Maksimo Ivanovo daktaro disertacijos tema yra “Grūdų dydžio įtaka dielektrinėms feroelektrikų ir relaksorių savybėms”. Šio darbo tikslas yra ištirti, kaip keramikų ar miltelių grūdų dydis įtakoja makroskopiškai stebimas kelių feroelektrikų bei feroelektrinių relaksorių dielektrines savybes. Tirtos medžiagos yra klasikinio relaksoriuas PbMg⅓Nb⅔O3 (PMN) milteliai, relaksoriaus su savaiminiu feroelektriniu faziniu virsmu PbSc½Nb½O3 (PSN) keramikos, feroelektriko 0.36BiScO3-0.64PbTiO3 keramikos, bei Ba2SnO4 keramiką, kuri buvo palyginta su kiek labiau žinoma BaSnO3 keramika. Tyrimai buvo atlikti plačiame dažnių (100 Hz – 55 GHz) bei temperatūrų (30 K – 1000 K) intervaluose. Eksperimentiniai tyrimai bei modeliavimai parodė, kad polinės nanosritys labai stipriai įtakoja tūrines relaksorių savybes, tačiau jų neapsprendžia. Medžiagos morfologija (antai keramikų grūdų dydis) lemia nanosričių augimą bei tarpusavio sąveikas, tokiu būdų įtakojamos tūrinės savybės. Be to, efektyvios terpės aproksimacija gali paaiškinti feroelektrikų ir relaksorių dielektrinių savybių priklausomybę nuo grūdų dydžio tik, jei žinomas sąryšis tarp dydžio ir tūrinių savybių. Įdomiausia yra tai, kad feroelektrikuose yra objektų, kurie nėra feroelektriniai domenai ir yra panašūs į relaksorių polines nanosritis. Jų indėlis į dielektrinę skvarbą gali būti palyginamas su visų kitų procesų (pvz. domenų sienelių ir polinių modų) indėliais. Šių objektų dydis priklauso nuo grūdų dydžio pagal Kittel'io dėsnį. / The aim of doctoral dissertation „Grain Size Effect on Dielectric Properties of Ferroelectrics and Relaxors“ by Maksim Ivanov is to investigate, how grain size of ceramics and powders of a few ferroelectrics and relaxors influences macroscopic dielectric properties. The studied materials are powders of a relaxor PbMg⅓Nb⅔O3 (PMN), ceramics of a relaxor with a spontaneous phase transition PbSc½Nb½O3 (PSN), ceramics of a ferroelectric 0.36BiScO3-0.64PbTiO3, and ceramics of Ba2SnO4, which were compared to a better investigated BaSnO3. Investigations were performed in broad frequency (100 Hz – 55 GHz) and temperature (30 K – 1000 K) ranges. Experimental investigations and modelling showed, that bulk properties of relaxor materials are heavily influenced by polar nanoregions, but they do not fully determine them. Morphology of the material (i.e. grain size of ceramics) determines growth and interactions of the nanoregions, thus influencing bulk properties. Moreover, effective medium approximation can explain evolution of dielectric properties of ferroelectrics and relaxors only if dependence of bulk properties on grain size is known. The most interesting result is, that there exist polar entities in ferroelectrics, which are different from ferroelectric domains and are similar to polar nanoregions in relaxors. Their contribution to dielectric permittivity can be comparable to all other contributions. Dimensions of these entities depend on grain size in accordance with Kittel's law.

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No. of bitstreams: 1 teseDSFV.pdf: 5669668 bytes, checksum: 165ca1333f2345e5e1406b40a2c0548c (MD5) Previous issue date: 2016-04-08 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / In the case of composites materials, which presents the magnetoelectric coupling, there are so many questions about the influence of the microstructure on the coupled and uncoupled properties. Thereby, this work proposes the study of the grain size influence, of ferroelectric and magnetic phases, on the magnetic, electric and coupled properties in particulate magnetoelectric composites. In order to study the relationship property/microstructure in volumetric magnetoelectric composites, it was necessary to prepare samples with different average grain sizes, ie, the phases having average grain size in the order of micrometers or nanometers. To perform this study it was used as magnetic phase the CoFe2O4 and, as the ferroelectric phase, the [0,675] Pb(Mg1/3Nb2/3)O3 – [0,325] PbTiO3, mixed in molar ratio 20/80 %, using techniques like micro ball milling (to reduce particle size) and high energy ball milling (to mixture the phases) during the powder syntheses, and Spark Plasma Sintering (SPS) to densify the bulk. Due the presence of secondary phases in sintered composites generated during the application of SPS, a processing protocol was created based on a post sintering thermal treatment, in PbO rich atmosphere, which allowed the reduction of secondary phases without, however, changing the average size of grain. These procedures resulted in composite material suitable for the investigations of the electrical, magnetic and magnetoelectric properties, considering the different microstructures obtained. The magnetoelectric composites showed the self-biased effect arising from the intrinsic strain generated in one phase over the other. It was observed the properties dependence, coupled or uncoupled, with average grain size of both phases. Furthermore, the magnetoelectric composites presented the selfbiased effect that appears due to intrinsic strain generated from one phase over the other. Moreover, the evolution of the magnetoelectric coefficient behavior as a function of temperature and applied magnetic field has been more dependent on the variation of the average grain size of the ferroelectric phase than the ferrimagnetic phase, which can be attributed to the largest amount of ferroelectric phase in the structure compound. / No caso de materiais compósitos, que apresentam o acoplamento magnetoelétrico, existem muitas questões sobre influência da microestrutura nas propriedades independentes e acopladas das fases. Dessa forma, este trabalho propõe o estudo da influência do tamanho médio de grão, tanto da fase ferroelétrica quanto da magnética, nas propriedades elétricas, magnéticas e de acoplamento em compósitos magnetoelétricos particulados. Com o objetivo de estudar a relação propriedade / microestrutura em compósitos magnetoelétricos volumétricos, fez-se necessária a preparação de amostras com tamanho médio de grão distintos, ou seja, as fases possuindo tamanho médio de grão na ordem de micrômetros ou nanômetros. Para a realização deste estudo, foi utilizada como fase magnética o CoFe2O4 e, como fase ferroelétrica, o [0,675] Pb(Mg1/3Nb2/3)O3 – [0,325] PbTiO3, misturados na proporção molar 20/80, utilizando técnicas como micromoagem (para redução de partículas) e moagem de altas energias (para a mistura homogênea das fases) durante a síntese dos pós, e a de Spark Plasma Sintering (SPS) para a densificação dos corpos cerâmicos compósitos. Devido à presença de fases secundárias nos compósitos sinterizados, geradas durante a aplicação de SPS, foi criado um protocolo de processamento baseado em tratamento térmico pós-sinterização, com atmosfera rica em PbO, o qual possibilitou a redução das fases secundárias sem, contudo, alterar os tamanhos médios de grão. Tais procedimentos resultaram em material compósito adequado à realização das investigações das propriedades elétricas, magnéticas e magnetoelétricas, considerando as diferentes microestruturas obtidas. Os compósitos magnetoelétricos apresentaram o efeito self-biased, que aparece devido ao strain intrínseco gerado de uma fase sobre a outra. Foi observada a dependência das propriedades, acopladas ou não, com o tamanho médio de grão de ambas as fases. Além disso, a evolução do comportamento do coeficiente magnetoelétrico em função da temperatura e do campo magnético se mostrou mais dependente da variação do tamanho médio de grão da fase ferroelétrica do que da ferrimagnética, fato que pode ser atribuído à maior quantidade de fase ferroelétrica na estrutura do composto.

Cette Thèse traite de la ferroélectricité magnétiquement induite dans deux pérovskites quadruples de manganèse: (LaMn3)Mn4O12 et (YMn3)Mn4O12. Tous deux possèdent une monovalence et une structure antiferromagnétique commensurable des sites B. Ces caractéristiques simples, font de ces composés métastables et stabilisés sous haute pression, des systèmes modèles pour identifier la contribution des interactions d’échange symétrique et antisymétrique à la polarisation. YMO est une nouvelle phase dans laquelle le faible rayon de l’ion Y3+ augmente la pression chimique et donc l’interaction d’échange. L’orientation magnétique des sites B à lieu 30 degrés plus haut que dans LMO, à TN,B=108 K. En revanche, nous mesurons une polarisation, P = 0.54 μC cm-2, identique dans les deux composés. Il s’agit d’une valeur record dans les ferroélectriques magnétiques. De façon inattendue, la nature de la ferroélectricité magnétique est très différente dans chaque composé. Dans LMO, la ferroélectricité apparaît à la transition magnétique des sites B, à TN,B = 78 K, mais nous ne reportons pas de brisure du centre d’inversion par diffraction de rayons X ou par spectroscopie Raman et IR. Nous tentons d’expliquer ce résultat inattendu dans le cadre de la théorie phénoménologique des ferroélectriques impropres. D’un autre côté, dans YMO, nous observons l’apparition de la ferroélectricité à T* = 70 K bien que T* ne corresponde pas à une transition magnétique. En effet, à T* nous reportons seulement une anomalie magnétique suggérant un ordre magnétique latent. La transition structurale, qui a lieu à Ts, pourrait être responsable de l’alignement des domaines ferroélectriques. / In the present Thesis, we study the large ferroelectricity induced by magnetism in two quadruple perovskite compounds: (LaMn3)Mn4O12 and (YMn3)Mn4O12, which both display single-valent properties and a commensurate C-type antiferromagnetic structure of the B-sites. These simple features offer a playground to elucidate the contribution of the symmetric and antisymmetric exchange interactions to the polarization. Both compounds are metastable and stabilized under high-pressure. YMO is a new phase, where the small Y3+ ion exerts a large chemical pressure, which is expected to enhance the exchange interaction and, thus, the spontaneous polarization. We find an ordering temperature of the B-sites, 30 K higher than in LMO. On the other hand, we surprisingly find identical values of the spontaneous polarization, P = 0.54 μC cm-2, in both compounds. It is a record value for magnetic ferroelectrics. In spite of the similarities, the nature of magnetic ferroelectricity appears to be very different in the two compounds. In LMO, ferroelectricity is induced by the magnetic ordering of B-sites, although no indication of inversion symmetry breaking is detected. We argue that this puzzling observation is consistent with prediction of domain structure in improper ferroelectrics. On the other hand, in YMO, the occurrence of ferroelectricity at T*=70 K is consistent with a polar structural modulation below Ts=200 K, however T* does not correspond to any long-range magnetic transition. Indeed, T* marks a magnetic anomaly suggesting a latent magnetic phase. We put forward the hypothesis that the above polar distortion of the crystal structure may force the alignment of polar domains.

Orientador: Aldo Eloizo Job / Banca: Marcos Augusto de Lima Nobre / Banca: Marcelo Ornaghi Orlandi / O Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da Unesp / Resumo: O objetivo deste trabalho foi estudar "efeitos de escala e tamanho" através de medidas de permissividade dielétrica de partículas nanométricas e/ou nanoestruturadas do óxido policátion niobato de potássio dopado com estrôncio (K'Sr IND. 2' N'b IND. 5' 'O IND. 15'), um óxido ferroelétrico da família tetragonal tungstênio bronze (TTB). A determinação da constante dielétrica de nanopartículas foi realizada através da técnica de mistura. Nesta técnica, nanopartículas de permissividade dielétrica desconhecidas são dispersas em um meio de permissividade dielétrica conhecida. A partir da resposta dielétrica da mistura a permissividade dielétrica das nanopartículas foram calculadas utilizando modelagem numérica via circuitos elétricos equivalentes. A fase K'Sr IND. 2' N'b IND. 5' 'O IND. 15, foi preparada por rota química (método poliol modificado), a temperatura de calcinação necessária à obtenção de nanopartículas foi otimizada e partículas nanoestruturadas de K'Sr IND. 2' N'b IND. 5' 'O IND. 15' monofásico foram avaliadas. A caracterização estrutural foi realizada utilizando as técnicas de difratometrica de raiox x (DRX), espectroscopia vibracional de absorção na região do infravermelho (FTIR) e ultravioleta visível (UV/vis) a partir das quais se avaliaram os parâmetros de rede, cristalinidade relativa, tamanho médio de cristalito, volume da cela unitária e energia de gap. Utilizando a técnica de espectroscopia de impedância entre 5 Hz e 3 MHz foram estudadas as propriedades dielétricas das amostras através de medidas de permissividade confirmando a existência de ferroeletricidade nas amostras e foi identificado o efeito de tamanho nas propriedades dielétricas das nanopartículas. / Abstract: This work aimed the study of "size effect" through permittivity measurements of nanometric particles and/or nanostructured of niobate oxide K'Sr IND. 2' N'b IND. 5' 'O IND. 15', a ferroelectric oxide belonging to the tetragonal tungsten bronze family (TTB). Determination of nanoparticles dielectric constant was done using the mixture technique. In this technique, nanoparticles of unknown dielectric permittivity are dispersed in a medium of know dielectric permittivity. From the dielectric response of the mixture, the dielectric permittivity of the nanoparticles is calculated using numerical modeling by means of equivalent circuits. The K'Sr IND. 2' N'b IND. 5' 'O IND. 15' phase was prepared using a chemical route (polyol modified method) and the optmization of the temperature calcination was performed aiming to obtain nanoparticles and the resulting nanostructured particles were evaluated. The structural characterization was carried out by X-ray diffraction (XRD), infrared absorption spectroscopy (FTIR) and UV-Vis spectroscopy allowing the evaluation of cell parameters, relative crystallinity and crystallite size, unitary cell volume and gap energy. The impedance spectroscopy technique in the range from 5 Hz to 3 MHz to study dielectric properties of samples was used. The presence of ferroelectric phase in samples was confirmed and the size effect was identified on nanoparticles. / Mestre

Integration of complex optical functionalities with high performance will lead to a huge development in the field of nanophotonics for a broad range of applications. Silicon photonics is currently the leading technology for the implementation of low-cost photonic integrated devices. The great potential of this technology relies on its compatibility with the mature silicon integrated circuits manufacturing based on complementary metal-oxide semiconductor (CMOS) processes widely used in microelectronic industry and the availability of high quality silicon-on-insulator wafers, an ideal platform for creating planar waveguide circuits that offers strong optical confinement due to the high index contrast between silicon (n=3.45) and silicon dioxide (n=1.45). In order to keep improving the performance of photonic devices on silicon, the integration of CMOS compatible materials with unique properties shows up as an excellent opportunity to overcome the current limitations in silicon while offering unprecedented and novel capabilities to the silicon platform. In this way, barium titantate (BaTiO3) stands out as one of the most disruptive candidates. The work developed in this thesis is essentially focused on the design, fabrication and characterization of an electro-optic modulator based on a hybrid BaTiO3 on silicon structure for the implementation of high performance electro-optic functionalities with beyond state-of-the art performance that currently cannot be afforded in silicon photonics technology. / La integración de funcionalidades ópticas con alto rendimiento llevará a un gran desarrollo en el campo de la nanofotónica para un amplio abanico de aplicaciones. Actualmente, la fotónica de silicio es la tecnología líder para la implementación de dispositivos fotónicos integrados a bajo coste. El gran potencial de esta tecnología reside en su compatibilidad con las maduras técnicas de fabricación de circuitos integrados de silicio basadas en los procesos "complementary metal-oxide semiconductor" (CMOS) ampliamente utilizados en la industria microelectrónica y la disponibilidad de disponer de obleas de silicio sobre aislante de alta calidad, una plataforma ideal para crear circuitos de guía de ondas planas que ofrecen un fuerte confinamiento óptico debido al alto contraste índices entre el silicio (n=3,45) y el dióxido de silicio (n=1,45). Para poder mejorar el rendimiento de dispositivos fotónicos en silicio, la integración de materiales con propiedades excepcionales y compatibles con los procesos de fabricación CMOS surge como una excelente oportunidad para superar las actuales limitaciones de la tecnología de silicio al mismo tiempo que ofrece oportunidades novedosas y sin precedentes en la plataforma de silicio. En este sentido, el material titanato de bario (BaTiO3) se postula como uno de los candidatos más prometedores. El trabajo desarrollado en esta tesis está esencialmente enfocado en el diseño, fabricación y caracterización de un modulador electro-óptico basado en una estructura híbrida de BaTiO3 en silicio para la implementación de funcionalidades electro-ópticas de alto rendimiento más allá del estado del arte de las que no se puede disponer actualmente en la tecnología de fotónica de silicio. / La integració de funcionalitats òptiques amb alt rendiment portarà a un gran desenvolupament en el camp de la nanofotònica per a un ampli ventall d'aplicacions. Actualment, la fotònica de silici és la tecnologia capdavantera per a la implementació de dispositius fotònics integrats a baix cost. El gran potencial d'aquesta tecnologia resideix en la seva compatibilitat amb les madures tècniques de fabricació de circuits integrats de silici basades en els processos "complementary metal-oxide semiconductor" (CMOS) amplament utilitzats en la indústria microelectrònica i la disponibilitat de disposar d'hòsties de silici sobre aïllant d'alta qualitat, una plataforma ideal per crear circuits de guia d'ones planes que ofereixen un fort confinament òptic a causa de l'alt contrast d'índexs entre el silici (n=3,45) i el diòxid de silici (n=1,45). Per poder millorar el rendiment de dispositius fotònics en silici, la integració de materials amb propietats excepcionals i compatibles amb els processos de fabricació CMOS sorgeix com una excel·lent oportunitat per superar les actuals limitacions de la tecnologia de silici al mateix temps que ofereix oportunitats noves i sense precedents en la plataforma de silici. En aquest sentit, el material titanat de bari (BaTiO3) es postula com un dels candidats més prometedors. El treball desenvolupat en aquesta tesi està essencialment enfocat en el disseny, fabricació i caracterització d'un modulador electro-òptic basat en una estructura híbrida de BaTiO3 en silici per a la implementació de funcionalitats electro-òptiques d'alt rendiment més enllà de l'estat de l'art de les quals no es pot disposar actualment a la tecnologia de fotònica de silici. / Castera Molada, P. (2017). Development of new photonic devices based on barium titanate in silicon [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/86197 / TESIS

<p>U poslednjih nekoliko decenija, feroelektrici su prepoznati kao dobri kandidati<br />za &scaron;irok spektar primene. Barijum titanat je jedan od najče&scaron;će istraživanih<br />perovskitnih materijala usled tipičnog feroelektričnog pona&scaron;anja na sobnoj<br />temperaturi, sa histerezisnom zavisnosti između polarizacije i električnog polja.<br />Specifična svojstva BaTiO₃ čine ovaj materijal veoma korisnim u proizvodnji<br />kondenzatora, memorija, senzora i dr. Pored toga, paraelektrična faza BaTiO₃<br />ima primenu u proizvodnji mikrotalasnih tunabilnih uređaja. BaTiO₃ pokazuje<br />paralaketrično pona&scaron;anje iznad Kirijeve temperature (120&deg;C) i veliki je izazov<br />modifikovati materijal da bude paraelektričan na sobnoj temperaturi. Delimičnom<br />supstitucijom Ba²⁺ ili Ti⁴⁺ jona, jonima Sr²⁺ ili Zr⁴⁺, respektivno, snižava<br />se Kirijeva temperatura i dolazi do pomeranja fazne transformacije feroelektri<br />čno/paraelektrično na niže temperature. Monolitna keramika na bazi bazi<br />BaTiO₃ je često istraživana kako bi se ispitale promene u strukturi i svojstvima<br />uzrokovane dodatkom dopanata, međutim ne postoji praktična primena ovih<br />materijala u mikrotalasnim tunabilnim tehnologijama. S druge strane, u trendu<br />sveop&scaron;te minijaturizacije elektronskih komponenti, tanki filmovi su prepoznati<br />kao dobri kandidati za proizvodnju tunabilnih uređaja. Jedan od ciljeva ovog<br />istraživanja bio je ispitivanje strukturnih i funkcinalnih svojstava tankih filmova na<br />bazi barijum titanata, BaTiO₃, Ba_1-xSr_xTiO₃ (x=0,1, 0,2, 0,3 i 0,4) i BaTi_1-xZr_xO₃<br />(x=0,1 i 0,2), pripremljenih hemijskom depozicijom iz tečne faze. Metalne<br />soli BaCO₃, C₄H₆O₄Sr i ZrOCl₂ H₂O i Ti(OCH₂CH₂CH₂CH₃)₄ rastvorene su<br />odvojeno u kiseloj sredini i pome&scaron;ane u transparentne solove. Inkdžet &scaron;tampa<br />i spin tehnika kor&scaron;ćene su za depoziciju funkcionalnih filmova na bazi BaTiO₃.<br />Nakon deponovanja, pripremljeni filmovi su termički tretirani na različitim<br />temperaturama do 1000&deg;C. U zavisnosti od koncentracije pripremljeni su filmovi<br />debljine od 100 do 700 nm,sa veličinom zrna od nekoliko desetina nanometara. Strukturna karakterizacija potvrdila je promene u strukturi BaTiO₃ tankih filmova<br />sa dodatkom Sr²⁺ i Zr⁴⁺. Tetragonalna (feroelektrična) faza BaTiO₃ je potvrđena<br />rentgenostrukturnom analizom i Ramanovom spektroskopijom. S druge strane,<br />smanjenje tetragonalnosti je primećeno kod dopiranih uzoraka. Promene u<br />funkcionalnosti dopiranih BaTiO₃ filmova analizirane su na osnovu dielektričnih<br />i feroelektričnih merenja. Izvr&scaron;ene analize su potvrdile feroelektrično pona&scaron;anje<br />kod BaTiO₃, dok se feroelektrični odgovor u tankim filmovima smanjuje sa<br />dopiranjem. Priprema elektroda specifičnih geometrija pogodnih za tunabilna<br />merenja različitim tehnikama depozicije bio je drugi cilj istraživanja. Tehnika spaterovanja u kombinaciji sa laserskim uklanjanjem, inkdžet &scaron;tampa i fotolitografija<br />su kori&scaron;ćene za pripremu kružnih i koplanarnih elektroda na povr&scaron;ini barijum<br />titanatnih tankih filmova. Uticaj procesnih parametara svake od pomenutih<br />metoda na dimenzionalnu preciznost pripremljenih elektroda je bio predmet istraživanja<br />u okviru teze. Odabrani laser se pokazao kao neprikladan za pripremu<br />elektroda na pripremljenim barijum titanatnim tankim filmovima. Inkdžet &scaron;tampa<br />se pokazala kao korisna u pripremi elektroda mikrometarskih fimenzija, dok je za<br />pripremu sofisticiranijih geometrija fotolitografija pokazala najbolje performanse.</p> / <p>In past few decades, ferroelectrics are recognized as good candidates for wide<br />range of applications. Barium titanate is one of the most investigated perovskite<br />materials due to typical ferroelectric behavior at room temperature, with hysteresis<br />dependence of the polarization and electric field. Specified properties<br />of BaTiO₃ make this material useful in production of capacitors, memories,<br />sensors, etc. Nevertheless, paraelectric phase of BaTiO₃ may have application<br />in production of microwave tunable devices. Barium titanate shows paraelectric<br />behavior at temperatures above the Currie temperature (120 &deg;C) and it is great<br />challenge to make material paraelectric at room temperature. Partial substitution<br />of Ba²⁺ or Ti⁴⁺ ions, by Sr²⁺ or Zr⁴⁺, respectively, decreases the Currie temperature<br />of barium titanate and moves phase transition ferroelectric/paraelectric<br />to lower temperatures. Bulk BaTiO₃was often investigated in order to improve<br />structure and functionality by addition of dopant, but do not have practical<br />application in microwave tunable technologies. On the other hand, with the<br />trend of overall miniaturization of electronic devices, thin films are recognized<br />as good candidates for production of tunable devices. One of the aims in this<br />research was investigation of structural and functional properties of barium<br />titanate based thin films, BaTiO₃, Ba_1-xSrxTiO₃ (x=0,1, 0,2, 0,3 i 0,4) and<br />BaTi_1-xZrxO₃ (x=0,1 i 0,2), prepared by chemical solution deposition. Metal<br />salts of BaCO₃, C₄H₆O₄Sr and ZrOCl₂ H₂O and Ti(OCH₂CH₂CH₂CH₃)₄were<br />separetly disolved in acetic environment and mixed in clear transparent sols.<br />Inkjet printing and spin coating were used for deposition of functional BaTiO3<br />based fims. After deposition prepared films were thermally treated at different<br />temperatures up to 1000 &deg;C. In dependence of sol concentration thickness of<br />obtained films is from 100 to 700 nm and grain size is few tens of nanometers. Structural characterization confirmed changes in structure of barium titanate thin<br />films by addition of Sr^₂₊ i Zr⁴⁺. Tetragonal (ferroelectric) phase of BaTiO₃ is<br />confirmed by X-ray diffraction and Raman spectroscopy. On the other hand, decreasing<br />of tetragonality was noticed in doped samples. Changes in functionality<br />of doped BaTiO₃ thin films were analyzed by dielectric and ferroelectric measurements.<br />Performed analysis confirmed ferroelectric behavior of barium titanate<br />thin films, and decrease in ferroelectric answer of doped films. Investigation of<br />possibility of complex shaped electrodes preparation,suitable for tunability measurements, by different deposition techniques was the second goal of this research.<br />Sputtering technique in combination with laser removal, inkjet printing and photolithography were used for preparation of complex circular and coplanar electrodes<br />on the surface of barium titanate based thin films. Influence of processing<br />parameters for the each of mentioned technique on dimensional precision of prepared<br />electrodes was investigated. Selected laser was not appropriate for production<br />of electrodes on prepared barium titanate based thin films. Inkjet printing<br />was useful for production of electrodes in micrometer range, but for more sophisticated<br />geometries photolithography shows the best performance.</p>

The rapid progress on developing new and improved multifunctional materials, for optoelectronic and spin based phenomena/devices, have increased the importance of the fundamental understanding of their coherent states and nonlinear optical properties. This study is aimed at characterizing, modeling, and controlling the fundamental electronic, phononic, and spin properties of several classes of materials through nonequilibrium and nonlinear light-matter interactions, coupled with a novel design of the material phases, interfaces, and heterostructures. This research directly addresses the Grand Challenges identified in the Basic Energy Sciences Advisory Committee report "Directing Matter and Energy: Five Challenges for Science and the Imagination" (Hemminger, 2007) [1], in particular, the area: "Matter far beyond equilibrium" and addresses the questions, "How do remarkable properties of matter emerge from complex correlations of the atomic or electronic constituents and how can we control these properties?" and "How do we design and perfect atom- and energy-efficient synthesis of revolutionary new forms of matter with tailored properties?". The knowledge gained from these fundamental studies can provide new information for a broad community to provide concepts for the next generation of multifunctional materials and devices, and resulted in several publications and conference presentations. The materials studied in this dissertation included multiferroic BaTiO3-BiFeO3 [2], ferroelectric Pb0.52Zr0.48TiO3 (PZT), InAs/AlAsSb multi-quantum-well [3], lead halide perovskite [4], n-type InAsP films [5, 6], and nanolaminate plasmonic crystals [7]. Probing multiferroics, which are materials that can exhibit ferromagnetic, ferroelectric, and ferroelastic orders simultaneously in a single phase, was a main focus of this study. BiFeO3 (BFO) is the most widely investigated multiferroic due to its high Neel and Curie temperatures and has antiferromagnetic and ferroelectric properties [8]. An inherent drawback of BFO is its large leakage currents. In this project, (1 − x)BaTiO3-(x)BiFeO3, x = 0.725 (BTO-BFO) heterostructures were investigated [9], where the conductivity of the solid solution can be reduced by adding another perovskite material, BaTiO3 [2]. We aimed to study optically induced coherent states in our BTO-BFO structures. Time resolved pumpprobe spectroscopic measurements were performed at room temperature as well as at low temperature (100 K) up to 10 T. Coherent acoustic phonons were observed both in a film and nanorods, resulting in coherent phonon frequencies of 27 and 33 GHz, respectively [2]. Coherent phonon spectroscopy is a sensitive tool to characterize the interfaces and can be employed as an effective ultrasensitive quantum sensor [10]. Furthermore, in the nanorods arrays of BTO-BFO, an additional oscillation with frequency in the range of 8.1 GHz was observed. This frequency is close to a theoretically predicted magnon frequency which could indicate the coexistence of coherent phonons and magnons in the nanorods arrays [2]. In an analogy to photonics which relies on electromagnetic waves, magnonics utilizes spin waves to carry and process information, offering several advantages such as an operation frequency in the THz range. Recently, "a quantum tango" [11] was reported where coupled coherent magnon and phonons modes were formed on a surface patterned ferromagnet. Furthermore, BTO-BFO heterostructures were probed using transient birefringence and magneto-optical Kerr effect spectroscopy. The results demonstrated that the magnetic field dependence of coherent phonons, measured by these two techniques, exhibits more sensitivity to the external magnetic fields compared to the differential reflectivity technique [2]. Moreover, nonlinear optical properties of this structure were investigated via second harmonic generation spectroscopy, where wavelength and polarization dependence of this nonlinear observation will be discussed in this dissertation. As part of this study, another class of multiferroic materials, with strong ferroelectric and piezoelectric properties, Pb0.52Zr0.48TiO3 (PZT) was studied [12]. In this project, the nonlinear optical properties of PZT nanorod arrays were investigated. Clear signatures of second harmonic generations from 490-525 nm (2.38-2.53 eV) at room temperature, were observed. Furthermore, time resolved differential reflectivity measurements were performed to study dynamical properties in the range of 690-1000 nm where multiphoton processes were responsible for the photoexcitations. We compared this excitation scheme, which is sensitive mainly to the surface states, to when the photoexcited energy (∼ 3.1 eV) was close to the bandgap of the nanorods. Our results offer promises for employing these nanostructures in nonlinear photonic applications. Furthermore, the established techniques during my research provided new insights on optical properties of InAs/AlAsSb multi-quantum-well [3], lead halide perovskite [4], n-type InAsP films [5, 6], and nanolaminate plasmonic crystals [7], and the results will be briefly presented in this dissertation. / Doctor of Philosophy / My research activities have explored multifunctional materials and heterostructures with strongly enhanced coupled electric and magnetic orders and optical properties. In particular, pursuing novel heterostructure designs such as multiferroics can provide control over electric and magnetic ordering in mixed dimensionality. This, together with control at the level from lattice structure to electron spin states can give rise to improved or even qualitatively new and robust materials properties. For example, a better understanding of the phenomena associated with the spin degree of freedom of electrons allows for advancement in spintronic device applications such as storage, logic, and sensors, which are associated with quantum computers and quantum communications [13, 14, 15]. Overarching questions and goals of my activities included: What are the microscopic origins and mechanisms of nonlinear response in strongly coupled nanostructured materials and its relationship to electronic, spin, and lattice degrees of freedom? (2) What are the effects of dimensionality and quantum confinement on optical properties? (3) How do we control and manipulate the coherent states, such as coherent phonons and magnons using external and internal fields, material composition, and morphology to achieve maximal efficiency and tunability? Addressing many of the challenges in the fast-paced technological world requires continued developments of new materials with enhanced optical properties. The knowledge gained from my fundamental studies can provide new information for the next generation of multifunctional materials and devices with advanced optical properties and resulted in several publications and conference presentations.

Microwave materials have been widely used in a variety of applications ranging from communication devices to military satellite services, and the study of materials properties at microwave frequencies and the development of functional microwave materials have always been among the most active areas in solid-state physics, materials science, and electrical and electronic engineering. In recent years, the increasing requirements for the development of high speed, high frequency circuits and systems require complete understanding of the properties of materials function at microwave frequencies. Ferroelectric materials usually have high dielectric constants, and their dielectric properties are temperature and electric field dependent. The change in permittivity as a function of electric field is the key to a wide range of applications. Ferroelectric materials can be used in fabrication capacitors for electronic industry because of their high dielectric constants, and this is important in the trend toward miniaturization and high functionality of electronic products. The simple tunable passive component based on ferroelectric films is a varactor which can be made as a planar structure, and electrically tunable microwave integrated circuits using ferroelectric thin films can be developed. Therefore, it is very important to characterize the dielectric constant and tunability of ferroelectric thin films. This thesis shows experimental results for growth, crystalline properties and microwave characterization of Na0.5K0.5NbO3 (NKN), AgTa0.5Nb0.5O3 (ATN), Ba0.5Sr0.5TiO3 (BST) as well as AgTaO3 (ATO), AgNbO3 (ANO) thin films. The films were grown by Pulsed Laser Deposition (PLD) and rf-magnetron sputtering of a stoichiometric, high density, ceramic NKN, ATN, BST target onto single crystal LaAlO3(LAO), Al2O3 (sapphire), and Nd:YAlO3, and amorphous glass substrates. By x-ray diffractometry, NKN, ATN, BST films on LAO substrates were found to grow epitaxially, whereas films on r-cut sapphire substrates were found to be preferentially (00l) oriented. Coplanar waveguide interdigital capacitor (CPWIDC) structures were fabricated by standard photolithography processing and metal lift-off technique. Microwave properties of the NKN/Sapphire and ATN/Sapphire with CPW structures were characterized using on-wafer microwave measurement technique. Measurement setup is composed of network analyzer, probe station, and microwave G-S-G probes. External electric field through the connection between network analyzer and power supply was applied to measure voltage tunability. Measured S-parameter were used for the calculation of capacitance, loss tanδ, tunability and K-factor. The NKN films interdigital capacitors with 2 μm finger gap on Nd:YAlO3 showed superior performance compared to ATN in the microwave range from 1 to 40 GHz. Within this range, the voltage tunability (40V, 200 kV/cm) was about 29%, loss tangent ∼ 0.13, K-factor = tunability/tanδ from 152% @ 10GHz to 46% @ 40GHz. The microwave performance of ATN film CPWIDC with 2 μm finger gap on sapphire substrate in the microwave range from 1 to 40 GHz showed that frequency dispersion is about 4.3%, voltage tunability was 4.7% @ 20GHz and 200 kV/cm, loss tangent ∼ 0.068 @ 20GHz, K-factor = tunability/tanδ is ranged from 124% @ 10GHz to 35% @ 40GHz. The BST films CPWIDC with 2μmfinger gap on Al2O3 substrate showed frequency dispersion of capacitance in the microwave range from 1 to 40 GHz about 17%, voltage tunability = 1 - C(40V)/C(0) ∼ 22.2%, loss tangent ∼ 0.137 @ 20GHz, and K-factor = tunability/tanδ from 281% @ 10GHz to 95% @ 40GHz. / QC 20101207

Growing environmental concerns, coupled with increasing regulatory restrictions, are requiring industries to develop non-lead-based compositions of ferroelectric and piezoelectric materials. These materials—now widely used in sensors, actuators, and transducers—are for the most part lead-based compounds such as Pb(Zr,Ti)O₃ (PZT). Indeed, PZT represents the dominant market share for use in these technologies. Moreover, next generation compounds, which include Pb(Mg_1/3Nb_2/3)O₃-xat%PbTiO₃ (PMN-x%PT) crystals with ultrahigh piezo-/electromechanical properties, are also Pb-based systems and thus are problematic for meeting more restrictive environmental standards. As alternative, Pb-free ferroelectrics such as NBT-derived single crystals represent viable next-generation materials for use in ferro-/piezoelectric applications. Development of these types of NBT-based crystals has made important advancements in the last decade. In fact, the performances of NBT-based materials are beginning to approach the properties of the widely used commercial PZT ceramic material. Nonetheless, additional studies are needed before it being able to compete with PMN-x%PT and PZN-x%PT crystals in next-generation applications. As a new type of piezoelectric material, much remains to be learned about Pb-free piezoelectric crystals. For instance, in addition to enhancing our understanding the nature of the piezoelectric third-rank tensor coefficients such as d₃₃ and d₁₅, a thorough knowledge of the Curie temperature, leakage current, and electromechanical properties is also essential for increasing the applications potential of these crystals. As detailed herein, multiple dopants may have to be incorporated into NBT to modify its microstructure and properties to meet these specific requirements, which may further complicate its chemical structure-property relationships. This study, therefore, was designed to investigate the heterogeneous structure of NBT-based single crystals, using x-ray diffraction, transmission electron microscopy, and neutron inelastic scattering, with the goal of investigating the mechanism coupling of morphotropic phase boundary (MPB) and the maximum property responses in A-site disordered perovskite Pb-free piezoelectric systems. Using the framework of polar nanoregions and adaptive phase theory, I sought to determine how the nanostructure of these single crystals change with temperature and composition—and how these factors impact its properties. Diffuse scattering, domain morphology, and phonon dispersions were used to investigate both the static and dynamic properties of these heterogeneous structures. / Ph. D.

Ferroelectric materials are characterized by a reversible spontaneous polarization in the absence of an electric field. The characteristic polarization response of a ferroelectric material to an applied electric field enables a binary state device in the form of a thin film ferroelectric capacitor that can be used to store digital information. This provides opportunities for the development of high speed, low cost and low power, nonvolatile memory devices. The development of commercial ferroelectric memory devices has however been hampered by (a) several reliability issues including fatigue, leakage current, aging, time dependent dielectric breakdown, retention and imprint and (b) processing problems including the development of a low temperature thin film deposition process and the development of a patterning technology. Lead zirconate titanate (PZT) is now widely considered as the most promising material for ferroelectric memory applications as a result of its excellent ferroelectric properties and wide operating temperature range. However, it is commonly found that metal electroded-PZT capacitors (e.g., Pt/PZT/Pt) show a loss of switchable polarization with cumulative switching cycles. This phenomenon is known as fatigue and is the one of the critical problems affecting the lifetime of ferroelectric memories. This research is primarily focused on the problem of fatigue. On the basis of a quantitative model, various guidelines to minimize the degradation problem have been derived. The model attributes fatigue to domain pinning by space charge that is caused by defect (e.g. oxygen vacancy) entrapment at various interface sites such as electrode-ferroelectric interface, domain boundaries and grain boundaries. Two different approaches to minimize the problem have been outlined : (a) control of the defect density and (b) control of the interface state. The control of interface state was achieved by replacing the metal electrodes with conducting oxide electrodes such as RuO₂. The oxide electrode/PZT capacitors were characterized for their diffusion barrier properties, perovskite phase formation, interface nature and ferroelectric properties. The results indicate that these oxide electroded PZT films are good candidates for nonvolatile memory applications. However, the leakage current levels at the operating voltages are far higher than the metal counterparts. Simultaneous minimization of fatigue and leakage current in PZT films was achieved by using multilayer metal/conducting oxide electrodes (e.g., Pt/RuO₂). The control of defect density was attained by (a) donor doping to compensate for the oxygen vacancies (e.g, Nb doping in PZT) and (b) utilizing ferroelectric materials that have a low intrinsic defect concentration. As a result of the latter approach, novel ferroelectric materials belonging to the layer-structure family of oxides have been identified as excellent candidates for fatigue free nonvolatile memory applications. Laser ablated SrBi₂(Ta_xNb_{1 - x})₂O₉ (0<x<1) films showed very good hysteresis characteristics (remnant polarization value of 11 µC/cm², coercive field of 60 kV/cm), no fatigue was observed up to 10⁹ switching cycles and very low leakage current densities. Furthermore, the formation and properties of these films were characterized. It was found grain size and orientation played a major role in determining the properties of these films. C-axis oriented films were found to exhibit almost no polarization. An additional objective of this research was to identify an etching technology (process integration issue) for patterning of the ferroelectric capacitors. The etching process should provide high etch rates, good etch anisotropy, high etch selectivity and minimal post etch residues. It has been shown that a reactive ion etch process with CCl₂F₂/O₂ as the etch gas mixture can meet these requirements. A detailed process study has been conducted to determine the mechanism of etching. / Ph. D.