Session D6: Focus Session: Domain Walls, Surfaces, Interfaces

Universal intrinsic origin for ferroelectric domain wall motion

The existence of domain walls in ferroelectric materials can have a profound influence on the properties of ferroelectrics [1]. We explored the dynamics of the 90$^\circ$ domain walls in PbTiO$_3$ with molecular dynamics simulations~[2] under a wide range of temperatures and electric fields. We found an intrinsic ``creep-depinning'' transition for the temperature- and field-dependence of the wall velocity, resulted from the nucleation-and-growth mechanism. By mapping non-180$^\circ$ domain walls to a 180$^\circ$ domain wall, we proposed an analytical model that is able to quantify the dynamics of all types of domain walls in various ferroelectrics, enabling rapid estimation of finite-temperature coercive fields with first-principles inputs. This work offers a unified picture for domain wall motion and an efficient framework for computational optimization of ferroelectrics. \\[4pt] [1] Seidel, J., J. Phys. Chem. Lett. {\bf3}, 2905 (2012). \\[0pt] [2] S. Liu, I. Grinberg, H. Takenaka, and A. M. Rappe, Phys. Rev. B {\bf 88}, 104102 (2013).   

Anomalous Dielectric Loss at Ferroelectric Domain Walls Revealed by Microwave Impedance Microscopy

Domain walls (DWs) in multiferroic materials, within which the ferroic order parameter changes its orientation, may possess emergent properties that are absent in the bulk domains. Combining the standard piezo-force microscopy (PFM), conductive atomic-force microscopy (C-AFM), and a novel microwave impedance microscopy (MIM) technique, we observed strong dielectric loss at the domain walls and vortex cores on the (001) charge neutral surface of hexagonal manganite YMnO$_{\mathrm{3}}$. The DW contrast was detected for a broad frequency range between 100MHz and 3GHz. The equivalent DW conductivity inferred from the MIM signals is estimated to be five orders of magnitude higher than that of the bulk YMnO$_{\mathrm{3}}$, which cannot be explained within the existing theoretical framework. By applying a DC bias on the MIM probe, we have also observed the transition from DW contrast to domain contrast in the impedance images. The MIM technique provides a unique opportunity to probe the nanoscale electronic anomalies in various topological defects, which will be crucial for future device applications of multiferroics.   

All-Epitaxial Ferroelectric Tunnel Junctions with Ultrathin BaTiO3

Ferroelectric tunnel junctions (FTJs) are a promising route toward the development of high density, non volatile memories with non-destructive readout [1]. The principle of operation is polarization-dependent tunneling electroresistance (TER). The direction of polarization in the ferroelectric layer defines high and low resistance states. So far, the most impressive results regarding TER ON/OFF ratios have been either without a top electrode, or using a top electrode of a non-oxide metal. However, defects in the ferroelectric [2] or interfacial layer can reduce performance. To overcome these limitations, we have fabricated fully-strained epitaxial FTJs using perovskite oxides for all layers: La$_{\mathrm{0.67}}$Sr$_{\mathrm{0.33}}$MnO$_{\mathrm{3}}$/BaTiO$_{\mathrm{3}}$/SrRuO$_{\mathrm{3}}$//SrTiO$_{\mathrm{3}}$. The heterostructures are grown by pulsed laser deposition and show high structural quality, sharp interfaces, and very smooth surfaces. Top electrodes are patterned using e-beam lithography. Piezoresponse force microscopy shows that ferroelectricity is maintained for a barrier thickness as low as 3 unit cells. We present our results on TER performance and the dependence of switching properties on the BaTiO$_{\mathrm{3}}$ thickness. [1] V. Garcia, et al. Nature 460, 81-84 (2009). [2] M. Dawber, K. M. Rabe, J.F. Scott, Rev. Mod. Phys. 77, 1083 (2005).   

Direct visualization of magnetoelectric domains in hexagonal manganites

Multiferroics are materials with coexisting magnetic and ferroelectric orders, where the cross-coupling between two ferroic orders can result in strong magnetoelectric effects [1-4]. Therefore, it is of both fundamental and technological interest to visualize cross-coupled magnetoelectric domains and domain walls in multiferroics. Recently, intriguing topological defects with six interlocked structural antiphase and ferroelectric domains merging into a vortex core were revealed in multiferroic hexagonal \textit{RE}MnO$_3$ (\textit{RE}=rare earths) [5, 6]. Many emergent phenomena, such as enhanced conduction and unusual piezoelectric response, were observed in charged ferroelectric domain walls protected by these topological defects [7-9]. More interestingly, alternating uncompensated magnetic moments were discovered at coupled structural antiphase and ferroelectric domain walls in hexagonal manganites using cryogenic magnetic force microscopy (MFM) [10], which demonstrates the cross-coupling between ferroelectric and magnetic orders. Using a newly-developed Magnetoelectric Force Microscopy (MeFM), which combines MFM with in-situ modulating high electric fields, we directly visualize the magnetoelectric response of the multiferroic domains in hexagonal manganites. The development of MeFM opens up explorations of emergent phenomena in multifunctional materials with multiple coupled orders [11, 12].\\[4pt] [1] N. A. Spaldin, and M. Fiebig, Science 309, 391 (2005).\\[0pt] [2] W. Eerenstein, N. D. Mathur, and J. F. Scott, Nature 442, 759 (2006).\\[0pt] [3] S-W. Cheong, and M. Mostovoy, Nat. Mater. 6, 13 (2007).\\[0pt] [4] N. A. Spaldin, S.-W. Cheong, and R. Ramesh, in Physics Today2010), pp. 38.\\[0pt] [5] T. Choi et al., Nature Materials 9, 253 (2010).\\[0pt] [6] T. Jungk et al., Appl. Phys. Lett. 97, 012904 (2010).\\[0pt] [7] E.B. Lochocki et al., Appl. Phys. Lett. 99, 232901 (2011).\\[0pt] [8] D. Meier et al., Nat. Mater. 11, 284 (2012).\\[0pt] [9] W. Wu et al., Phys. Rev. Lett. 108, 077203 (2012).\\[0pt] [10] Y. Geng et al., Nano Letters 12, 6055?6059 (2012).\\[0pt] [11] Y. Geng, and W. Wu, Rev. Sci. Instrum. 85, 053901 (2014).\\[0pt] [12] Y. Geng et al., Nat. Mater. 13, 163 (2014).   

Dynamical magnetoelectric effects associated with ferroelectric domain walls

Molecular dynamics simulations using a first-principles-derived effective Hamiltonian are conducted on lead zirconium titanate ultrathin films possessing nanoscale ferroelectric domains and being under GHz electric field. Pulses of magnetization are predicted to occur in this system, when sudden changes of morphology of these nano domains occur. A simple equation relating the magnetization and product between the electrical polarization and its time derivative is further derived from a simple model (via the relation between the magnetization and time derivative of the so-called electrical toroidal moment). This equation naturally explains our numerical findings, as well as previously observed magnetoelectric effects in {\it moving} ferroelectric domain walls/phase boundaries in ferroelectrics and magnetoelectrics.   

First-principles study of the effect of oxygen vacancies around the 180$^\circ$ ferroelectric domain walls of tetragonal PbTiO$_3$

People have extensively studied the dynamics of ferroelectric materials to apply them to nonvolatile memory devices. One of the issues in ferroelectric random access memory is the fatigue effect, which results from the presence of oxygen vacancy. Many cycles of polarization switching increase the density of oxygen vacancy around ferroelectric domain walls, and it makes the ferroelectric energy barrier higher to slow down the switching rate. In this presentation, we examine the domain dynamics around the 180$^\circ$ ferroelectric domain walls of tetragonal PbTiO$_3$ with and without the oxygen vacancy by using the first-principles calculations. We estimate the energy barriers of several possible reaction paths with the nudged elastic band method. Compared to the oxygen vacancy far away from domain walls, the oxygen vacancy around ferroelectric domain walls tends to be thermodynamically stable with lower energy barriers.Finally, we expect that by controling of oxygen vacancy density around the ferroelectric domain wall could be the solution for solving fatigue problem in ferroeletric materials.   

Pinning of Ferroelectric Domain Walls in Nanostructured CoFe$_{2}$O$_{4}$-BiFeO$_{3}$ Composite

Ferroelectric domain walls in BiFeO$_{3}$(BFO) thin films have attracted interest due to the observation of enhanced electronic transport at the domain walls in an otherwise insulating material. To understand the properties of domain walls in nanostructured thin films having matrix-pillar morphology, thin films of CoFe$_{2}$O$_{4}$ (CFO)-BFO were grown epitaxially by pulsed electron deposition on SrTiO$_{3}$ (STO) substrates. Piezoresponse force microscopy (PFM) measurements indicate that the vertically-oriented CFO nanopillars act as pinning sites for the in-plane domain walls. The pinning effect is most likely due to misfit dislocations at the matrix/pillar interface which have been identified in transmission electron microscopy images. The ability to produce ordered nanocomposites by directed self-assembly offers potential for more extensive investigation of domain wall behavior.   

Ferroelectric Striped Nanodomains in PbTiO$_{3}$/SrTiO$_{3}$ Superlattice Islands

The ferroelectric remnant polarization of isolated unscreened ferroelectric layers in ultrathin films or ferroelectric/dielectric superlattices spontaneously forms striped domains to minimize the total electrostatic energy. The X-ray scattering patterns of domains in a PbTiO$_{3}$/SrTiO$_{3}$ ferroelectric/dielectric superlattice indicate that the striped domains have a highly disordered arrangement with an average period of approximately 8 nm. Isolated superlattice islands have been fabricated with lengths of 2 $\mu$m and widths ranging from 150 nm to 800 nm using focused ion beam (FIB) patterning. Coherent x-ray scattering patterns show that the striped domain pattern is preserved in the nanopatterned islands. Neither the width nor the in-plane coherence length of the domains have shown any meaningful dependence on the width of the island. In addition, the correlation of the coherent scattering patterns indicates that the temporal fluctuation of the domain patterns is different from what have been previously observed in the unpatterned areas. We expect that such difference be due to the mechanical boundary introduced by the FIB etching.   

Electrical and Magnetic Properties of PbTiO$_{3}$/SrRuO$_{3}$ superlattices

Theoretical calculations on PbTiO$_{3}$/SrRuO$_{3}$ (PTO/SRO) superlattices suggest that the SRO layer should retain metallic character even when their thickness is only a single unit cell. They further suggest that when the SRO layer is 2 unit cells or thicker there is coupling between spin and the electric polarization. Here we report on the electrical transport properties of PTO/SRO superlattices, which were fabricated using off-axis RF magnetron sputtering. In the out of plane direction, the samples demonstrate ferroelectricity and tunneling current characteristics that confirm the metallicity of the SrRuO$_{3}$ layers. We also studied the impact of the compositionally broken inversion symmetry and magnetic field on the capacitance-voltage characteristic of our superlattices. Changes in the dielectric constant were induced by the application of magnetic field at low temperatures.   

Novel in-situ x-ray diffraction measurement of ferrroelectric superlattice properties during growth

Ferroelectric domains, surface termination, average lattice parameter and bilayer thickness were monitored by in-situ x-ray diffraction during the growth of BaTiO$_3$/SrTiO$_3$ (BTO/STO) superlattices by off-axis RF magnetron sputtering. A new x-ray diffraction technique was employed which makes effective use of the custom growth chamber, pilatus detector and synchrotron radiation available at beamline X21, NSLS, BNL. The technique allows for scan times substantially faster than the growth of a single layer of material, allowing continuous monitoring of multiple structural parameters as the film grows. The effect of electric boundary conditions was investigated by growing the same superlattice alternatively on STO substrates and 20nm SrRuO$_3$ (SRO) thin films grown on STO substrates. Besides the fundamental knowledge gained from these studies, being able to monitor the structural parameters of a growing ferroelectric superlattice at this level of detail provides numerous insights which can guide the growth of higher quality ferroelectric superlattices in general.   

Fluctuating defects in the incipient relaxor K$_{1-x}$Li$_x$TaO$_3$ ($x=0.02$)

We have measured the structural correlations associated with the apparent relaxor transition near 70 K in K$_{0.98}$Li$_{0.02}$TaO$_3$ (KLT(0.02)) with neutrons. No elastic diffuse scattering or soft mode anomaly is observed, a situation that diverges from that in other relaxors like PMN. The structural correlations in KLT(0.02) are dynamic at all temperatures with timescales of $\sim$ THz. The fluctuations are overdamped, non-propagating, spatially uncorrelated, and absent in the parent material KTaO$_3$. The temperature dependence correlates with the dielectric response, implying that the fluctuations are associated with local, ferroelectric regions induced by the Li-doping. The ferroelectric transition induced by the introduction of sufficient Li cations is thus characterized by quasistatic fluctuations, which is a stark contrast to the soft-harmonic-mode-driven transition observed in perovskite ferroelectrics like PbTiO$_3$. The glass-like structural correlations in KLT(0.02) are much faster than those in random-field, lead-based relaxors, which occur on the $\sim$ GHz timescale, and they are better correlated spatially. Our results support the view that random fields give rise to the relaxor phenomena, and that the glassy dynamics observed here represent a nascent response.   

Polar State in Freestanding Strontium Titanate Nanoparticles

Monodispersed strontium titanate nanoparticles were prepared and studied in detail. It is found that $\sim$ 10 nm as-prepared stoichiometric nanoparticles are in a polar structural state (with possibly ferroelectric properties) over a broad temperature range. A tetragonal structure, with possible reduction of the electronic hybridization is found as the particle size is reduced. In the 10 nm particles, no change in the local Ti-off centering is seen between 20 and 300 K. This work is supported by DOE Grant DE-FG02-07ER46402.   

Effect of C6$+$ Ion Irradiation on structural and electrical properties of Yb and Eu doped Bi1.5Zn0.92Nb1.5O6.92 pyrochlores

Pyrochlore general formula of A$_{2}$B$_{2}$X$_{7}$ where A and B are cations and X is an anion Pyrochlore compounds exhibit semiconductor, metallic or ionic conduction properties, depending on the doping, compositions/ substituting variety of cations and oxygen partial pressure. Ion beam irradiation can induce the structural disordering by mixing the cation and anion sublattices, therefore we aim to inevestigate effects of irradiation in pyrochlore compounds. In this study, Eu and Yb-doped Bi$_{1.5}$Zn$_{0.92}$Nb$_{1.5}$O$_{6.92}$ (Eu-BZN, Yb-BZN) Doping effect and single phase formation of Eu-BZN, Yb-BZN was characterized by X-ray diffraction technique (XRD). Radiation-induced effect of 85 MeV C$^{6+}$ ions on Eu-BZN, Yb-BZN was studied by XRD, scanning electron microscopy (SEM) and temperature dependent dielectric measurements at different fluences. XRD results revealed that the ion beam-induced structural amorphization processes in Eu-BZN and Yb-BZN structures. Our results suggested that the ion beam irradiation induced the significant change in the temprature depndent dielectric properties of Eu-BZN and Yb-BZN pyrochlores due to the increased oxygen vacancies as a result of cation and anion disordering.