BTO films and bare SRO by STM. The surface reconstruction of SRO bottom electrode is shown to affect the interface of films deposited subsequently which could be reflected in ultrathin film properties. The in-situ LEED I-V structural studies on 1-2 ML BTO interface have been performed without SRO layer, which kept ultrathin BTO films from the preclusion of reconstructed SRO films.   

Ellipsometric study of SrTiO$_{3}$ thin film grown Si(100)

Recently, a new method to grow SrTiO$_{3}$ thin ferroelectric film directly on Si(100) has been demonstrated by \textit{M. P. Warusawithana, et al}. We use ellipsometry to study the film and a model based on inhomogenous gap to oxide deficiency was made to interpret the data.   

Valence Fluctuation in UltrathinTi$_{1+x}$O$_{2}$ on Rutile TiO$_{2}$

The physical properties of metal oxides (TMO) can change drastically depending on the non-stoichiometry of oxygen, for which rutile TiO$_{2}$ single crystals were self-implanted with Ti to acquire a layers of $\sim $6 nm populated with extra Ti found in +2, +3, and +4 charge states as revealed by X-ray photoelectron spectroscopy (XPS). The formations of the TiO and Ti$_{2}$O$_{3}$ crystalline phases were verified by high-resolution transmission electron microscopy (HRTEM). Variable-temperature electrical resistivity measurement suggests the occurrence of charge ordering at low temperatures, where weak localization of the charges take hold, commonly observed in heavily doped semiconductors. The magneto-transport behaviors follow mixed scenario of band conduction and fixed range hopping, albeit with different energy scales and weighting factors demarcated by a transition temperature of MR sign change, a signature of valence fluctuation in which the ordered low-temperature phase would melt into the high-temperature disordered phase, or by condensation in reverse. This work was supported by the National Science Council, the Ministry of Education, Taiwan, the National Science Foundation, and the State of Texas through Texas Center for Superconductivity.   

Atomic Layer-by-Layer Growth of Homoepitaxial SrTiO$_{3}$ Films by Laser MBE

Two most effective techniques for oxides film growth are reactive MBE and laser MBE. With alternating monolayer growth, reactive MBE has shown the capacity of stoichiometry control and crystalline perfection in SrTiO$_{3}$ films on SrTiO$_{3}$, while most works of laser MBE, using compound targets, often shows non-stoichiometry and lattice defects. In order to control layer-by-layer growth to atomic level, we carried out laser MBE from separate oxide targets, for example, growing SrTiO$_{3}$ from SrO and TiO$_{2}$ targets, such that the SrO and TiO$_{2}$ layers were deposited alternatively one atomic layer at a time. X-ray diffraction spectra showed that the stoichiometric SrTiO$_{3}$ film peak overlapped with and was indistinguishable from the SrTiO$_{3 }$substrate peak, while the off stoichiometric SrTiO$_{3}$ films, either Sr rich or poor, showed lattice expansion. We conclude that laser MBE from separate oxide targets can achieve the same stoichiometry control as reactive MBE.   

Preparation of Atomically Flat SrTiO$_3$ Surfaces Using Non-acid Etching and Thermal Annealing Process

The growth of epitaxial thin films and heterostructures requires atomically flat surfaces of substrates. So far chemical etching processes by acidic etchant have been widely used for the surface preparation of oxide substrates. Here we show that atomically flat surfaces of single crystalline SrTiO$_{3}$ substrates of both the (100) and (111) orientations can be prepared using non-acidic etching and thermal annealing techniques. Atomic force microscopy confirms the evolution of the surface of the substrates from rough to a step-terrace structure. Scanning tunneling microscopy shows that SrO segregation on the surface is removed by our etching process. This new technique replaces the use of acidic oxide etchant as the primary method for etching SrTiO$_{3}$ substrates.   

Effect of Mg$^{2+}$ on the structure of amorphous CaCO$_3$ -- A molecular dynamics simulation

Molecular dynamics (MD) simulations of amorphous calcium carbonate (ACC) were carried out to investigate the effect of Mg$^{2+}$ ions on the structure of CaCO$_3$ crystal nucleus formed from ACC\@. Our systems contained 432 CaCO$_3$ with several concentrations of MgCO$_3$. In this study, our original ion model of Mg$^{2+}$ was developed and combined with Raiteri model of Ca$^{2+}$ and rigid CO$_3$$^{2-}$ [1]. The simulations indicated that the fraction of vaterite-like ion arrangement was much larger than those of calcite-like and aragonite-like ion arrangements in pure ACC\@. However, as the Mg$^{2+}$ concentration increased, the faction of vaterite-like ion arrangements decreased, which suggests that Mg$^{2+}$ ions play as inhibitors of vaterite nucleation. The result explains why calcite or aragonite is preferentially nucleated in the presence of Mg$^{2+}$, whereas vaterite is nucleated in the absence of them.[1] P. Raiteri, J.~D. Gale, D. Quigley, and P.~M. Rodger, J.~Phys.\ Chem.\ C 114, 5997 (2010).