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Fomichov YM, Yudin PV, Tyunina M, Dejneka A. Theory for anisotropic local ferroelectric switching. Nanotechnology 2023; 35:04LT01. [PMID: 37863078 DOI: 10.1088/1361-6528/ad0595] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/11/2023] [Accepted: 10/20/2023] [Indexed: 10/22/2023]
Abstract
Theoretical modeling of polarization switching around a biased tip contact is important for fundamental understanding and advanced applications of ferroelectrics. Here we propose a simple in-plane two-dimensional model that considers surface charge transport and the associated evolution of the electric field driving domain growth. The model reproduces peculiar domain shapes ranging from round to faceted in KTiOPO4(C2vsymmetry) and LiNbO3(C3vsymmetry). This is done through modulation of dielectric permittivity, which mimics domain wall pinning on the lattice. In contrast to previous works, which attempted to justify domain anisotropy by means of point symmetry invariants, here we illustrate the necessity of taking translational symmetry into account. The results are pertinent to ferroelectric racetrack memories and other applications requiring domain tailoring.
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Affiliation(s)
- Y M Fomichov
- Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 18221 Praha 8, Czech Republic
| | - P V Yudin
- Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 18221 Praha 8, Czech Republic
- Kutateladze Institute of Thermophysics, Siberian Branch of Russian Academy of Science, Lavrent'eva av. 1, Novosibirsk, Russia
| | - M Tyunina
- Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 18221 Praha 8, Czech Republic
- Microelectronics Research Unit, Faculty of Information Technology and Electrical Engineering, University of Oulu, PO Box 4500, FI-90014 Oulu, Finland
| | - A Dejneka
- Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 18221 Praha 8, Czech Republic
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2
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Tyunina M, Savinov M, Pacherova O, Dejneka A. Small-polaron transport in perovskite nickelates. Sci Rep 2023; 13:12493. [PMID: 37528184 PMCID: PMC10394062 DOI: 10.1038/s41598-023-39821-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2023] [Accepted: 07/31/2023] [Indexed: 08/03/2023] Open
Abstract
Knowledge of the explicit mechanisms of charge transport is preeminent for a fundamental understanding of the metal-to-insulator transition in ABO3-type perovskite rare-earth nickelates and for potential applications of these technologically promising materials. Here we suggest that owing to intrinsic Jahn-Teller-driven carrier localization, small-polaron transport is innate in nickelates. We demonstrate experimental evidence for such transport by investigating AC conductivity over a broad range of temperatures and frequencies in epitaxial SmNiO3 films. We reveal the hopping mechanism of conductivity, Holstein-type activation energy for hopping, nonclassical relaxation behavior, and nonclassical consistency between activation and relaxation. By analyzing these observations, we validate small-polaron transport. We anticipate that our findings can lead to precise tailoring of the DC and AC conductivity in nickelates as requested for fruitful employment of these materials. We also believe that further investigations of self-trapped small polarons are essential for a comprehensive understanding of nickelates.
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Affiliation(s)
- M Tyunina
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18220, Prague, Czech Republic.
- Microelectronics Research Unit, Faculty of Information Technology and Electrical Engineering, University of Oulu, P. O. Box 4500, Fl-90014, Oulu, Finland.
| | - M Savinov
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18220, Prague, Czech Republic
| | - O Pacherova
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18220, Prague, Czech Republic
| | - A Dejneka
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18220, Prague, Czech Republic
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3
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Rusevich LL, Tyunina M, Kotomin EA, Nepomniashchaia N, Dejneka A. The electronic properties of SrTiO 3-δ with oxygen vacancies or substitutions. Sci Rep 2021; 11:23341. [PMID: 34857848 PMCID: PMC8639995 DOI: 10.1038/s41598-021-02751-9] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2021] [Accepted: 11/17/2021] [Indexed: 12/14/2022] Open
Abstract
The electronic properties, including bandgap and conductivity, are critical for nearly all applications of multifunctional perovskite oxide ferroelectrics. Here we analysed possibility to induce semiconductor behaviour in these materials, which are basically insulators, by replacement of several percent of oxygen atoms with nitrogen, hydrogen, or vacancies. We explored this approach for one of the best studied members of the large family of ABO3 perovskite ferroelectrics - strontium titanate (SrTiO3). The atomic and electronic structure of defects were theoretically investigated using the large-scale first-principles calculations for both bulk crystal and thin films. The results of calculations were experimentally verified by studies of the optical properties at photon energies from 25 meV to 8.8 eV for in-situ prepared thin films. It was demonstrated that substitutions and vacancies prefer locations at surfaces or phase boundaries over those inside crystallites. At the same time, local states in the bandgap can be produced by vacancies located both inside the crystals and at the surface, but by nitrogen substitution only inside crystals. Wide-bandgap insulator phases were evidenced for all defects. Compared to pure SrTiO3 films, bandgap widening due to defects was theoretically predicted and experimentally detected.
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Affiliation(s)
- L L Rusevich
- Institute of Solid State Physics, University of Latvia, Kengaraga Str. 8, Riga, 1063, Latvia.
| | - M Tyunina
- Microelectronics Research Unit, Faculty of Information Technology and Electrical Engineering, University of Oulu, P. O. Box 4500, 90014, Oulu, Finland.
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221, Prague, Czech Republic.
| | - E A Kotomin
- Institute of Solid State Physics, University of Latvia, Kengaraga Str. 8, Riga, 1063, Latvia
- Max Planck Institute for Solid State Research, Heisenberg Str. 1, 70569, Stuttgart, Germany
| | - N Nepomniashchaia
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221, Prague, Czech Republic
| | - A Dejneka
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221, Prague, Czech Republic
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4
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Tyunina M, Pacherova O, Kocourek T, Dejneka A. Anisotropic chemical expansion due to oxygen vacancies in perovskite films. Sci Rep 2021; 11:15247. [PMID: 34315921 PMCID: PMC8316387 DOI: 10.1038/s41598-021-93968-1] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2021] [Accepted: 06/29/2021] [Indexed: 11/09/2022] Open
Abstract
In scientifically intriguing and technologically important multifunctional ABO3 perovskite oxides, oxygen vacancies are most common defects. They cause lattice expansion and can alter the key functional properties. Here, it is demonstrated that contrary to weak isotropic expansion in bulk samples, oxygen vacancies produce strong anisotropic strain in epitaxial thin films. This anisotropic chemical strain is explained by preferential orientation of elastic dipoles of the vacancies. Elastic interaction of the dipoles with substrate-imposed misfit strain is suggested to define the dipolar orientation. Such elastic behavior of oxygen vacancies is anticipated to be general for perovskite films and have critical impacts on the film synthesis and response functions.
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Affiliation(s)
- M Tyunina
- Microelectronics Research Unit, Faculty of Information Technology and Electrical Engineering, University of Oulu, P. O. Box 4500, 90014, Oulu, Finland.
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221, Prague, Czech Republic.
| | - O Pacherova
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221, Prague, Czech Republic
| | - T Kocourek
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221, Prague, Czech Republic
| | - A Dejneka
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221, Prague, Czech Republic
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5
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Tyunina M. Conductivity in Ferroelectric Barium Titanate: Electrons Versus Oxygen Vacancies. IEEE Trans Ultrason Ferroelectr Freq Control 2021; 68:296-302. [PMID: 32149630 DOI: 10.1109/tuffc.2020.2978901] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Mobile oxygen vacancies are increasingly widely believed to be responsible for electrical conductivity in perovskite oxide ferroelectrics. Here, this hypothesis is debated. The small-signal conductivity is investigated in oxygen-deficient films of barium titanate, where oxygen vacancies are epitaxially clamped and immobile. The observed behavior of conductivity as a function of temperature and frequency evidences pure electronic processes. Importantly, it is shown that these processes mimic motion of oxygen vacancies, which are immobile. It is also demonstrated that under the applied dc electric field, the electronic processes lead to such effects as coloration and degradation, which before were plausibly ascribed to migration of oxygen vacancies. Finally, it is concluded that the hypothesis of mobile oxygen vacancies is misleading.
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Yudin P, Shapovalov K, Sluka T, Peräntie J, Jantunen H, Dejneka A, Tyunina M. Mobile and immobile boundaries in ferroelectric films. Sci Rep 2021; 11:1899. [PMID: 33479382 PMCID: PMC7820330 DOI: 10.1038/s41598-021-81516-w] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2020] [Accepted: 01/05/2021] [Indexed: 11/17/2022] Open
Abstract
The intrinsic mobile interfaces in ferroelectrics—the domain walls can drive and enhance diverse ferroelectric properties, essential for modern applications. Control over the motion of domain walls is of high practical importance. Here we analyse theoretically and show experimentally epitaxial ferroelectric films, where mobile domain walls coexist and interact with immobile growth-induced interfaces—columnar boundaries. Whereas these boundaries do not disturb the long-range crystal order, they affect the behaviour of domain walls in a peculiar selective manner. The columnar boundaries substantially modify the behaviour of non-ferroelastic domains walls, but have negligible impact on the ferroelastic ones. The results suggest that introduction of immobile boundaries into ferroelectric films is a viable method to modify domain structures and dynamic responses at nano-scale that may serve to functionalization of a broader range of ferroelectric films where columnar boundaries naturally appear as a result of the 3D growth.
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Affiliation(s)
- P Yudin
- Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 18221, Praha 8, Czech Republic. .,Kutateladze Institute of Thermophysics, Siberian Branch of Russian Academy of Science, Lavrent'eva av. 1, Novosibirsk, Russia.
| | - K Shapovalov
- Institutut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus UAB, 08193, Bellaterra, Spain.,CNRS, Université de Bordeaux, ICMCB, UPR, 9048, 33600, Pessac, France
| | - T Sluka
- CREAL SA, Chemin du Paqueret 1A, CH-1025, Saint-Sulpice, Switzerland
| | - J Peräntie
- Microelectronics Research Unit, University of Oulu, P.O. Box 4500, 90014, Oulu, Finland
| | - H Jantunen
- Microelectronics Research Unit, University of Oulu, P.O. Box 4500, 90014, Oulu, Finland
| | - A Dejneka
- Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 18221, Praha 8, Czech Republic
| | - M Tyunina
- Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 18221, Praha 8, Czech Republic.,Microelectronics Research Unit, University of Oulu, P.O. Box 4500, 90014, Oulu, Finland
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7
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Tyunina M, Pacherova O, Nepomniashchaia N, Vetokhina V, Cichon S, Kocourek T, Dejneka A. In situ anion-doped epitaxial strontium titanate films. Phys Chem Chem Phys 2020; 22:24796-24800. [PMID: 33107506 DOI: 10.1039/d0cp03644g] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Misfit strains arising from a film-substrate mismatch can induce novel phases and properties in the epitaxial films of perovskite oxides. Here we employ yet another effect, namely, strain-assisted formation of oxygen vacancies. We demonstrate the misfit-promoted presence of oxygen vacancies and related substitutional incorporation of anion dopants in the epitaxial films of archetypal perovskite oxide SrTiO3. Both the oxygen vacancies and hydrogen or nitrogen dopants are introduced in situ during the pulsed-laser deposition of the films using compressive substrates. The films exhibit peculiar chemical expansion and optical properties, which are consistent with substitutional anion doping.
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Affiliation(s)
- M Tyunina
- Microelectronics Research Unit, Faculty of Information Technology and Electrical Engineering, University of Oulu, P. O. Box 4500, FI-90014, Finland.
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Abstract
Allying epitaxial strain and synthesis conditions allows for the introduction of specific point defects in perovskite oxide films. In ferroelectric films, such defects lead to essential polar and electronic properties, which can enable advanced applications. Here, to elucidate the nature of the defects, optical constants are investigated in the spectral range of 0.7-8.8 eV in epitaxial ferroelectric BaTiO3 films, which are synthesized under different conditions. It is demonstrated that oxygen-vacancy-related defects are responsible for a peculiar transition below the bandgap at ∼2.7-2.9 eV and significant blueshifts of ∼0.3-0.4 eV of the gap and the main interband transitions. These observations suggest that the defects are dipolar complexes comprising titanium cations and oxygen vacancies (Ti3+-VO).
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Affiliation(s)
- M Tyunina
- Microelectronics Research Unit, Faculty of Information Technology and Electrical Engineering, University of Oulu, P. O. Box 4500, FI-90014, Finland.
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9
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Tyunina M, Pacherova O, Peräntie J, Savinov M, Jelinek M, Jantunen H, Dejneka A. Perovskite ferroelectric tuned by thermal strain. Sci Rep 2019; 9:3677. [PMID: 30842509 PMCID: PMC6403324 DOI: 10.1038/s41598-019-40260-y] [Citation(s) in RCA: 21] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2018] [Accepted: 02/12/2019] [Indexed: 12/05/2022] Open
Abstract
Modern environmental and sustainability issues as well as the growing demand for applications in the life sciences and medicine put special requirements to the chemical composition of many functional materials. To achieve desired performance within these requirements, innovative approaches are needed. In this work, we experimentally demonstrate that thermal strain can effectively tune the crystal structure and versatile properties of relatively thick films of environmentally friendly, biocompatible, and low-cost perovskite ferroelectric barium titanate. The strain arises during post-deposition cooling due to a mismatch between the thermal expansion coefficients of the films and the substrate materials. The strain-induced in-plane polarization enables excellent performance of bottom-to-top barium titanate capacitors akin to that of exemplary lead-containing relaxor ferroelectrics. Our work shows that controlling thermal strain can help tailor response functions in a straightforward manner.
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Affiliation(s)
- M Tyunina
- Microelectronics Research Unit, University of Oulu, P.O. Box 4500, FI-90014, Oulu, Finland. .,Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221, Prague, Czech Republic.
| | - O Pacherova
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221, Prague, Czech Republic
| | - J Peräntie
- Microelectronics Research Unit, University of Oulu, P.O. Box 4500, FI-90014, Oulu, Finland
| | - M Savinov
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221, Prague, Czech Republic
| | - M Jelinek
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221, Prague, Czech Republic
| | - H Jantunen
- Microelectronics Research Unit, University of Oulu, P.O. Box 4500, FI-90014, Oulu, Finland
| | - A Dejneka
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221, Prague, Czech Republic
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Yao L, Inkinen S, Pacherova O, Jelinek M, van Dijken S, Tyunina M. Chemical-bond effect on epitaxial strain in perovskite sodium niobate. Phys Chem Chem Phys 2018; 20:4263-4268. [PMID: 29364292 DOI: 10.1039/c7cp08449h] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Epitaxial films and heterostructures of perovskite oxides attract tremendous scientific interest because of the unique phenomena therein. Especially important is the epitaxial growth of films subjected to substrate-induced misfit strain. We show here that in contrast to conventional misfit-controlled epitaxy, chemical bonds determine the crystal stability and strain in epitaxial films of sodium niobate on different cubic substrates. Strain relaxation in sodium niobate is independent of misfit magnitude and proceeds through perovskite-specific tilting of oxygen octahedra in addition to common defect formation. The observed structural relaxation evidences a major role of a large internal strain that originates from chemical bonds in the perovskite cell. The effect of chemical bonds on film strain is anticipated to also control the epitaxy of other perovskite oxides and related compounds.
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Affiliation(s)
- L Yao
- NanoSpin, Department of Applied Physics, Aalto University School of Science, P. O. Box 15100, FI-00076 Aalto, Finland
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11
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Chernova E, Pacherova O, Kocourek T, Jelinek M, Dejneka A, Tyunina M. Optical Properties of Ferroelectric Epitaxial K0.5Na0.5NbO3 Films in Visible to Ultraviolet Range. PLoS One 2016; 11:e0153261. [PMID: 27074042 PMCID: PMC4830626 DOI: 10.1371/journal.pone.0153261] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2016] [Accepted: 03/26/2016] [Indexed: 12/02/2022] Open
Abstract
The complex index of refraction in the spectral range of 0.74 to 4.5 eV is studied by variable-angle spectroscopic ellipsometry in ferroelectric K0.5Na0.5NbO3 films. The 20-nm-thick cube-on-cube-type epitaxial films are grown on SrTiO3(001) and DyScO3(011) single-crystal substrates. The films are transparent and exhibit a significant difference between refractive indices Δn = 0.5 at photon energies below 3 eV. The energies of optical transitions are in the range of 3.15–4.30 eV and differ by 0.2–0.3 eV in these films. The observed behavior is discussed in terms of lattice strain and strain-induced ferroelectric polarization in epitaxial perovskite oxide films.
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Affiliation(s)
- E. Chernova
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, Prague 8, Czech Republic, 18221
- Czech Technical University, Technicka 2, Prague 6, Czech Republic, 166 27
- * E-mail: (EC); (MT)
| | - O. Pacherova
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, Prague 8, Czech Republic, 18221
| | - T. Kocourek
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, Prague 8, Czech Republic, 18221
| | - M. Jelinek
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, Prague 8, Czech Republic, 18221
| | - A. Dejneka
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, Prague 8, Czech Republic, 18221
| | - M. Tyunina
- Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, Prague 8, Czech Republic, 18221
- Microelectronics and Materials Physics Laboratories, University of Oulu, P. O. Box 4500, FI-90014 Oulun yliopisto, Finland
- * E-mail: (EC); (MT)
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Tyunina M, Pintilie I, Iuga A, Stratulat MS, Pintilie L. Frustration of ferroelectricity in epitaxial film of relaxor ferroelectric PbSc1/2Nb1/2O3. J Phys Condens Matter 2014; 26:325901. [PMID: 25030065 DOI: 10.1088/0953-8984/26/32/325901] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
Relaxor-to-ferroelectric transformations induced by varying electric fields and temperatures are studied experimentally in acube-on-cubetype epitaxial PbSc(1/2)Nb(1/2)O3 film grown on La(1/2)Sr(1/2)CoO3/MgO(001). Dielectric response, quasi-static and dynamic polarization, and dynamic current-voltage characteristics evidence the absence of spontaneous relaxor-to-ferroelectric transition. The electricfield-induced transformation from a glass-like relaxor state to a new dynamic polar state is detected at low temperatures below 100 K only. The frustration of ferroelectricity is discussed in relation to orientational anisotropy of the dipolar system in the epitaxial (001) film.
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Affiliation(s)
- M Tyunina
- Microelectronics and Materials Physics Laboratories, University of Oulu, PO Box 4500, FI-90014 Oulunyliopisto, Finland
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13
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Tyunina M, Dejneka A, Rytz D, Gregora I, Borodavka F, Vondracek M, Honolka J. Ferroelectricity in antiferroelectric NaNbO3 crystal. J Phys Condens Matter 2014; 26:125901. [PMID: 24594846 DOI: 10.1088/0953-8984/26/12/125901] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
Sodium niobate (NaNbO3, or NNO) is known to be antiferroelectric at temperatures between 45 and 753 K. Here we show experimentally the presence of the ferroelectric phase at temperatures between 100 and 830 K in the NNO crystals obtained by top-seeded solution growth. The ferroelectric phase and new phase transitions are evidenced using a combination of thermo-optical studies by variable angle spectroscopic ellipsometry, Raman spectroscopy analysis, and photoelectron emission microscopy. The possibility for strain-induced ferroelectricity in NNO is suggested.
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Affiliation(s)
- M Tyunina
- Microelectronics and Materials Physics Laboratories, University of Oulu, PO Box 4500, FI-90014 Oulun yliopisto, Finland. Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic
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14
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Oja R, Tyunina M, Yao L, Pinomaa T, Kocourek T, Dejneka A, Stupakov O, Jelinek M, Trepakov V, van Dijken S, Nieminen RM. d0 ferromagnetic interface between nonmagnetic perovskites. Phys Rev Lett 2012; 109:127207. [PMID: 23005984 DOI: 10.1103/physrevlett.109.127207] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/04/2012] [Indexed: 06/01/2023]
Abstract
We use computational and experimental methods to study d(0) ferromagnetism at a charge-imbalanced interface between two perovskites. In SrTiO(3)/KTaO(3) superlattice calculations, the charge imbalance introduces holes in the SrTiO(3) layer, inducing a d(0) ferromagnetic half-metallic 2D hole gas at the interface oxygen 2p orbitals. The charge imbalance overrides doping by vacancies at realistic concentrations. Varying the constituent materials shows ferromagnetism to be a general property of hole-type d(0) perovskite interfaces. Atomically sharp epitaxial d(0) SrTiO(3)/KTaO(3), SrTiO(3)/KNbO(3), and SrTiO(3)/NaNbO(3) interfaces are found to exhibit ferromagnetic hysteresis at room temperature. We suggest that the behavior is due to the high density of states and exchange coupling at the oxygen t(1g) band in comparison with the more studied d band t(2g) symmetry electron gas.
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Affiliation(s)
- R Oja
- COMP Centre of Excellence, Department of Applied Physics, Aalto University, Helsinki, Finland
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15
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Tyunina M, Narkilahti J, Plekh M, Oja R, Nieminen RM, Dejneka A, Trepakov V. Evidence for strain-induced ferroelectric order in epitaxial thin-film KTaO3. Phys Rev Lett 2010; 104:227601. [PMID: 20867203 DOI: 10.1103/physrevlett.104.227601] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/31/2010] [Indexed: 05/29/2023]
Abstract
In perovskite-structure epitaxial films, it has been theoretically predicted that the polarization and the coherence of polar order can increase with increasing crystallographic strain. Experimental evidence of strain-induced long-range ferroelectric order has not been obtained thus far, posing the fundamental question of whether or not strain can induce the long-range polar order. Here we demonstrate the existence of strain-induced ferroelectric order in quantum paraelectric KTaO3 by combining experimental investigations of epitaxial KTaO3 films and density-functional-theory calculations. The long-range ferroelectric order does exist under a large enough epitaxial strain. We suggest that a region of short-range polar order might appear between paraelectric and ferroelectric states in the strain-temperature phase diagrams.
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Affiliation(s)
- M Tyunina
- Microelectronics and Materials Physics Laboratories, University of Oulu, P.O. Box 4500, FI-90014 Oulun yliopisto, Finland
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16
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Tyunina M, Narkilahti J, Levoska J, Chvostova D, Dejneka A, Trepakov V, Zelezny V. Ultrathin SrTiO(3) films: epitaxy and optical properties. J Phys Condens Matter 2009; 21:232203. [PMID: 21825577 DOI: 10.1088/0953-8984/21/23/232203] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
Ultrathin (12-15 nm) SrTiO(3) films are grown by pulsed laser deposition on various single-crystal substrates. The crystal structure, orientation, and strain state of the films are studied by x-ray diffraction. The room-temperature optical properties of the films are experimentally determined using ellipsometric spectroscopy in the 1-6 eV spectral range. Epitaxial films with biaxial in-plane strain are obtained on LaAlO(3), DyScO(3), and KTaO(3) substrates, with the critical thickness for pseudomorphic growth being less than 10 nm. Abrupt strain relaxation has been detected. The optical properties of the films with different microstructure are compared with each other and with those of single-crystal SrTiO(3). Based on the comparison, surface effects are suggested to be dominant in the visible range, while the interband transitions are smeared and suppressed due to small film thickness and the presence of biaxial strain. The energies of gaps can increase due to strain-induced polarization. The absorption edge is affected by all the factors mentioned (surface, thickness, strain, and polarization).
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Affiliation(s)
- M Tyunina
- Microelectronics and Materials Physics Laboratories, University of Oulu, PL4500, FI-90014 Oulun yliopisto, Finland
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