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Morozovska, Anna N.,Eliseev, Eugene A.,Genenko, Yuri A.,Vorotiahin, Ivan S.,Silibin, Maxim V.,Cao, Ye,Kim, Yunseok,Glinchuk, Maya D.,Kalinin, Sergei V. American Physical Society 2016 Physical Review B Vol.94 No.17
<P>We explore the role of flexoelectric effect in functional properties of nanoscale ferroelectric films with mixed electronic-ionic conductivity. Using a coupled Ginzburg-Landau model, we calculate spontaneous polarization, effective piezoresponse, elastic strain and compliance, carrier concentration, and piezoconductance as a function of thickness and applied pressure. In the absence of flexoelectric coupling, the studied physical quantities manifest well-explored size-induced phase transitions, including transition to paraelectric phase below critical thickness. Similarly, in the absence of external pressure flexoelectric coupling affects properties of these films only weakly. However, the combined effect of flexoelectric coupling and external pressure induces polarizations at the film surfaces, which cause the electric built-in field that destroys the thickness-induced phase transition to paraelectric phase and induces the electretlike state with irreversible spontaneous polarization below critical thickness. Interestingly, the built-in field leads to noticeable increase of the average strain and elastic compliance in this thickness range. We further illustrate that the changes of the electron concentration by several orders of magnitude under positive or negative pressures can lead to the occurrence of high-or low-conductivity states, i.e., the nonvolatile piezoresistive switching, in which the swing can be controlled by the film thickness and flexoelectric coupling. The obtained theoretical results can be of fundamental interest for ferroic systems, and can provide a theoretical model for explanation of a set of recent experimental results on resistive switching and transient polar states in these systems.</P>
Kim, Young-Min,Morozovska, Anna,Eliseev, Eugene,Oxley, Mark P.,Mishra, Rohan,Selbach, Sverre M.,Grande, Tor,Pantelides, S. T.,Kalinin, Sergei V.,Borisevich, Albina Y. Nature Publishing Group, a division of Macmillan P 2014 NATURE MATERIALS Vol.13 No.11
The development of interface-based magnetoelectric devices necessitates an understanding of polarization-mediated electronic phenomena and atomistic polarization screening mechanisms. In this work, the LSMO/BFO interface is studied on a single unit-cell level through a combination of direct order parameter mapping by scanning transmission electron microscopy and electron energy-loss spectroscopy. We demonstrate an unexpected ~5% lattice expansion for regions with negative polarization charge, with a concurrent anomalous decrease of the Mn valence and change in oxygen K-edge intensity. We interpret this behaviour as direct evidence for screening by oxygen vacancies. The vacancies are predominantly accumulated at the second atomic layer of BFO, reflecting the difference of ionic conductivity between the components. This vacancy exclusion from the interface leads to the formation of a tail-to-tail domain wall. At the same time, purely electronic screening is realized for positive polarization charge, with insignificant changes in lattice and electronic properties. These results underline the non-trivial role of electrochemical phenomena in determining the functional properties of oxide interfaces. Furthermore, these behaviours suggest that vacancy dynamics and exclusion play major roles in determining interface functionality in oxide multilayers, providing clear implications for novel functionalities in potential electronic devices.
Chang, Hye Jung,Kalinin, Sergei V.,Morozovska, Anna N.,Huijben, Mark,Chu, Ying‐,Hao,Yu, Pu,Ramesh, Ramamoorthy,Eliseev, Evgeny A.,Svechnikov, George S.,Pennycook, Stephen J.,Borisevich, Albina Y WILEY‐VCH Verlag 2011 Advanced Materials Vol.23 No.21
<P><B>Direct atomic displacement mapping at ferroelectric interfaces</B> by aberration corrected scanning transmission electron microscopy(STEM) (a‐STEM image, b‐corresponding displacement profile) is combined with Landau‐Ginsburg‐Devonshire theory to obtain the complete interface electrostatics in real space, including separate estimates for the polarization and intrinsic interface charge contributions. </P>
Mechanical Control of Electroresistive Switching
Kim, Yunseok,Kelly, Simon J.,Morozovska, Anna,Rahani, Ehsan Kabiri,Strelcov, Evgheni,Eliseev, Eugene,Jesse, Stephen,Biegalski, Michael D.,Balke, Nina,Benedek, Nicole,Strukov, Dmitri,Aarts, J.,Hwang, I American Chemical Society 2013 Nano letters Vol.13 No.9
<P>Hysteretic metal–insulator transitions (MIT) mediated by ionic dynamics or ferroic phase transitions underpin emergent applications for nonvolatile memories and logic devices. The vast majority of applications and studies have explored the MIT coupled to the electric field or temperarture. Here, we argue that MIT coupled to ionic dynamics should be controlled by mechanical stimuli, the behavior we refer to as the piezochemical effect. We verify this effect experimentally and demonstrate that it allows both studying materials physics and enabling novel data storage technologies with mechanical writing and current-based readout.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2013/nalefd.2013.13.issue-9/nl401411r/production/images/medium/nl-2013-01411r_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl401411r'>ACS Electronic Supporting Info</A></P>
Kalinin, Sergei V,Kim, Yunseok,Fong, Dillon D,Morozovska, Anna N IOP 2018 Reports on progress in physics Vol.81 No.3
<P>For over 70 years, ferroelectric materials have been one of the central research topics for condensed matter physics and material science, an interest driven both by fundamental science and applications. However, ferroelectric surfaces, the key component of ferroelectric films and nanostructures, still present a significant theoretical and even conceptual challenge. Indeed, stability of ferroelectric phase <I>per se</I> necessitates screening of polarization charge. At surfaces, this can lead to coupling between ferroelectric and semiconducting properties of material, or with surface (electro) chemistry, going well beyond classical models applicable for ferroelectric interfaces. In this review, we summarize recent studies of surface-screening phenomena in ferroelectrics. We provide a brief overview of the historical understanding of the physics of ferroelectric surfaces, and existing theoretical models that both introduce screening mechanisms and explore the relationship between screening and relevant aspects of ferroelectric functionalities starting from phase stability itself. Given that the majority of ferroelectrics exist in multiple-domain states, we focus on local studies of screening phenomena using scanning probe microscopy techniques. We discuss recent studies of static and dynamic phenomena on ferroelectric surfaces, as well as phenomena observed under lateral transport, light, chemical, and pressure stimuli. We also note that the need for ionic screening renders polarization switching a coupled physical–electrochemical process and discuss the non-trivial phenomena such as chaotic behavior during domain switching that stem from this.</P>
Interplay of Octahedral Tilts and Polar Order in BiFeO<sub>3</sub> Films
Kim, Young‐,Min,Kumar, Amit,Hatt, Alison,Morozovska, Anna N.,Tselev, Alexander,Biegalski, Michael D.,Ivanov, Ilya,Eliseev, Eugene A.,Pennycook, Stephen J.,Rondinelli, James M.,Kalinin, Sergei V. WILEY‐VCH Verlag 2013 ADVANCED MATERIALS Vol.25 No.17
<P><B>Heterointerface stabilization of a distinct nonpolar BiFeO<SUB>3</SUB> phase</B> occurs simultaneously with changes in octahedral tilts. The resulting phase arises via suppression of polarization by a structural order parameter and can thus be identified as anti‐ferroelectric (Fe displacements – bottom panel). The phase is metastable and can be switched into a polar ferroelectric state (top panel) under an applied electric bias.</P>