Phase transitions and properties of ferroics in the form of single crystals, ceramics and thin films

Resume : The most frequent structural distortions in perovskite oxides involve rotations of the oxygen octahedra that form the backbone of the lattice. These tilting modes are present in the vast majority of compounds and control many of their key properties -- from magnetic interactions to elecronic conductivity --, which warrants the sustained interest of the community in them. In this talk I will summarize our recent first-principles works on rotational distortions in perovskites, covering topics that range from the most basic (e.g., the reasons why a particular tilting pattern combining in-phase and antiphase rotations is majoritarily preferred [1]) to the less obvious (e.g., the dual competing-colaborative nature of the interactions between tilting and polar modes [2] or pressure effects [3]). Further, I will describe our current efforts to understand the domain walls (DWs) associated to these rotational distortions. In particular, I will show how chemical substitution, of the kind observed experimentally in the DWs of TbMnO3 films [4], may allow us to obtain new (ferroelectric, magnetic, electronic) properties by design, suggesting a promising direction for wall functionalizion. Work done in collaboration with many colleagues, particularly H.J. Zhao and P. Chen (LIST) and the groups of H.J. Xiang (Fudan) and L. Bellaiche (Arkansas). Work at LIST funded by Luxembourg National Research Fund. [1] Energetics of oxygen-octahedra rotations in perovskite oxides from first principles, Peng Chen, Mathieu N. Grisolia, Hong Jian Zhao, Otto E. González-Vázquez, L. Bellaiche, Manuel Bibes, Bang-Gui Liu and Jorge Íñiguez, Physical Review B 97, 024113 (2018). [2] Cooperative couplings between octahedral rotations and ferroelectricity in perovskites, Teng Gu, Timothy Scarbrough, Yurong Yang, Jorge Íñiguez, L. Bellaiche and H.J. Xiang, Physical Review Letters 120, 197602 (2018). [3] Rules and mechanisms governing octahedral tilts in perovskites under pressure, H.J. Xiang, Mael Guennou, Jorge Íñiguez, Jens Kreisel and L. Bellaiche, Physical Review B 96, 054102 (2017). [4] Artificial chemical and magnetic structure at the domain walls of an epitaxial oxide, S. Farokhipoor, C. Magen, S. Venkatesan, J. Íñiguez, C.J.M. Daumont, D. Rubi, E. Snoeck, M. Mostovoy, C. de Graaf, A. Mueller, M. Doeblinger, C. Scheu and B. Noheda, Nature 515, 379 (2014).

Resume : The conductivity of domain walls has been a topic of intense interest in the past decades, and it has been demonstrated, for instance in BiFeO3 [1,2] or (Pb1-xLax)(Zr1-yTiy)O3 (PLZT) [3], or in peculiar ferroelectric textures such as vortices/90˚ domain wall junctions [4]. In particular, in neutral domain walls, that are often most stable than their charged counterparts for electrostatic reasons, the question of the origin of such conductivity (if any), is unclear. Several explanations have been put forward, such as the concentration of oxygen vacancies at domain walls [5–7]. To address some of these questions, we performed Density Functional Theory calculations to estimate the formation energy of single point defects in 180˚ and 90˚ domain walls in the classical ferroelectric PbTiO3 using the Local Density Approximation Functional. We show that, for any type of vacancies, in any charge state, it is always more favorable in the domain walls, confirming that defects tend to aggregate to domain walls when given the opportunity (and conversely, have the tendency to pin domain walls) [8]. Furthermore, the question of whether the domain wall is able to retain charge carriers (specifically holes in this work) under the form of small trapped hole (STH) polarons is discussed in the framework of an original LDA+U scheme [9,10]. It is shown that the self-trapping energy of small holes polarons is greatly reduced at the domain wall. Very recently, there has been a surge of interest about the dynamical conductivity of domain walls [11,12]. Using a first-principle based effective Hamiltonian [13] and Molecular Dynamics simulations [14], it is revealed that the ac conductivity associated to displacement currents is stronger in the domain wall than in the adjacent domains [15]. Local mapping of the dielectric susceptibility shows that new, low frequency, vibration modes associated with the domain wall appear compared to a monodomain material [15,16], and most likely contribute to the enhancement of the ac conductivity. Acknowledgements We acknowledge the support of National Computing Center for Higher Education (CINES) for granting access to its computing facility (project c2015097227), CNRS and CEA agencies and ONR Grants No. N00014-12-1-1034 and N00014-17-1-2818. CP and BD thank a public grant overseen by the French National Research Agency (ANR) as part of the ‘Investissements d’ Avenir’ program (reference: ANR-10-LABX-0035, Labex NanoSaclay). L.B. also thanks the DARPA Grant No. HR0011727183‐D18AP00010. S.P. appreciates support of RMES 3.1649.2017/4.6 and RFBR 18-52-00029 Bel_a. References [1] Farokhipoor, S. & Noheda, B. Conduction through 71° Domain Walls in BiFeO3 Thin Films. Phys. Rev. Lett. 107, 127601 (2011). [2] Seidel, J. et al. Conduction at domain walls in oxide multiferroics. Nat. Mater. 8, 229–234 (2009). [3] Guyonnet, J., Gaponenko, I., Gariglio, S. & Paruch, P. Conduction at Domain Walls in Insulating Pb(Zr0.2Ti0.8)O3 Thin Films. Adv. Mater. 23, 5377–5382 (2011). [4] Balke, N. et al. Enhanced electric conductivity at ferroelectric vortex cores in BiFeO3. Nat. Phys. 8, 81–88 (2011). [5] Farokhipoor, S. & Noheda, B. Local conductivity and the role of vacancies around twin walls of (001)−BiFeO3 thin films. J. Appl. Phys. 112, 052003 (2012). [6] Gaponenko, I., Tückmantel, P., Karthik, J., Martin, L. W. & Paruch, P. Towards reversible control of domain wall conduction in Pb(Zr0.2Ti0.8)O3 thin films. Appl. Phys. Lett. 106, 162902 (2015). [7] Stolichnov, I. et al. Persistent conductive footprints of 109° domain walls in bismuth ferrite films. Appl. Phys. Lett. 104, 132902 (2014). [8] Paillard, C., Geneste, G., Bellaiche, L. & Dkhil, B. Vacancies and holes in bulk and at 180° domain walls in lead titanate. J. Phys. Condens. Matter 29, 485707 (2017). [9] Erhart, P., Klein, A., Åberg, D. & Sadigh, B. Efficacy of the DFT+U formalism for modeling hole polarons in perovskite oxides. Phys. Rev. B 90, 035204 (2014). [10] Geneste, G., Amadon, B., Torrent, M. & Dezanneau, G. DFT+U study of self-trapping, trapping, and mobility of oxygen-type hole polarons in barium stannate. Phys. Rev. B 96, 134123 (2017). [11] Tselev, A. et al. Microwave a.c. conductivity of domain walls in ferroelectric thin films. Nat. Commun. 7, 11630 (2016). [12] Wu, X. et al. Low-energy structural dynamics of ferroelectric domain walls in hexagonal rare-earth manganites. Sci. Adv. 3, e1602371 (2017). [13] Prosandeev, S., Wang, D., Ren, W., Íñiguez, J. & Bellaiche, L. Novel Nanoscale Twinned Phases in Perovskite Oxides. Adv. Funct. Mater. 23, 234–240 (2013). [14] Wang, D., Weerasinghe, J. & Bellaiche, L. Atomistic Molecular Dynamic Simulations of Multiferroics. Phys. Rev. Lett. 109, 067203 (2012). [15] Prosandeev, S., Yang, Y., Paillard, C. & Bellaiche, L. Displacement Current in Domain Walls of Bismuth Ferrite. npj Comput. Mater. 4, 8 (2018). [16] Hlinka, J., Paściak, M., Körbel, S. & Marton, P. Terahertz-Range Polar Modes in Domain-Engineered BiFeO3. Phys. Rev. Lett. 119, 057604 (2017).

Resume : Ferroelectric Rashba semiconductors (FERSC), in which Rashba spin-splitting can be controlled and reversed by an electric field, have recently emerged as a very exciting new class of functional materials. However, full exploitation of the concept is still hampered by the lack of known robust ferroelectric compounds showing large Rashba spin splitting. Here, we rationalize the search of efficient FERSC within the family of perovskite oxides relying on first-principles calculations. We first highlight that the coexistence of large spontaneous polarisation and spin-orbit coupling is not sufficient to have good FERSC properties and we establish why simple ferroelectric oxide perovskites with transition metal at the B-site are typically not suitable candidates. Then, we show how this intrinsic limitation can be by-passed through interface engineering of the electronic band structure in layered perovskites and identify the ferroelectric Bi2WO6 Aurivillius phase as a promising FERSC compound. The role of distinct atomic distortions on the Rashba spin splitting is discussed. We further show that Bi2WO6 preserves its ferroelectric properties over substantial n-doping. This last feature also makes it attractive for other applications, which will be briefly discussed. Work done in collaboration with H. Djani, A.C. Garcia Castro, W.-Y. Tong, E. Bousquet, P. Barone and S. Picozzi and supported by the ARC project AIMED and M-ERA.NET project SIOX.

Resume : Conduction at domain-boundaries, ferroelectric/insulator like SrTiO3/LaAlO3 (STO/LAO), and free surface is a hot topic in physics of ferroics. Although, as predicted*, these conductions are often attributed to polarization discontinuity of spontaneous polarization Ps, the importance of the extrinsic origins as defects, impurities, and field-induced defects have been established, and emphasized by Szot and Roleder.** Indeed, these mechanisms exist in most of conduction phenomena. In STO/LAO the defects and intermixtures are unavoidable because of the growth at high temperature in vacuum. In the low-mobility and artificially-formed domain-boundaries, the pinning by defects and high-field-induced defects should exist, respectively; the domain boundaries are formed along the path of the high field. Moreover, most of the clean surfaces of metal oxides are prepared through the annealing in vacuum that forms defects such as oxygen vacancy. On the other hand, the Ps–induced conduction at domain boundaries and ferroelectric/insulator boundary was earlier predicted* to show its fundamental impacts on size effect, super-ferroelectricity and domains. Therefore, the rigorous examinations whether Ps is the dominant origin of these conductions are of primary importance. The intrinsicness of Ps–induced conduction in the view of sample preparation, measurement methods and conduction characteristics will be discussed together with their implications. * Watanabe,PRB57,789(1998);PRL 86,332(2001),**IMF Abst. 245 (2017).

Resume : Our lecture focuses on the important role of dislocations in the insulator-to-metal transition and for redox processes, which can be preferentially induced along of dislocations using chemical and electrical gradients for two models ternary oxides with perovskite structure (here SrTiO3 and BaTiO3) . It is surprising that independently of the growth techniques the density of dislocations in surface layers of both prototypical oxides is high (109/cm2 for epipolished surfaces and up to 1012/cm2 for the rough surface). The TEM and LCAFM measurements show that the dislocations create a network with the character of a hierarchical tree. The distribution of the dislocations in the plane of the surface is in principle inhomogeneous. A strong tendency for bundling and the creation of arrays or bands in the crystallographic < 100> and < 110> directions can be observed. The analysis of the dislocation cores using STEM techniques (such as EDX with atomic resolution, EELS) shows unequivocally that the cores possesses a different crystallographic structure, electronic structure and chemical composition relatively to the matrix. Because their Burgers vectors are per se invariant, the network of dislocations (with additional d1 electrons) causes an electrical short-circuit of the matrix. This behavior is confirmed by LCAFM measurements for the stoichiometric crystals, moreover a similar dominant role of dislocations in channeling of the current after thermal reduction of the crystals or during resistive switching can be observed. In our opinion the easy transformation of the chemical composition of the surface layer of both model oxides should be associated with the high concentration of extended defects in this region. Another important insight for the analysis of the physical properties in real oxide crystals (matrix + dislocations) is coming from the studies of the nucleation of dislocations via in situ STEM indentation, namely the dislocations can be simply nucleated under mechanical stimulus and can be easily moved at room temperature.

Resume : The creation of quasi-two-dimensional s electron gas at the interface and the ability to control such states by magnetic and electric fields is impossible without the use of new materials and without the development of new design interfaces. Unique properties of functional materials are achieved due to the effects associated with the complex composition of the interface structure. Such new materials include oxide heterointerfaces between two nonconducting oxides in which, owing to strong electronic correlations, unique transport properties are observed. A high-mobility electron gas was first observed in 2004 [1] at the interface of heterostructure LaAlO3 (LAO) and SrTiO3 (STO). Such heterointerfaces involving two insulating nonmagnetic oxides were comprehensively studied. In particular, it was found that the metallic phase (quasi-two-dimensional electron gas, 2DEG) is formed in the STO layers at the LAO/STO interface when the number of LAO layers is larger than three [2]. Such a system undergoes a transition to a superconducting state at temperatures below 300 mK [3]. We investigate the properties of 2DEG at the interface between ferroelectric oxide and insulating oxide in heterostructures, isostructural to BaTiO3/LaMnO3. The numerical simulations of the structural and electronic characteristics of the BaTiO3/LaMnO3 ferroelectric-antiferromagnet heterostructure have been performed. The temperature dependence of the electrical resistance has been studied for heterostructures formed by antiferromagnetic LaMnO3 single crystals of different orientations with epitaxial films of ferroelectric Ba0.8Sr0.2TiO3 (BSTO) deposited onto them. The measured electrical resistance is compared to that exhibited by LaMnO3 (LMO) single crystals without the films. It is found that, in the samples with the film, for which the axis of polarization in the ferroelectric is directed along the perpendicular to the surface of the single crystal, the electrical resistance decreases significantly with temperature, exhibiting metallic behavior below 160 K [4]. The transition to the state with 2DEG at the interface is demonstrated. The effect of a magnetic field on heterostructure BSTO/LMO haves been investigated. It is sown that magnetic field change the resistivity properties of the interface BSTO/LMO very strong. The new properties of the interfaces of some other heterostrocture will have been presented. The reported study was funded by Russian Scientific Foundation, research project No. 18-12-00260. References [1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004); [2] S. Thiel, G. Hammerl, A. Schmehl, C. W. Schneider, and J. Mannhart, Science 313, 1942 (2006). [3] N. Reyren, S. Thiel, A. D. Caviglia, L. Fitting Kourkoutis, G. Hammerl, C. Richter, C. W. Schneider, T. Kopp, A.-S. Ruetschi, D. Jaccard, M. Gabay, D. A. Muller, J.-M. Triscone, and J. Mannhart, Science 317, 1196 (2007). [4] D. P.Pavlov, I. I. Piyanzina, V. I. Muhortov, A. I. Balbashov, D. A.Tauyrskii, I. A. Garifullin, R. F. Mamin, JETP Letters, том 106, вып. 7, 440 – 444 (2017)

Resume : The disorder of relative Ti-O displacements in BaTiO3, more than 50 years after it has been revealed by observation of electron and x-ray diffuse scattering [1, 2], is still a matter of scientific debate [3]. The persisting question is that of the time-scale of the displacements - can they be still described as phonons or shell they be treated as an additional polar mode of a relaxational character? To this end we have investigated three dimensional distribution of the x-ray diffuse scattering intensity by comparison of synchrotron data and results of molecular dynamics simulations [4]. Together these have allowed the details of the disorder in paraelectric BaTiO3 to be clarified. The narrow sheets of diffuse scattering, related to longitudinal correlations of Ti-O displacements, are shown to be caused by the overdamped anharmonic soft phonon branch. This finding demonstrates that the occurrence of diffuse scattering agrees with a displacive picture of the cubic phase of this textbook ferroelectric material. The methodology that we used allows one to go beyond the harmonic approximation in the analysis of phonons and phonon-related scattering and we show other examples to illustrate this. [1] J. Harada and G. Honjo, J. Phys. Soc. Jpn. 22, 45 (1967). [2] R. Comes, M. Lambert, and A. Guinier, Solid State Commun. 6, 715 (1968). [3] M. S. Senn, D. A. Keen, T. C. A. Lucas, J. A. Hriljac, and A. L. Goodwin, Phys. Rev. Lett. 116, 207602 (2016). [4] M. Pa?ciak, T. R. Welberry, J. Kulda, S. Leoni, and J. Hlinka, Phys. Rev. Lett. 120, 167601 (2018)

Resume : We have investigated in detail the lattice dynamics of five different perovskite titanates ATiO3 (A=Ca, Sr, Ba, Pb, Eu) where the A sites are occupied by +2 ions. In spite of the largely ionic character of these ions, the properties of these compounds differ substantially. They range from order/disorder like, to displacive ferroelectric, quantum paraelectric, and antiferromagnetic. All compounds crystallize in the cubic structure at high temperature and undergo structural phase transitions to tetragonal symmetry, partly followed by further transitions to lower symmetries. Since the TiO6 moiety is the essential electronic and structural unit, the question arises, what makes the significant difference between them. It is shown that the lattice dynamics of these compounds are very different, and that mode-mode coupling effects give rise to many distinct properties. In addition, the oxygen ion nonlinear polarizability plays a key role since it dominates the anharmonicity of these perovskites and determines the structural instability.

Resume : BiFeO3 is perhaps the most famous example of a ferroelectric photovoltaic - nonetheless fundamental questions, such as, how does the photovoltaic effect in BiFeO3 work exactly? are still under debate. Based on experiments, ferroelectric domain walls were identified as the origin of the large photovoltage [1], then discarded completely in favor of the bulk photovoltaic effect [2]. Calculations of electrostatic potentials and charge carrier distributions at domain walls, using a combination of first-principles and semiclassical methods, apparently support an active role of domain walls in the photovoltaic effect [3]. On the other hand, both experiments [2] and first-principles calculations of the bulk photovoltaic effect [4] show strong evidence that bulk contributions are at least the major factor. Here we calculated electronic potentials and charge-carrier distributions at domain walls directly from first principles, in order to further narrow down the role of domain walls in the photovoltaic effect in BiFeO3. We find that neutral, pristine domain walls selectively capture excess electrons in strongly localized trap states, creating a strongly repulsive potential for further electrons, which should be beneficial for charge-carrier separation. In the case of the 71° and 109° walls the potential is asymmetric and could possibly cause a net photocurrent. [1] J. Seidel, D. Fu, S.-Y. Yang, E. Alarcón-Lladó, J. Wu, R. Ramesh, and J. W. Ager III, Physical review letters 107, 126805 (2011). [2] A. Bhatnagar, A. R. Chaudhuri, Y. H. Kim, D. Hesse, and M. Alexe, Nature Communications 4, 2835 (2013). [3] J. Seidel, L. W. Martin, Q. He, Q. Zhan, Y.-H. Chu, A. Rother, M. Hawkridge, P. Maksymovych, P. Yu, M. Gajek, et al., Nat. Mater. 8, 229 (2009). [4] S. M. Young, F. Zheng, and A. M. Rappe, Phys. Rev. Lett. 109, 236601 (2012).

Resume : Antiferroelectrics are characterized by a spontaneous zero net polarization that can be switched into a ferroelectric state with an external electric field. Moreover, their dielectric constant undergoes a Curie-like anomaly at the phase transition, where it increases sharply. They have not being as researched as their ferroelectric counterparts, partly due to their centrosymetric and non-polar ground state, which make them less obvious for applications. Here we present experimental results on the anomalous electrocaloric and flexoelectric responses of these materials. The electrocaloric effect of antiferroelectrics has been researched due to their anomalous (negative) response, in which the samples decrease their temperature when a voltage pulse is applied. We have examined the response close to and beyond their Curie temperature, where we measured the negative-to-positive electrocaloric transition with an infrared camera. Meanwhile, flexoelectricity- the coupling between polarization and strain gradient- is a universal property of all materials, but it has never been experimentally characterized in antiferroelectrics. And, yet, it has been hypothesized that the antiferroelectric phase might be stabilized by the influence of flexocoupling on the free energy of the system, thus pointing into a higher flexocoupling value than their ferroelectric or non-polar counterparts with the same lattice structure. We show that this flexocoupling is not higher than in simple dielectrics; however, unexpectedly, this flexocoupling is not constant as a function of temperature but increases sharply at the antiferroelectric-paraelectric phase transition

Resume : High-performance piezoelectrics are key components of various smart devices and, recently, it has been discovered that (Ba,Ca)(Ti,Zr)O3 (BCTZ) solid solutions show large electromechanical response. Nevertheless, the microscopic origin of such feature is still unclear and theoretical characterizations of BCTZ remain very limited. Accordingly, we present here a first-principles study of the structural and dynamical properties of different compositions of (Ba,Ca)(Ti,Zr)O3 solid solutions and related parent compounds in order to identify the microscopic mechanisms tuning the ferroelectric properties of the system. Specifically, we focus on the distinct effects arising from the Ca and Zr substitutions in the (Ba,Ca)TiO3 and Ba(Ti,Zr)O3 parent binary-systems, respectively. When going from BaTiO3 to CaTiO3 in (Ba,Ca)TiO3 , the main feature is a gradual transformation from B-site to A-site ferroelectricity due to steric effects that largely determines the behavior of the system. In particular, for low Ca-concentration we found out that steric effects arising from the partial substitution of Ba by Ca not only promote the emergence of Ca-driven ferroelectricity, but also weakens the Ti-driven ferroelectricity through the transformation of some key interatomic interactions. Such an interplay lowers the energy barrier between different polar states, which eventually results into an isotropic polarization around 12.5% Ca-concentration. A sizeable enhancement of the piezoelectric response directly results from these features. When going from BaTiO3 to BaZrO3 in Ba(Ti,Zr)O3, in contrast, the behavior is dominated by cooperative Zr-Ti motions and the local electrostatics. In particular, low Zr-concentration produces the further stabilization of the R3m-phase. Then, the system shows the tendency to globally reduce the polar distortion with increasing Zr-doping. Nevertheless, ferroelectricity can be locally preserved in Ti-rich regions. Moreover, we also found out an unexpected polar activation of Zr as a function of specific atomic ordering explained via a basic electrostatic model based on BaZrO3 /mBaTiO3 superlattice. Therefore, the mixing of (Ba,Ca)TiO3 and Ba(Ti,Zr)O3 with low concentration of Ca and Zr, like in the interesting composition-range of BCTZ, could allow the system to experience different polar states separated by low energy barrier. Further investigations are in progress to verify the effect of such energy landscape on the piezoelectric response.

Resume : Ferroelectric materials have attracted enormous interest on account of their novel physical properties as well as technological applications. In ferroelectric ultrathin films, the strong coupling between strain and electrical polarization can be utilized to tune the film properties and thus induce novel polar phases and functionalities. The effects of biaxial strain and temperature on ferroelectric phase stability have been intensively investigated for a variety of materials, both experimentally and theoretically. Among the numerous candidates, lead titanate based ferroelectrics have become one of the most investigated materials, especially in the tensile strain regime where complex domain patterns have been predicted and observed. However, the compressive region is less well-probed. We have studied the strain accommodation mechanism of lead titanate thin films with relative large compressive strain. A complex evolution of surface morphology and structure was observed as film thickness increases. Initially, a relatively flat surface was formed in strained lead titanate films. As thickness increased, deep pits were formed and thus surface roughness increased dramatically. When the film thickness increased further, film surfaces recovered to become flat and smooth again. At thicknesses above the critical thickness, relaxation started to occur, accompanied by the formation of pits. For even larger thicknesses, the surfaces became flat again, while still being relaxed.

Resume : Lead-zirconate titanate (PbZr1−xTixO3, PZT) is one of the most actively studied and widely used ferroelectric materials. One of the reasons of the interest is a complex phase diagram of lead zirconate and lead titanate solid solution, which defines variety of physical properties and crystal structures depending on titanium concentration. Other reasons of popularity are high piezolelectric properties, demonstrated by PZT around morthotropic phase boundary [1], and prospects of application of antiferrolectric properties [2], demonstrated by PZT with low Ti concentration. Pure lead zirconate (x=0) is the prototypical antiferroelectric material. Between the cubic perovskite paraelectric phase and the antiferroelectric phase, in the narrow temperature range, intermediate ferroelectric phase exists [3]. Addition of titanium increases the temperature range of stability of this phase. Cubic-to-intermediate phase transition is accompanied by doubling of the cell parameters of the paraelectric cubic lattice along two directions [4] and results in the appearance of M-superstructure with coordinate (H±1/2 K±1/2 0) in the diffraction pattern. Observation of additional satellites around M-point by electron diffraction [4] results in conclusion about complex domain pattern, characterized by antiphase domain boundary in lead displacement. Recent studies [5, 6] of pure lead zirconate reveals complex pattern of dynamical correlations in paraelectric phase. Diffuse scattering distribution indicates disordering of oxygen octahedral tilts and Pb displacements is shown in the high-temperature cubic phase. To study temperature behavior of these correlations X-ray diffuse scattering measurements have been done in wide temperature range in PZT with small titanium concentration (x < 0.04). To characterize dynamical origin of DS lattice dynamics have been studied using inelastic X-ray scattering. Obtained temperature evolution of DS and pre-transitional dynamical peculiarities will be shown in presentation and discussed in the context of mode coupling. Andronikova D. acknowledges support by Russian President Grants No. SP-3762.2018.5 References [1]. G. H. Haertling, Journal of the American Ceramic Society 82(4), 797 (1999). [2] K. M. Rabe, Functional metal oxides: New science and novel applications, (Wiley-VCH Verlag GmbH & Co.) [3] Michiyoshi Tanaka, Ryuichi Saito, and Kaoru Tsuzuki. Japanese Journal of Applied Physics, 21(2R):291, 1982. [4] J Ricote, D L Corker, R W Whatmore, S A Impey, A M Glazer, J Dec, and K Roleder. Journal of Physics: Condensed Matter, 10(8):1767, 1998. [5] M. Pasciak et.al. Phase Transitions, 2014 [6] Zhang N., et.al. J. Appl. Cryst. (2015). 48, 1637–1644 1.

Resume : YMnO3 is the prototypical multiferroic hexagonal manganite and an important improper ferroelectric model system. Improper ferroelectricity in YMnO3 give rise to topologically protected domains which exhibit a rich plethora of physical phenomena. This spans from topological defect formation emulating the Kibble-Zurek scenario of cosmic string formation to domain walls with magnetic and electrical properties with potential for nanoelectronic circuitry. The local crystal structure and structural coherence across the ferroelectric transition is not well understood, even after decades of studies. Here we use a combination of spallation neutron total scattering and first principles calculation to reveal the evolution of the local structure across the improper ferroelectric transition at the Curie temperature of ~1250 K. Our results show that at room temperature, the local and average structures are consistent with the established ferroelectric P63cm symmetry. On heating, both local and average structural analyses show striking anomalies from ∼800 K up to the Curie temperature consistent with increasing fluctuations of the order parameter angle. These fluctuations result in an unusual local symmetry lowering into a continuum of structures on heating. This local symmetry breaking persists into the high-symmetry non-polar phase, constituting an unconventional type of order-disorder transition [1]. [1] SH Skjærvø, Q Meier, M Feygenson, NA Spaldin, SJL Billinge, E Bozin, SM Selbach arXiv:1707.09649 [cond-mat] (2017).

Resume : Majority of reports on the PbZr1-xTixO3 (PZT) solid solutions concern ceramics, especially from so-called Morphotropic Phase Boundary [1]. Only few describe the properties of PZT single crystals, mainly because of technological difficulties to grow them. Moreover, recent papers [2-5] suggest that the Jaffe's phase diagram has to be re-visited. Whatmore et al. [6] already stated that the diagram for ceramics differs from that for crystals especially in the vicinity of the tricritical point (x close to 0.06). We have succeeded to grow a homogeneous single crystal of PbZr0.93Ti0.07O3 by means of solution growth method. Optical, dielectric and piezoelectric properties of this crystal have been studied in a function of temperature. Results obtained seem to confirm validity of the diagram reported by Whatmore et al. [6]. [1] B. Jaffe, W. R. Cook and H. Jaffe, Piezoelectric Ceramics, Academic, London (1971). [2] F. Cordero, F. Trequattrini, F. Craciun and C. Galassi, Phys. Rev. B 87, 094108 (2013). [3] M. J. Li, L. P. Xu, K. Shi, J. Z. Zhang, X. F. Chen, Z. G. Hu, X. L. Dong and J. H. Chu, J. Phys. D: Appl. Phys. 49, 275305 (2016). [4] N. Zhang, H. Yokota, A. M. Glazer, D. A. Keen, S. Gorfman, P. A. Thomas, W. Rena and Z.-G. Ye, doi.org/10.1107/S2052252517016633 (2018). [5] N. Zhang, H. Yokota, A. M. Glazer, Z. Ren, D. A. Keen, D. S. Keeble, P. A. Thomas and Z. - G. Ye, Nat. Commun. 5, 5231 (2014). [6] R. W. Whatmore, R. Clarke and A. M. Glazer, Tricritical behaviour in PbZrxTi1-xO3 solid solutions, J. Phys. C: Solid State Phys, 11, 3089 (1978) Acknowledgements: This research has been supported by the National Science Centre, Poland, within the project 2016/21/B/ST3/02242.

Resume : Lead hafnate single PbHfO3 crystals of good quality were obtained using flux growth method [1]. In literature its structure [1,3], lattice dynamics [2,3] and electric properties [2] are described assuming existence of two antiferroelectric phases below TC = 204oC. However, the antiferroelectric character of the phase appearing directly below TC, i.e. in the temperature range 159°C-204°C, has appeared not to be so obvious. Due to observation of weak reversible pyroelectric signals and quasi-static piezoelectric effect, this intermediate phase might be treated as that of ferroelectric character. [1] S. Huband, A. M. Glazer, K. Roleder, A. Majchrowski and P. A. Thomas J. Appl. Cryst. 50, 378 (2017) [2] A. Bussmann-Holder, T. H. Kim, B. W. Lee, J-H Ko, A. Majchrowski, A. Soszy?ski, and K. Roleder J. Phys.: Condens. Matter 27, 105901 (2015) [3] R. G. Burkovsky, D. Andronikova, Yu Bronwald, M. Krisch, K. Roleder, A.Majchrowski, A. V. Filimonov, A. I. Rudskoy and S. B. Vakhrushev J. Phys.: Condens. Matter 27, 335901 (2015) Acknowledgements: This research has been supported by the National Science Centre, Poland, within the project 2016/21/B/ST3/02242.

Resume : Relaxor ferroelectrics with diffuse phase transitions (relaxors) have been subject to intensive research. Interest in these compounds is determined by a combination of ferroelectric, piezoelectric and optical properties and the ability to use these materials in optoelectronics and data storage systems. The distinguishing features of relaxors are a strongly diffuse maximum in the temperature behavior of permittivity, the shift of this maximum toward higher temperatures with rising measuring field frequency, and a strong frequency dependence of permittivity at very low frequencies. Numerous experimental data show that the properties of the low-temperature phase depend on the history of samples, so nonergodic behavior is observed in the low-temperature phase [1]. In an applied electric field, the transition to a uniform state of polarization is observed in the low-temperature phase after zero-field cooling. Such a phase transition was observed in [1] after a sufficiently long delay time had passed from the beginning of field application. The dependences of delay time t0 of the phase transition on temperature T and external electric field E were established. The observed regularities have been discussed using an approach [2] developing on the basis of the model of diffuse phase transition in the system with defects [3]. It is shown that in the frame of that approach the delay phase transition in polar phase in relaxor could be explain if the dynamic of electron system would be take in consideration [2]. For examine that model we investigate the effect of illumination on the delay time t0 of the phase transition in PbMg1/3Nb2/3O3 in [110] orientation. The delay time t0 of the phase transition have been measured for different temperatures and applied electric field. The photoconductivity also has been investigate and correlation of observed results with developed model is discussed. The reported study was partially supported by RFBR, research project No. 18-02-00675a. References [1] E.V. Colla, E.Yu. Koroleva, N.M. Okuneva, S.B. Vakhrushev, PRL 74, 1681-1684 (1995). [2] R.F. Mamin, R. Blinc, Physics of the Solid State 45, 942-945 (2003). [3] R.F. Mamin, Physics of the Solid State 43, 13141319 (2001).

Resume : For the fast and low power consumption phase change memory, we have suggested In3SbTe2 doped with Bi and Y and calculated doping formation energy, the phase transition behaviours, lattice distortion, and vacancy concentration. As a result, it is found that thermodynamic stability is improved due to negative formation energy and the energy difference between the amorphous and the crystalline IST is 0.26 eV/atom and that of the Y doped IST (Y-IST) is 0.18 eV/atom, which means that the phase transition of Y-IST from crystalline to amorphous is as fast as 2.83 times of that of the IST. The doping element provides higher interfacial energy during the formation of metastable grain boundaries of InSb and InTe, which results in fast transition with lower energy. Lattice images and density functional calculation of the doped IST indicate that the distortion and vacancy concentration cause fast set/reset operations of phase change memory.

Resume : In the calculations of tetragonal BaTiO3, some exchange-correlation (XC) energy functionals such as local density approximation (LDA) have shown good agreement with experiments at RT, and superiority compared with other XC functionals. This is due to error compensation of RT effect and, hence, will be ineffective in the heavily strained case such as domain boundaries. Here, ferroelectrics under large strain at RT are approximated as those at 0 K, because the strain effect surpasses the RT effects. To find effective XC functionals for accurate calculations of strained BaTiO3 we propose a new criterion. This criterion is the properties at 0 K given by the Ginzburg-Landau (GL) theory, because GL theory is a thermodynamic description of experiments and works under the same symmetry-constraints as ab initio calculations. With this criterion, we conclude that three XC functionals are suitable for the calculations of BaTiO3. Indeed, the calculations of homogeneously strained tetragonal BaTiO3 by these XC functionals agree excellently with experimental lattice parameters [1]. Consequently, in a definite manner, the present results show much more enhanced ferroelectricity at large strain than the previous reports. This conclusion is expected valid also for other perovskite oxide ferroelectrics, because of its methodology. In addition, we show also that the effective critical temperature Tc under strain < -0.01 is > 1000 K from an approximate method combining ab initio results with GL theory. [1] Y. Watanabe, J. Chem. Phys.148, 194702 (2018).

Resume : We?ve found that the plots of Ps vs. certain crystallographic parameters under different strains calculated with several exchange-correlation (XC) functionals lies precisely on a single curve, despite widely varying Ps [1], which can be used for crystallographic estimation of local Ps. For example, TEM uses the displacement of Ti atom to identify Ps and its direction. Here, each XC functional tended to yield specific deviations of crystallographic properties from ideal experimental ones. We think that such deviations exist locally in experiments owing to ambient temperature, defects and local strains. Because of the precision of the present Berry phase method, the ab initio Ps deviating from ideal experiments can be reinterpreted as exact calculations of Ps in crystals deviating from ideal ones. That is, the correlations between PS and crystallographic properties calculated with different XC functionals correspond to those in experiments, which uncover analytical expressions of the local Ps using crystallographic parameters, e.g., Ps (C/m2) ~ 9.77 zTi-O2. In addition, these expressions show the primary origin of BaTiO3 ferroelectricity as oxygen displacements. [1] Y. Watanabe, J. Chem. Phys.148, 194702 (2018).

Resume : The paper deals with a piezoelectric harvester based on micro-electro-mechanical system (MEMS) devices and piezoelectric material fabrication, thin film deposition and patterning technology, focusing on small-scale power energy harvesting techniques (1-100μW) using thin-film and MEMS approach for autonomous operation systems. Different materials, as PZT, pristine and Mn- or V-doped ZnO with various morphologies have been synthesized, deposited as thin films on Pt/Ti/SiO2/Si by chemical (sol-gel, hydrothermal), physical (PLD, e-gun, ALD) and printing techniques. They were characterized, analyzed and compared related to the piezoelectric transduction function. The obtaining of well crystallized, nanostructured materials were proved by XRD and SEM, while the thickness and the permittivity were estimated by spectroscopic ellipsometry (SE). The presence of the dopant in the films matrix was proved by EDX and XRF, and its chemical state by XPS. IR and XRD have proved the removal of the organics from the films matrix. XRD did not found any signal from the dopant (Mn) because of the small amount and low thickness (in the case of sol-gel films). The chemical state of the dopants (Mn or V) was found to strongly depend on the preparation route as proved by XPS. The piezoelectric effect was assessed by using several geometries for the harvester, as for example the Mazzalay configuration, while the measurements were made by using several types of measurements: piezometer testings, electrical polarizability and laser interferometry.

Resume : Thin films of Na0.5Bi0.5TiO3 (NBT) are promising for practical use due to high values of polarization and piezoelectric coefficients. Despite the progress in NBT films production, the nature of the observed large values of the leakage current, which masks the real values of the polarization, is still unclear. The influence of technological factors on values of the leakage current in NBT films deposited by RF magnetron sputtering is considered. NBT films are deposited in atmosphere of argon and oxygen taken in different proportions at 10 mTorr pressure on Si substrates. NBT ceramics, as well as crushed and pressed NBT crystals is used as targets. Film crystallization is carried out by annealing at a temperature of 700 degree Celsius for 1 hour in air. XRD analysis is used to control the films structure. Observed leakage current densities is in 1-10 uA/(cm^2) range. The greatest values of leakage current are observed in films deposited in pure Ar atmosphere, while the smallest values are observed at the Ar/O2 ratio 1/3. It is assumed that presence of electronic conductivity cannot explain large values of the leakage current. Most probably, the increase in leakage currents is associated with the formation of Bi3+ vacancies (due to the Bi volatility), which stimulate the appearance of O2- localized vacancies on grain boundaries. Even relatively small number of Bi3+ and O2- vacancies leads to significant increase of leakage currents.

Resume : PbHfO3 and PbZrO3 are representative antiferroelectrics with relatively similar phase transition temperatures. Incorporation of Sn ions may be considered as a kind of chemical pressure and thus its effect on phase transition behaviors is an interesting topic. Electrostrictive response of two antiferroelectric single crystals PbHf1-xSnxO3 with two different compositions of x = 0.025 and 0.3 grown by a flux method, was studied in wide temperature range. The temperature dependences of the field related electrostrictive coefficient M11 for PbHf1-x SnxO3 with high Sn concentration differs significantly from M11(T) for the crystal with low Sn concentration. The M11(T) response of PbHf0.975Sn0.025O3. is of nearly classical response. In the PbHf0.7Sn0.3O3 single crystals crystal M11(T) is of strongly nonlinear behavior. It was assumed that the non-linear M11(T) dependence around Temax and is associated with the presence of local polarity. Observed non-linearity results from specific coupling between electrostrictive and elastic constants and attributed to enhanced polarization fluctuations caused by increased lattice distortions due to Sn substitution. Resultant polarization fluctuations are strongly enhanced in the specific temperature range between Temax and T1, where the existence of intermediate ferroelastic phase in PbHf0.7Sn0.3O3 was found on the basis of Brillouin light scattering measurements.

Resume : Poly(vinylidene fluoride) (PVDF) based ferroelectric polymers with their typical polarization hysteresis loop is one of the promising candidates for next generation non-volatile memory devices. However, the high roughness of the interface between PVDF layer and semiconductor channel is a serious obstacle to the memory device performance. In this work, grafting polystyrene (PS) or poly(2,3,4,5,6-pentafluorostyrene) (PPFS) onto the chain of PVDF has been firstly demonstrated to be an effective way to improve the interface smoothness as well as to enhance the ferroelectric properties of the PVDF film. The ferroelectric behaviour of spin-cast PVDF-g-PS and PVDF-g-PPFS films is highly influenced by sample preparation methods such as solvent and thermal annealing condition. The ferroelectric phase, chain and dipole orientation, surface morphology, and polarization of those films are characterized by FTIR-grazing incident reflection absorption spectroscopy (GIRAS), grazing incident wide angle X-ray diffraction (GIWAXD), SEM, AFM, and polarization-electric field (P-E) hysteresis measurements. The β-phase of PVDF-g-PS and PVDF-g-PPFS can be obtained by simply annealing the spin-cast films at 150˚C in 2 hours. Ferroelectric FET non-volatile memory devices using a rubrene thin film and PVDF-g-PS or PVDF-g-PPFS ferroelectric films were fabricated. The high performance of the memory devices based on those graft polymers are very promising in real applications.

Resume : I will describe some work on new ferroelectrics from our group at St. Andrews. These include SnTiO3, PbTiO3:Pd, LaTaO4, and K3Li2Ta5O15. If time permits I will also show a very precise analysis of Barkhausen pulses in ferroelectrics.

Resume : In magnetic perovskite oxides ABO3, first-neighbour antiferromagnetic super-exchange interactions usually dominate, but may coexist with other terms such as ferromagnetic double-exchange or Dzyaloshinskii-Moriya interactions at B-O-B and A-O-A bonds. This often produces non-collinear spin configurations leading to weak ferromagnetism or to spatially modulated spin structures. A prototypical non-collinear magnetic oxide is multiferroic BiFeO3 that shows a cycloidal order with a 64 nm period in the bulk [1]. In this talk, I will show how epitaxial strain can be used to tailor the magnetic order of BiFeO3 thin films [2,3] and present real-space images of the cycloidal structure, as well as its manipulation by an electric field [4]. In a second part, I will report the observation of a very large topological Hall effect (THE) in thin films of a lightly electron-doped manganite. Magnetic force microscopy reveals the presence of small magnetic bubbles, whose density vs. magnetic field peaks near the THE maximum, as is expected to occur in skyrmion systems. The THE critically depends on carrier concentration and diverges at low doping, near the metal-insulator transition. I will discuss this observation of a THE in a weak-coupling regime with non-adiabatic contributions [5] beyond the conventional picture, and the strong amplification of this topological phenomenon by correlation effects. [1] D. Sando, A. Barthélémy, and M. Bibes, J. Phys. Condens. Matter 26, 473201 (2014). [2] D. Sando et al., Nature Mater. 12, 641 (2013). [3] A. Agbelele et al., Adv. Mater. 29, 1602327 (2017). [4] I. Gross et al., Nature 549, 252 (2017). [5] K. Nakazawa, M. Bibes, and H. Kohno, J. Phys. Soc. Japan 87, 33705 (2018).

Resume : The search for alternative solid-state refrigeration materials to hazardous gases in conventional and cryogenic cooling devices is a very active field of condensed matter [1,2]. The use of phase transitions is a powerful tool to achieve giant caloric effects in ferroic materials in which magnetization, polarization, strain and/or volume can be strongly tuned under a moderate external stimulus. Here, we explored various aspects of ferroelectrics to reveal their potentialities as solid state coolers such as the ferroeletrci phase transitions, the multiphase points composition, the stress-sensitivity through elasto- and baro-caloric responses, the inverse electrocaloric effect evidenced for instance in antiferroelectrics, the asymmetric effect arising from non-ergodic state, the use of dual-stimuli by taking advantage of multicaloric effects combining stress and electric field in ferroelectrics or magnetic and electric fields in multiferroics, as well as non-ergodicity and defect as well as the use of defects [3-12]. References: [1] X. Moya, S. Kar-Narayan, N. D. Mathur, Nat. Mater. 13, 439 (2014) [2] T. Correia and Q. Zhang (Eds.). Electrocaloric Materials, Springer: Berlin, 2014 [3] Y. Liu, J.F. Scott, B. Dkhil, APL Materials 4, 064109 (2016) [4] Y. Liu, B. Dkhil, E. Defay, ACS Energy Lett. 1, 521 (2016) [5] Y. Liu, L. C. Phillips, M. Bibes, A. Barthélémy, B. Dkhil, Nat. Comm. 7, 11614 (2016) [6] Y. Liu, J.F. Scott, B. Dkhil, Appl. Phys. Reviews 3, 031102 (2016) [7] Y. Liu, G. Zhang, Q. Li, L. Bellaiche, J.F. Scott, B. Dkhil, Q. Wang, Phys. Rev. B 94, 214113 (2016) [8] R Faye, H Strozyk, B Dkhil, E Defay Journal of Physics D: Applied Physics 50 (46), 464002 [9] M Sanlialp, Z Luo, VV Shvartsman, X Wei, Y Liu, B Dkhil, DC Lupascu, Appl. Phys. Lett. 111, 173903 (2017) [10] X Wang, J Wu, B Dkhil, B Xu, X Wang, G Dong, G Yang, X Lou, Appl. Phys. Lett. 110, 063904 (2017) [11] T. Li et al., to be published [12] J. Li et al., to be published B.D. acknowledges Fonds National de la Recherche (FNR) du Luxembourg through the InterMobility project 16/1159210 "MULTICALOR"

Resume : We have used low energy and photoemission electron microscopy (LEEM and PEEM, respectively) to carry out a microscopic characterization of the surface charge and domain structure of BaTiO3(001). Ferroelectric and ferroelastic domains and tweed dominate the surface structure of BaTiO3 at room temperature. Heating the sample to temperature (~550 K), well above the transition ferroelectric temperature (393 K) maintains ferroic signatures at the surface. This surface proximity effect ensures the memory of the bulk ferroelectric domain arrangement up to 150K above TC and thus can be considered as a robust fingerprint of the FE-state. Annealing at higher temperature, above the tweed freezing temperature, triggers the dynamic tweed and in turn allows a full reorganization the ferroic domain configuration. During the ferroelectric-paraelectric phase transition, transient surface structures appear slightly above the Curie temperature, roughly perpendicular to the intersections between polar domain walls and the sample surface. These transient surface structures coarsen and then disappear over a temperature range of less than 2 K and are consistent with a surface induced elastic anomaly similar to that observed in martensitic transitions.

Resume : As promising materials for novel electronic, catalytic, and energy applications, transition metal oxides have attracted great attention. In particular, the discovery of the resistive switching effect has generated enormous interest as it can be exploited for future memristive data storage and neuromorphic computing. In order to understand the fundamental mechanisms leading to a change of the resistance induced by external electrical stimuli, macroscopic electrodegradation experiments are a well-established method. While numerous research on this topic has been conducted, the influence of extended defects such as dislocations has not been studied in detail so far. Here, we present investigations of the prototypical transition metal oxides SrTiO3 and TiO2 as well as on the technologically relevant oxygen conductor Y-stabilized ZrO2. By employing optical and thermal microscopy we show that in the first stage of electrodegradation the current can be channeled along dislocations, which have been introduced mechanically by scratching or sawing. After prolonged degradation, also the matrix of the crystal gets electro-reduced and the influence of the initially present dislocations is diminished. In this later stage, stoichiometry polarization occurs resulting in an increase of the temperature at the anode. This does not only lead to a rearrangement of dislocations due to electrical and thermal stress, but can eventually cause, in particular in ternary oxides, a decomposition via incongruent sublimation.

Resume : The first-order metal-insulator transition (MIT) and phase separation which arise in perovskite rare-earth nickelate oxides have provided opportunities to explore not only the nature of phase-separated electronic states but also ?Mottronics? devices, e.g., synaptic multilevel devices analogous to human brains. Although the phase coexistence of metallic and insulating domains and their evolution across the MIT in NdNiO3 (NNO) thin films have been investigated by utilizing X-ray photoemission electron microscopy (XPEEM) [1], a direct resistance observation of the phase-separated electronic states has been still elusive. In this presentation, we show a clear evidence of phase coexistence in NNO thin films by using both conductive-tip atomic force microscopy (C-AFM) and XPEEM. The C-AFM images measured across the MIT temperature have revealed the nucleation of ~100-300 nm metallic domains in the insulating phase of NNO as well as their evolution in a percolative way [2]. This work received support from the ERC Consolidator grant no. 615759 MINT. [1] G. Mattoni et al. Nat. Commun. 7, 13141 (2016). [2] D. Preziosi et al. Nano Lett. 18, 2226 (2018).

Resume : Domain walls of continuous order parameters, as polarization, magnetization, and strain in ferroic materials, exhibit novel electronic properties localized on a naturally 2-dimensional nanoscale. Ferroelectric domain wall dynamic phenomena are attracting enormous attention lately [1,2]. Recent scanning impedance microscopy (SIM) measurements [3] use alternating electric field to excite domain wall-localized phonons, affecting a series of material properties, such as dielectric loss, electronic conductivity, mechanical stiffness, and changes in optical absorption. We address these dynamical domain wall-related phenomena by a combination of first-principles calculations and Ginzburg-Landau theory to characterize the variety of domain wall localized phonon modes. The results explain the puzzling SIM data on sliding mode excitation in 71-degree domain walls in BiFeO3, in which the polarization component along the field is the same in the domains across the wall. The wall tilting and electrostriction and flexoelectric couplings are identified to be responsible for the effect.

Resume : Multiferroic materials have attracted a great deal of attention because of their ferroelectric, ferromagnetic and ferroelastic properties in a single material. Due to its promising feature, they have gained a remarkable usage area in non-volatile information storage, spintronics, multiple state memories and sensors. In this study, one of the multiferroic material, BiFeO3 was investigated. The effect of temperature on phase transformation of bismuth ferrite (BFO) crystallites was shown. In-situ XRD results proved that starting point of crystallization of BFO was 200°C whereas this crystallization completed itself at almost 600°C. Moreover, change in crystal structure from rhombohedral to monoclinic was observed between 800°C ? 850 °C, which is consistent with its Curie temperature. These results were also confirmed with the help of simultaneously taken DSC/TGA/DTA measurements. Those analysis showed Neel temperature at around 370°C and Curie temperature at around 850°C, yielding in total d.3 weight percent loss due to decomposition of nitrate and evaporation of water arising from precursor materials. SEM analysis was conducted to observe the morphology as well as homogeneity of crystallites. XPS results demonstrated Fe-O & Bi-O bonds along with presence of Bi3 and Fe3 rather than Fe2 . The change in the polarization and remnant polarization values were measured for the crystallites and hysteresis curves were observed. The maximum polarization values were achieved above 0.5 ?C/cm2 at around 50 kV/cm. Lastly, VSM technique revealed remanent magnetization (Mr) nearly 0.02 (emu/g) at 6T. All these results proposed the outstanding intrinsic ferroelectric and magnetic behavior of BiFeO3 in a single phase. Keywords: Multiferroic, BiFeO3, ferroelectric/magnetic, thermal analysis, powders

Resume : In the search for multiferroics with strong magnetoelectric coupling, the design of thin film heterostructures and multilayers is a particularly promising and versatile strategy. Epitaxial multilayers of BaTiO3 and BiFeO3 deposited onto SrTiO3 substrates via pulsed laser deposition exhibit magnetoelectric voltage coefficients of up to 54.9 V*cm^-1Oe^-1 [1]. This represents an increase of one order of magnitude with respect to BiFeO3 single layers [2] and rivals the largest values reported in literature. The enhanced magnetoelectric coupling appears to be highly sensitive to factors like the total number of interfaces in a sample, as well as the density of interfaces per sample volume. Investigations with Mössbauer spectroscopy suggest an influence of structurally determined magnetic anisotropy to the enhanced longitudinal magnetoelectric effect [3]. To further the understanding and push the boundaries of magnetoelectric coupling in these heterostructures, we here present a detailed investigation into the effects of double-layer thickness, BaTiO3 to BiFeO3 ratio, and deposition conditions on the magnetoelectric voltage coefficient in BaTiO3-BiFeO3 multilayers. [1] S Hohenberger et al., J. Phys. D: Appl. Phys. 51 184002 (2018) [2] J Wang et al., Science 299 1719 (2003) [3] J Jochum et al., Nanoscale,10, 5574-5580 (2018)

Resume : In RFeO3, the interplay between magnetic moments of iron (Fe) and rare earth (R) ions gives rise to a rich magnetic landscape and various fascinating phenomena. Competitions between Fe–Fe, R–Fe, and R–R interactions induce spin-reorientations of the iron lattice stirred by the R magnetic moments. In turn, the R moments order in the exchange field of the magnetic sublattice of iron. At low temperature R-R interactions dominate and the R-R-sublattice orders. The latter can lead to an electric polarisation with a strong coupling to the magnetic orders. In fact, this constellation allows for a net-magnetisation that can be switched by an electric field, one of the main goals of magnetoelectrics and multiferroics research. Whether or not the ordering of the R-Fe interaction can already cause an overall symmetry breaking and eventually induce a polarisation remains an open question. To learn about the different interactions, we investigate two members of the RFeO3 family: (Dy,Tb)FeO3 and SmFeO3. In (Dy,Tb)FeO3, we demonstrate the full inversion of magnetic domain patterns by the application of an electric field. In addition, the different time scales of the coupling of R- and Fe-sublattices to the polarisation allow for the deterministic inversion of either magnetic sublattice.To scrutinize the effect of the Fe-R interaction, we choose SmFeO3 to perform Raman spectroscopy and optical second harmonic generation measurements, two techniques highly sensitive to symmetry breaking. These measurements reveal strong anomalies in the temperature evolution of the Raman signature and SHG signal well-above the classical ordering temperatures of the R–R sublattice. These anomalies suggest a symmetry breaking as a consequence of the R–Fe interactions at comparably high temperatures.

Resume : Two years after the invention of modern prototype solar cells, it was found that a ferroelectric material, BaTiO3, exhibits a photovoltaic effect distinct from that of p-n junctions, later called the bulk photovoltaic (BPV) effect. Under uniform illumination, a homogeneous ferroelectric material gives rise to a current under zero bias, i.e. short-circuit current (ISC), that depends on the polarization state of the incident light, and produces an anomalously large photo-voltage well exceeding the bandgap energy. The microscopic origins of this effect are still under debate. It supposed to originate from the asymmetric distribution of photoexcited non-equilibrium carriers in k-space, caused by absence of centrosymmetry in the material. In the recent past, the entire field of photo-ferroelectrics has been revitalized by the reports of photovoltaic effect in BiFeO3 (BFO), which is a ferroelectric/multiferroic material with one of the lowest band gap and significant semiconducting properties. The present talk will firstly present a short history and the basics of the bulk photovoltaic effect, tip enhancement, as well as the electronic origin of the anomalous BPV in some materials such as BiFeO3. Later, potential applications such as energy harvesting or light-induced reversible switching of ferroelectric polarization at room temperature. I will show how the tip-enhanced effect, i.e. enhancement of the short-circuit photocurrent density at an AFM tip contact area, may be at the basis of harvesting devices with efficiency exceeding the Schokely-Quesser limit. Finally, we will discuss a new photovoltaic effect which turns the BPV effect into a universal effect allowed in all semiconductors by mediation of the flexoelectric effect. [1] [1] M.-M. Yang D. J. Kim, & M. Alexe, Flexo-Photovoltaic Effect, Science (2018) DOI: 10.1126/science.aan3256

Resume : In semiconductors, electronic excitations interact with the lattice via electron-phonon coupling. Through these interactions, lattice dynamics play a pivotal role in hot carrier cooling and the transport of band-edge charge carriers. Hybrid organic-inorganic perovskites (HOIPs) have become popular materials for optoelectronic and other applications, and while computational and experimental studies have probed the lattice dynamics, little attention has been paid to phonon lifetimes. We report high-precision measurements of acoustic phonon lifetimes in methylammonium lead iodide (MAPI) using triple-axis neutron spectroscopy to provide high energy resolution and fully deuterated single crystals to reduce the incoherent scattering from hydrogen. Our measurements reveal extremely short lifetimes on the order of a few picoseconds, corresponding to acoustic phonon mean free paths of a few nanometers. These ultra-short mean free paths suggest that acoustic phonons are unable to dissipate heat efficiently in MAPI and have profound implications for carrier-lattice interactions that are relevant to optoelectronic devices. These results highlight a fundamental and profound difference between HOIPs and conventional inorganic semiconductors.

Resume : Bismuth ferrite BiFeO3 (BFO) is one of the most studied single-phase multiferroic material. It undergoes a ferroelectric-paraelectric phase transition at TC = 830°C and an antiferromagnetic-paramagnetic phase transformation at Néel temperature TN = 370 °C. Despite these wonderful properties of BFO, there are some drawbacks associated with this material including the formations of impurity phases, weak magnetic properties, weak magnetoelectric coupling, and large leakage current density. Therefore, the appropriate chemical modifications are required to improve the electrical and magnetic properties of BFO. In this work, the substitutions of rare earth (RE) ions, such as Dy3+, Er3+ and Yb3+ for the A-site Bi3+ ion have been performed. New multiferroic materials (1−x)BiFeO3-xDyFeO3 (denoted BDF-x) and (1-x)BiFe(1-y)Ti(y)O(3+y/2)-xDyFeO3 (denoted BDFT-x-y) were synthesized by solid-state reactions. Compared with pure BFO, the ferromagnetism in the BDF-x solid solution is substantially enhanced by the structural distortion and unpaired electrons due to A-site substitution of Dy3+ for Bi3+. Well-developed ferroelectric hysteresis loops are displayed in BDFT-x-y with a large remnant polarization Pr = 23 μC/cm2 at room temperature, which is significantly higher than the previously reported Pr = 3.5 μC/cm2 in pure BiFeO3 ceramic. Moreover, weak ferromagnetism is found in it at room temperature (Ms = 0.1 μB/f.u.). The structure-composition phase diagram of the BiFeO3-DyFeO3 system is established. Investigated were also (1-x)BiFe(1-y)Ti(y)O(3+y/2)-xLuFeO3, (1−x)BiFeO3-xYbFeO3, (1−x)BiFeO3-xErFeO3 and (1−x)BiFeO3-xEuFeO3 solid solutions. Their magnetic and ferroelectric properties will be presented. This work is supported by the Office of Naval Research (Grants and N00014-12-1-1045 and No. N00014-16-1-3106) and the Natural Sciences and Engineering Research Council of Canada (NSERC Grant No. 203773).

Resume : Understanding the crystal structure is essential to clarify its relationship with physical properties. For many years, the average structure of PbZr1-xTixO3 (PZT) has been extensively studied because of its high piezoelectric performance near the morphotropic phase boundary (MPB). With a help of theoretical calculation, the existence of lower symmetry phase (monoclinic phase) around the MPB is recognized as a key factor for an enhancement of piezo-activity. In spite of that, an average structure model cannot fully explain the increase of physical properties. Therefore, it is necessary to understand the local structure for revealing the origin of physical properties. Recent progress on third-generation synchrotron radiation sources enables us to study a local structure. The Fourier transform of total scattering intensity gives the pair distribution function (PDF), which describes the probability of finding any two atoms at a given inter-atomic distance. Here, we present a local structure analysis on PZT as a function of temperature. PDF is analysed by a small box modelling using PDFFIT and reverse Monte Carlo (RMC) modelling using RMCProfile. We observed a continuous change of local structure as a function of temperature.

Resume : When ultrathin ferroelectric layers of PbTiO3 are embedded in superlattices with a paraelectric material, such as SrTiO3, the interplay between elastic, electrostatic, and gradient energies produces complex patterns of the electrical polarization. In particular, nanometer scale of clock- and counterclock-wise rotating vortices arrays have been recently detected [1] and exotic properties such as the emergence of a negative capacitance have been measured [2]. In this work we carry out atomistic simulations to determine the properties of these emergent structures. Performing predictive simulations in these systems is difficult due to the long spatial scales involved in the formation of counter-rotating vortices pairs, the strong competition between a large number of phases and the sensitivity of the results to external perturbations like strain, periodicity, temperature or electric fields. In order to overcome these problems we employ a recently developed second-principles method [3,4] that can cope with all the degrees of freedom associated to a large number of atoms retaining high accuracy. Our simulations predict the existence of several quasi-degenerate phases at low energies each displaying different properties including net polarization, non-null topological constants and chirality. The later prediction supports the findings of optical activity in x-ray circular dichroism experiments [5]. Moreover, depending on the periodicity of the superlattice these chiral vortex phases coexist with ferroelectric phases and reversible phase transitions can be induced by external electric fields [6], and they can even transfer angular momentum to an electron beam. [1] A. K. Yadav et al., Nature 530, 198 (2016) [2] P. Zubko et al., Nature 534, 524 (2016) [3] J. Wojdel et al. J. Phys.: Condens. Matter. 25, 305401 (2013) [4] P. García-Fernández et al, Phys. Rev. B 93, 195137 (2016) [5] P. Shafer et al. Proceedings of the National Academy of Sciences 115, 915 (2018) [6] A. R. Damodaran et al., Nature Materials 16, 1003 (2017)

Resume : Scientific process begins with observations, followed by intuition, then construction of a quantitative theory that explains the observations, and subsequently, refinement of the theory based on new observations. Recent years, data science paradigm is becoming an essential part of material research portfolio, complementary with empirical observation, theoretical model and computational simulation paradigms. Data-Mining or Machine-Learning from reliable extant data can lead to discover those previously unknown qualitative and quantitative rules between properties, used to predict new materials faster and cheaper and reduce human effort than required by the benchmark simulation or experimental methods utilized to create data in the first place. To mine or learn quantitative correlations between materials and properties, the key and first step is to represent numerically various materials, i.e. to define descriptor for materials. The choice of descriptor for a problem at hand is not always straightforward or obvious, which needs adequate knowledge of the problem and goals (i.e., domain expertise or experience), and proceeds in an iterative manner. Thereafter, establishing a map between input descriptors and target property is becoming entirely numerical in nature. At last, an expert recommendation system that can continuously and adaptively improve is accomplished to predict new material for desirable application. In this work, we performed data-mining with material genome approach in perovskite-type oxides to search new multiferroic compounds targeted with parallel long-range electric and magnetic polarizations and with both ordering points above room temperature. The reduce mass of unit cell, tolerant factor/octahedral factor related to ionic size, displacements of cations from the high-symmetry lattice positions, and dm-dn spin dimers in oxide perovskites are attempted as descriptors to classify and data-mine the relationship between chemical composition and ferroic properties. The heavy and large elements of Bi, Pb and Ba on A-site and the small ones of Fe, Ti, Zn, Nb and Cr (archetypal but not all) on B-site are mined out for design promising ferrimagnetic-ferroelectric multiferroic compounds with both Curie temperatures above 300K. Using strategy of ternary solid solution, such ferrimagnetic-ferroelectric multiferroic perovskite-type oxides were sure synthesized using solid state reaction method. Our essay demonstrates data-mining driven design in a fast way to accelerate discovery of multiferroic and ferromagnetoelectric materials. For experimental characterizations and device applications, suitable synthesis method is necessarily required for processing high resistive and low dielectric loss samples. References 1. R. Ramprasad, R. Batra, G. Pilania, A. Mannodi-Kanakkithodi and C. Kim, Machine learning in materials informatics: recent applications and prospects. npj Comput. Mater. 3, 54 (2017). 2. P. V. Balachandran, J. Young, T. Lookman, and J. M. Rondinelli, Learning from data to design functional materials without inversion symmetry. Nature Comm. 8, 14282 (2017). 3. P. V. Balachandran, T. Shearman, J. Theiler, T. Lookman, Predicting displacements of octahedral cations in ferroelectric perovskites using machine learning. Acta Cryst. B 73(5), 962-967 (2017). 4. J. Yu, L. L. Zhang, X. B. Hou, Y. Lin and W. L. Zheng, Novel perovskite-type ferroelectrics with high curie temperature and piezoresponse. Proc. of 2016 ISAF/ECAPD/PFM. IEEE Xplore: DOI: 10.1109/ISAF.2016.7578087 5. J. Yu and J. H. Chu, Searching for New Perovskite-structured Ferroelectric Materials within Paradigm of Data Science. Sci. Technol. Rev. (2018). (to be submitted) [in chinese]

Resume : In 2012, an unusually strong spin-order induced ferroelectric polarization was discovered in CaMn7O12 below 90 K. To find general tendencies in structural, magnetic, ferroelectric and magnetoelectric properties, we prepared AMn7O12 (A=Cd, Sr, Pb) ceramics using high pressure and high temperature sintering. All these compounds exhibit a similar sequence of structural phase transitions: Near 400 K, they undergo a charge-ordering metal-insulator phase transition from a cubic to a rhombohedral structure. The next phase transition gives rise to Mn orbital ordering and an incommensurate structural modulation below ca. 250 K. On further cooling, the samples show two magnetic phase transitions near 90 K and 40 K with transition temperatures depending on the chemical composition. The AFM phases are helical and modulated. We investigated IR, Raman and THz spectra in all phases of AMn7O12. On cooling, we observed distinct phonon splitting accompanying both structural phase transitions. In THz spectra, many new modes activate in magnetic phases. These modes are strongly temperature dependent and they also shift their frequencies with magnetic field. For that reason we assign them to spin excitations. In CaMn7O12, we identified some of the modes as electromagnons, i.e. electrically active magnons contributing to dielectric permittivity due to a dynamic magnetoelectric coupling. Therefore we assume that at least some of the THz modes in AMn7O12 (A=Cd, Sr, Pb) are also electromagnons.

Resume : Antiferroelectric perovskites are key ingredients in high-performance piezoelectric ceramics and are prospective in memory and energy storage applications. Instability with respect to a set of different order parameters makes the physics of antiferroelectrics highly difficult for disentangling. We explore the role of antiferrodistortive instability in conditioning incommensurate phase in PbHfO3 crystals using diffraction and inelastic X-ray scattering spectroscopy. We show that the antiferrodistortive instability is characterized by the highest critical temperature among the other potential instabilities present and is decisive in triggering incommensurate instability in an avalanche-like process. We show the effect of separation of antiferrodistortive and incommensurate instabilities in temperature by either application of hydrostatic pressure or by forming solid solutions with PbSnO3, which can be considered as chemical pressure. The latter two factors enable direct observation of incommensurate critical scattering as a precursor of incommensurate phase transition.

Resume : SrxBa1-xNb2O6 (SBN) belongs among the most studied uniaxial ferroelectric materials which exhibits excellent electrooptic, piezoelectric and dielectric properties near room temperature. Moreover, these macroscopic material properties can be effectively tuned with Sr content, for example from ferroelectric- to relaxor-like dielectric response [1]. Even though the average crystal structure of SBN remains tetragonal for 0.35 < x < 0.72 [2], its local structure is strongly influenced by substitutional disorder which is inherently present in squared and pentagonal channels formed by NbO6 octahedra chains. This gives rise to local strains and electric dipoles that affect the ordering of polarization and incommensurately modulated octahedra tilts [3]. In spite of considerable effort the atomistic picture of polarization in SBN is not known in detail. The signatures of a short range order, however, are rather strong in the neutron and x-ray diffuse scattering [4,5]. We present results of the neutron and x-ray scattering experiments on SBN single crystals and powders. Above all, we performed a detailed analysis of neutron pair distribution functions which was supported with extensive first principle calculations. Using the combination of experimental and theoretical methods we can convincingly demonstrate how the local structural patterns comprising octahedra tilts and relative Nb-O displacements develops when lowering the temperature. [1] T. Lukasiewicz et al., J. Cryst. Growth 310, 1464 (2008). [2] R. Paszkowski et al., Cryst. Res. Technol. 48, 413 (2013) [3] T. Woike et al., Acta Cryst B 59, 28 (2003). [4] P. Ondrejkovic et al., Phys. Rev. Lett. 113, 167601 (2014). [5] M. Paściak et al., accepted for publishing in Phase Transitions (2018).

Resume : The search for good magnetoelectric multiferroics has been prevented by the intrinsic small responses of these materials and the often too low temperature where the magnetism is appearing. Improper ferroelectric mechanism has been proposed to overcome this problem in layered perovskites but it appeared that the coupling between the polarization and octahedra rotations is not robust and too malleable to systematically drive a strong coupling between the polarization and the weak magnetization induced by the octahedral rotation. Herewith, we report from first-principles calculations an ideal magnetization reversal through polarization switching in BaCuF4, which could be achieved close to room temperature. We show that this ideal coupling is present because it is driven by a single soft mode that embeds both polarization and octahedral rotation together and thus does not rely on improper coupling. This gives a perfect and ideal direct coupling between the polarization and the weak ferromagnetism present in BaCuF4. Our results allow thus to give a new direction in searching for good magnetoelectrics where electric field can tune the magnetization.

Resume : Multiferroics are promising candidates for future multifunctional devices such as four-state memories, magnetoelectric sensors. Since single phase multiferroics usually show weak magnetoelectric (ME) coupling, recently, self-assembled heterostructure films have aroused enormous research interest due to their strong ME coupling which is highly desired for practical devices. Here, we report a novel architecture of 3D structured ferromagnetic CoFe2O4 (CFO) nanocomposites embedded in ferroelectric Bi3.25La0.75Ti3O12 (BLT) with emerging strong ME effect. In order to fabricate such architecture films, we used Co, Fe incorporated BLT as a target for pulsed laser deposition. It is revealed nucleation and agglomeration of second phase CFO in BLT matrix occur at a critical thickness of ~10nm, and finally nanocup shaped composites are formed. This nanocup heterostructure film had a relatively large interface density because the both phases were connected epitaxially to each other with lateral and vertical lattice strains. Furthermore, CFO nucleation inside the BLT matrix led into excellent interface coherence and relaxed substrate clamping effect. Using our novel heterostructure films, we successfully demonstrated magnetically controlled dielectric switching with showing room temperature multiferroism. We believe our approach to form unique heterostructure may be applied to fabricate a series of nanocup heterostructure films with diverse functionalities.

Resume : The promise of novel domain-wall based nanoelectronic devices has recently motivated much work concerning strain-engineered ferroelectrics with dense ferroelastic domain structures [1]. Certain thin film systems display an hierarchal organization, whereby ferroelastic domains arrange in distinct ?superdomain? bundles [2,3]. The behavior of these superdomain structures under device-like conditions is still unclear; studies to date have been confined to local Piezoresponse Force Microscopy (PFM) imaging under highly inhomogeneous electric fields, or large area X-ray diffraction. Here we employ synchrotron Scanning X-ray NanoDiffraction (SXND) [4] to shed new light on the structure of an hierarchal ferroelectric thin film, as well as to investigate its response to in-plane applied electric field via a set of interdigititated electrodes deposited on its surface. We use PbTiO3 (PTO) // KTaO3 (KTO) thin films as a prototype system for this study due to the peculiar mixture of in-plane and out-of-plane domain bundles (the so-called ?a1a2/c? structure) that PTO exhibits at the value of misfit strain resulting from deposition onto KTO substrates. SXND allows us to non-invasively probe the long-range buried domain structure of the thin film with approximately 60nm resolution [5], and reveals a highly twinned ferroelastic arrangement resulting in the PTO unit cells being tilted in several directions with respect to the substrate surface. Comparison with PFM data allows us to describe the variations in polarization direction based on such tilts. We are then able to track these tilts across the film as a function of in-plane field, observing 90º domain wall rotations to accommodate 180º switching of the in-plane polarization component as the applied field polarity is inverted. PFM data collected on the same regions corroborates our interpretation. References [1] Martin, L. W., et al. (2017) Nature Reviews Materials, 2(2), p.16087 [2] Matzen, S., et al. (2014) Nature Communications, 5, 4415 [3] Chang, L. W., et al. (2013) Nano Letters, 13(6), 2553?2557 [4] Chahine, G. A., et al. (2014) J. Appl. Cryst. 47, 762?769 [5] S.J. Leak et al. Materials & Design, 119, 2017, 470-47

Resume : The problem of phase transitions (PT) in the PbZrO3 like antiferroelectric (PZ) single crystals has been studied for several years and it is still a challenging task because of the presence of competing interactions at PT between two instabilities: ferroelectric (FE) and antiferroelectric one (AFE). Recently an inelastic x-ray scattering (IXS) study of the critical dynamics revealed that the paraelectric phase of PZ is compatible with that expected for the crystals with an incommensurate (IC) phase. It reveals the possibility of formation of IC or AFE phases in the course of the first-order PT, although the IXS (Phys. Rev. B 90 144301 (2014)) data do not tell whether the AFE, IC or FE phase shall form. Among the other experimental techniques that are able to help us solve problem of PT mechanism and existence of the IC phase in PZ, appears to be Mössbauer spectroscopy (MS). Mössbauer spectra of materials can be recorded under a large range of conditions, including temperature changes. This enables in situ observations of small changes in atomic environment before, during and after phase transformations. MS was investigated for two AFE single crystals: PbZrO3 and PbHfO3 modified by Sn ions (PZS and PHS respectively). The temperature evolution of the MS for PZS and PHS showed clearly that there are two quadrupole dublets corresponding to normal phase-NC and IC phase. Thus the MS measurements confirms the existence of IC phases both in PZS as well as PHS single crystals.