This thesis presents experimental and related theoretical studies of pyrochlore titanate oxides, boron nitride nanotubes, and multiferroic bismuth ferrite. We have investigated these systems at high pressures and at low temperatures using Raman spectroscopy. Below, we furnish a synoptic presentation of our work on these three systems. In Chapter 1, we introduce the systems studied in this thesis, viz. pyrochlores, boron nitride nanotubes, and multiferroic BiFeO3, with a review of the literature pertaining to their structural, electronic, vibrational, and mechanical properties. We also bring out our interests in these systems. Chapter 2 includes a brief description of the theory of Raman scattering and infrared absorption. This is followed by a short account of the experimental setups used for Raman and infrared measurements. We also present the technical details of high pressure technique including the alignment of diamond anvil cells, gasket preparation, calibration of the pressure, etc. Chapter 3 furnishes the results of our pressure-and temperature-dependent studies of pyrochlore oxides which has been divided into eight different parts. In recent years, magnetic and thermodynamic properties of pyrochlores have received a lot of attention. However, not much work has been reported to address the quasiparticle excitations, e.g., phonons and crystal-field excitations in these materials. A material that shows exotic magnetic behavior and high degree of degenerate ground states can be expected to have low-lying excitations with possible couplings with phonons, thereby, finger-printing various novel properties of the system. Raman and infrared absorption spectroscopies can, therefore, be used to comprehend the novel role of phonons and their role in various phenomena of frustrated magnetic pyrochlores. Recently, there have been reports on various novel properties of these systems; for example, Raman and absorption studies [Phys. Rev. B 77, 214310 (2008)] have revealed a loss of inversion symmetry in Tb2Ti2O7 at low temperatures which has been suggested as the key reason for this frustrated magnet to remain in spin-liquid state down to 70 mK. Powder neutron-diffraction experiments [Nature 420, 54 (2002)] have shown that an application of isostatic pressure of about 8.6 GPa in spin-liquid Tb2Ti2O7 induces a long-range magnetic order of the Tb3+ spins coexisting with the spin-liquid phase ascribing this transition to the breakdown of the delicate balance among the various fundamental interactions. Moreover, Raman and x-ray studies have shown that Tb2Ti2O7,Sm2Ti2O7,and Gd2Ti2O7 undergo a structural transition followed by an irreversible amorphization at very high pressures (~ 40 GPa or above) [Appl. Phys. Lett. 88, 031903 (2006)]. In this chapter, therefore, we present our temperature-and pressure-dependent Raman studies of A2Ti2O7 pyrochlores, where ‘A’ is a trivalent rare-earth element (A = Sm, Gd,Tb, Dy,Ho, Er,Yb, and Lu; and also Y). Since all the group theoretically predicted Raman modes of this cubic lattice are due to oxygen vibrations only, in Part (A), we revisit the phonon assignments of pyrochlore titanates by performing Raman measurements on the O16 /O18 − isotope based Dy2Ti2O7 and Lu2Ti2O7 and find that the vibrations with frequencies below 250 cm−1 do not involve oxygen atoms. Our results lead to a reassignment of the pyrochlore Raman phonons thus proposing that the mode with frequency ~ 200 cm−1, which has earlier been known as an F2g phonon due to oxygen vibration, is a vibration of Ti4+ ions. Moreover, we have performed lattice dynamical calculations using Shell model that help us to assign the Raman phonons. In Part (B), we have explored the temperature dependence of the Raman phonons of spin-ice Dy2Ti2O7 and compared with the results of two non-magnetic pyrochlores, Lu2Ti2O7 and Y2Ti2O7. Our results reveal anomalous red-shift of some of the phonons in both magnetic and non-magnetic pyrochlores as the temperature is lowered. The phonon anomalies can not be understood in terms of spin-phonon and crystal field transition-phonon couplings, thus attributing them to phonon-phonon anharmonic interactions. We also find that the anomaly of the disorder activated Ti4+ Raman vibration (~ 200 cm−1) is unusually high compared to other phonons due to the large vibrational amplitudes of Ti4+-ions rendered by the vacant Wyckoff sites in their neighborhood. Later, we have quantified the anharmonicity in Dy2Ti2O7. We have extended our studies on spin-ice compound Dy2Ti2O7 by performing simultaneous pressure-and temperature-dependent Raman measurements, presented in Part (C). We show that a new Raman mode appears at low temperatures below TC ~ 110 K, suggesting a structural transition, also supported by our x-ray measurements. There are reports [Phys. Rev. B 77, 214310 (2008), Phys.Rev.B 79, 214437 (2009)] in the literature where the new mode in Dy2Ti2O7 at low temperatures has been assigned to a crystal field transition. Here, we put forward evidences that suggest that the “new” mode is a phonon and not a crystal field transition. Moreover, the TC is found to depend on pressure with a positive coefficient. In Part (D), we have presented our results of temperature-and pressure-dependent Raman and x-ray measurements of spin-frustrated pyrochlores Gd2Ti2O7, Tb2Ti2O7,and Yb2Ti2O7. Here, we have estimated the quasiharmonic and anharmonic contributions to the anomalous change in phonon frequencies with temperature. Moreover, we find that Gd2Ti2O7 and Tb2Ti2O7 undergo a subtle structural transition at a pressure of ~ 9 GPa which is absent in Yb2Ti2O7. The implication of this structural transition in the context of a long-range magnetically ordered state coexisting with the spin-liquid phase in Tb2Ti2O7 at high pressure (8.6 GPa) and low temperature (1.5 K), observed by Mirebeau et al. [Nature 420, 54 (2002)], has been discussed. As we have established in the previous parts that the anomalous behavior of pyrochlore phonons is due to phonon-phonon anharmonic interactions, we have tuned the anharmonicity in the first pyrochlore of the A2Ti2O7 series, i.e., Sm2Ti2O7,by replacing Ti4+-ions with bigger Zr4+-ions, presented in Part (E). Our results suggest that the phonon anomalies have a very strong dependence on the ionic size and mass of the transition element (i.e., the B4+-ion in A2B2O7 pyrochlores). We have also observed signatures of coupling between a phonon and crystal-field transitions in Sm2Ti2O7. In Part (F), we have studied spin-ice compound Ho2Ti2O7 and compared the phonon anomalies with the stuffed spin-ice compounds, Ho2+xTi2−xO7−x/2 by stuffing Ho3+ ions into the sites of Ti4+ with appropriate oxygen stoichiometry. We find that as more and more Ho3+-ions are stuffed, there is an increase in the structural disorder of the pyrochlore lattice and the phonon anomalies gradually disappear with increasing Ho3+-ions. Moreover, a coupling between phonon and crystal field transition has also been observed. In Part (G), we have examined the temperature dependence of phonons of “dynamical spin-ice” compound Pr2Sn2O7 and compared with its non-pyrochlore (monoclinic) counterpart Pr2Ti2O7. Our results conclude that the anomalous behavior of phonons is an intrinsic property of pyrochlore structure having inherent vacant sites. We also find a coupling between phonon and crystal-field transitions in Pr2Sn2O7. In the last part of this chapter, Part (H), we present our Raman studies of Er2Ti2O7. Here, we show that in addition to the anomalous phonons, there are modes that originate from photoluminescence transitions and some of these luminescence lines show anomalous temperature dependence which have been understood using the theory of optical dephasing in crystals, developed by Hsu and Skinner [J. Chem. Phys. 81, 1604 (1984)]. Temperature dependence of a few Raman modes and photoluminescence bands suggest a phase transition at 130 K. In Chapter 4, we furnish our pressure-dependent Raman studies of boron nitride multi-walled nanotubes (BNNT) and hexagonal boron nitride (h-BN) and compare the results with those of their carbon counterparts. Using Raman spectroscopy, we show that BNNT undergo an irreversible transition at ~ 12 GPa while the carbon counterpart, multi-walled carbon nanotubes, show a similar transition at a much higher pressure of ~ 51 GPa. In sharp contrast, the layered form of both the systems (i.e. h-BN and graphite) undergo a hexagonal to wurtzite phase at nearly similar pressure (~ 13 GPa of h-BN and ~ 15 GPa for graphite). A molecular dynamical simulation on boron nitride single-walled nanotubes has also been undertaken that suggests that the polar nature of the B−N bonds may be responsible for the irreversibility of the pressure-induced transformations. It is interesting to see that in hexagonal phase both the systems have almost similar mechanical property, but once they are rolled up to make nanotubes, the property becomes quite different. Chapter 5 presents the temperature dependence of the Raman modes of multiferroic thin films of BiFeO3 and Bi0.7Tb0.2La0.1O3. Though there have been several Raman investigations of BiFeO3 in literature, here we emphasize the observation of unusually intense second order Raman phonons. Our results have motivated Waghmare et al. to suggest a theoretical model to explain the anomalously large second order Raman tensor of BiFeO3 in terms of an incipient metal-insulator transition. In Chapter 6, we summarize our findings on the three different systems, namely, pyrochlores, boron nitride nanotubes, and BiFeO3 and highlight a few possible experiments that may be undertaken in future to have a better understanding of these systems.

Multiferroics are currently intensely investigated because the coexistence and coupling of ferroic arrangements brings about new physical effects and, for the few room-temperature examples, interesting prospects for applications in various fields. This interest is illustrated by the recent publication of several articles on multiferroics in high impact reviews over the last five years. The main goal of the thesis was to look for new multiferroics by exploiting overlooked and original polar and magnetic arrangements. We more precisely investigated compounds based on lead iron tungsten PbFe2/3W1/3O3 (PFW) and lead zirconate PbZrO3 (PZO) oxides. PFW displays long- and short-range both polar and magnetic orders (ferroelectric-relaxor and antiferromagnetic-spin-glass) while PZO is antiferroelectric with antiferrodistorsivity (oxygen tilts) and existence of ferroelectric instabilities. Combining various techniques from synthesis to electric, magnetic and structural characterizations, we demonstrated that it is possible to get a multiferroic compound (50%PFW-50%PZO) with coexistence of multiple ferroic and structural arrangements with room temperature properties of practical interest. This work opens new prospects in this rich field of multiferroics in peculiar by using antiferroelectrics.

Thin film multiferroic oxides with co-existing ferroelectric and ferromagnetic ordering have attracted much interest in recent years, partly as a result of the enhancements achieved through the adoption of strained thin film geometries. This thesis presents work on two such thin film oxides; lanthanide substituted BiFeO₃ and Fe substituted PbTiO₃. Coherent magnons and acoustic phonons were impulsively excited and probed in thin films of the room temperature multiferroic Bi_1-x-yDy_xLa_yFeO₃ using femtosecond laser pulses. The elastic moduli of rhombohedral, tetragonal and rare-earth doped BiFeO₃ were determined from acoustic mode frequencies in conjunction with spectroscopic ellipsometry. A weak ferromagnetic order, induced alternately by magnetization in the growth direction or by tetragonality, created a magnon oscillation at 75 GHz, indicative of a Dzyaloshinskii-Moriya interaction energy of 0.31 meV. Bulk crystals and thin films of PbTi_1-xFe_xO₃ (PTFO) are multiferroic, exhibiting ferroelectricity and ferromagnetism at room temperature. Here we report that the Ruddlesden-Popper phase Pb_n+1(Ti_1-xFe_x)_nO_3n+1 forms spontaneously during pulsed laser deposition of PTFO on LaAlO₃ substrates. High-resolution transmission electron microscopy, x-ray difraction and x-ray photoemission spectroscopy were utilised to perform a structural and ompositional analysis, demonstrating that n≃8 and x≃0.33. The complex dielectric function of the films was determined from far-infrared to ultraviolet energies using a combination of terahertz time-domain spectroscopy, Fourier transform spectroscopy, and spectroscopic ellipsometry. The simultaneous Raman and infrared activity of phonon modes, and the observation of second harmonic generation, establishes a non-centrosymmetric point group for Pb_n+1(Ti_0.67Fe_0.33)_nO_3n+1-δ consistent with ferroelectricity. No evidence of macroscopic ferromagnetism was found in SQUID magnetometry. The ultrafast optical response exhibited coherent magnon oscillations compatible with local magnetic order, and additionally was used to study photocarrier cooling on picosecond timescales. An optical gap smaller than that of BiFeO₃ and long photocarrier lifetimes may make this system interesting as a ferroelectric photovoltaic.

There has been significant interest in BiFeO₃ over the past decade. This interest has focused on the magnetic and electrical properties, which in the long term may prove useful in device applications. This thesis focuses on the synthesis, electrical characterisation, and structural origin of the electrical properties of rare earth doped bismuth ferrite. Two systems have been studied: BiFeO₃ doped with lanthanum and neodymium (Bi₁₋ₓREₓFeO₃ RE= La, Nd). Specific examples have been highlighted focusing on a detailed structural analysis of a lanthanum doped bismuth ferrite, Bi₀.₅La₀.₅FeO₃, and a neodymium analogue, Bi₀.₇Nd₀.₃FeO₃. Both adopt an orthorhombic GdFeO₃-type structure (space group: Pnma) with G-type antiferromagnetism. Structural variations were investigated by Rietveld refinement of temperature dependent powder neutron diffraction using a combination of both conventional “bond angle/bond length” and symmetry-mode analysis. The latter was particularly useful as it allowed the effects of A-site displacements and octahedral tilts/distortions to be considered separately. This in-depth structural analysis was complemented with ac-immittance spectroscopy using the multi-formulism approach of combined impedance and modulus data to correlate structural changes with the bulk electrical properties. This approach was essential due to the complex nature of the electrical response with contributions from different electroactive regions. The structural variations occur due to a changing balance between magnetic properties and other bonding contributions in the respective systems. This results in changes in the magnitude of the octahedral tilts, and A-site displacements giving rise to phenomena such as negative thermal expansion and invariant lattice parameters i.e., the invar effect. More specifically, analysis of Bi₀.₅La₀.₅FeO₃ highlights a structural link between changes in the relative dielectric permittivity and changes in the FeO₆ octahedral tilt magnitudes, accompanied by a structural distortion of the octahedra with corresponding A-site displacement along the c-axis; this behaviour is unusual due to an increasing in-phase tilt mode with increasing temperature. The anomalous orthorhombic distortion is driven by magnetostriction at the onset of antiferromagnetic ordering resulting in an Invar effect along the magnetic c-axis and anisotropic displacement of the A-site Bi³⁺ and La³⁺ along the a-axis. This contrasts with the neodymium analogue Bi₀.₇Nd₀.₃FeO₃ in which a combination of increasing A-site displacements in the ac-plane and decrease in both in-phase and anti-phase tilts combine with superexchange giving rise to negative thermal expansion at low temperature. The A-site displacements correlate with the orthorhombic strain. By carefully changing the synthesis conditions, a significant change in bulk conductivity was observed for a number for Bi₁₋ₓLaₓFeO₃ compositions. A series of Bi₀.₆La0.₄FeO₃ samples are discussed, where changes in the second step of the synthesis result in significantly different bulk conductivities. This behaviour is also observed in other compositions e.g. Bi₀.₇₅La₀.₂₅FeO₃. Changes in the electrical behaviour as a function of temperature are discussed in terms of phase composition and concentration gradients of defects. Activation energies associated with the conduction process(es) in Bi₁₋ₓLaₓFeO₃ samples, regardless of composition, fall within one of two broad regimes, circa. 0.5 eV or 1.0 eV, associated with polaron hopping or migration of charge via oxygen vacancies, respectively. The use of symmetry-mode analysis, in combination with conventional crystallographic analysis and electrical analysis using multi-formulism approach, presents a new paradigm for investigation of structure-property relationships in rare earth doped BiFeO₃.

Orientador: Eudes Borges de Araújo / Resumo: Foram preparados filmes finos, de Ferrita de Bismuto (BiFeO3), considerado um dos principais multiferróico que são classes de materiais que apresentam ferroeletricidade e ferromagnetismo simultaneamente. Os filmes foram preparados por um rota química chamada de Sol-gel modificado, variando-se a quantidade de % de mol do Bismuto, depositados em substratos de platina Pt/TiO2/SiO2/Si(100), variando-se a temperatura de cristalização entre 400°C a 600°C, com o objetivo de eliminar algumas fases indesejadas encontradas na literatura. Alguns filmes finos passaram pelo tratamento térmico em atmosférica de O2, com o intuito de diminuir a condutividade, causada pelas vacâncias de oxigênio no material. Pelos resultados obtidos foi possível conseguir filmes finos sem as fases indesejadas e com condutividade não tão alta, sendo possível realizar análises elétricas. Assim, tornou-se possível analisar o comportamento da permissividade, impedância e condutividade em função do campo aplicado e da temperatura. Com tais resultados mostra-se a indicação de polarização iônica nestes filmes. Eles apresentam uma energia de ativação parecida com filme finos encontrados na literatura. Além disso, também mostra que o comportamento das propriedades físicas são os mesmos quando varia a temperatura e o campo. / Abstract: Bismuth Ferrite (BiFeO3) thin films were prepared, considered one of the main multiferroic that are classes of materials that present ferroelectricity and ferromagnetism simultaneously. The films were prepared by a chemical path called modified sol-gel, varying the amount of Bismuth mol percentage, deposited on Pt/TiO2/SiO2/Si(100) platinum substrates, varying the crystallization temperature between 400 °C to 600 °C, with the aim of eliminating some unwanted phases found in literature. Some thin films underwent the thermal treatment in atmospheric O2, in order to reduce the conductivity, caused by the oxygen vacancies in the material. By the results obtained, it was possible to obtain thin films without the undesired phases and with not so high conductivity, being possible to perform electrical analysis. This way it was possible to analyze the behavior of the permissiveness, impedance and conductivity in function of the applied field and temperature. With these results, it is shown an indication of ionic polarization in these films. They have an activation energy similar to thin films found in literature. It is also shown that the behavior of the physical properties are the same when temperature and the field change. / Mestre

Nas últimas décadas, o consumo de dispositivos eletrônicos e a alta demanda por armazenamento de dados tem mostrado grandes oportunidades para a criação de novas tecnologias que garantam as necessidades mundiais na área de computação e desenvolvimento. Alguns materiais multiferroicos tem sido amplamente estudados e o BiFeO3, considerado o único material multiferroico em temperatura ambiente, ganhou destaque como candidato para produção de dispositivos lógicos e de memória. O uso de técnicas de crescimento como a deposição por laser pulsado permitiu a produção de filmes finos de BiFeO3 com elevado controle de qualidade. Heteroestruturas de filmes multiferroicos de BiFeO3 e LaBiFeO3 foram crescidas com diferentes espessuras sobre substratos de SrTiO3(100), DyScO3(110) e SrTiO3/Si(100) para avaliação e teste de suas propriedades elétricas e magnéticas. Filmes ferromagnéticos de Co0,9Fe0,1 foram depositados por sputtering sobre os filmes multiferroicos para avaliação da interação interfacial entre ordenamentos magnéticos. Técnicas como fotolitografia foram utilizadas para padronização de microdispositivos gravados sobre as amostras. Tanto os filmes finos de BiFeO3 como os de LaBiFeO3 foram crescidos epitaxialmente sobre os substratos já cobertos com uma camada buffer de SrRuO3 usado como contato elétrico inferior. A estrutura cristalina romboédrica das ferritas de bismuto foi confirmada pelos dados de difração de raios X, bem como a manutenção de tensão estrutural causada pela rede cristalina do substrato para amostras de 20 nm. Os valores de coeficiente do tensor piezelétrico d33 foram da ordem de 0,15 V (∼ 60 kV.cm-2) para amostras com 20 nm de espessura enquanto que os valores de voltagem coerciva para as análises de histerese elétrica foram da ordem de 0,5 V para as mesmas amostras. A relação de coercividade elétrica com a espessura corresponde ao perfil encontrado na literatura pela relação E≈d-2/3. As amostras de CoFe/BFO e CoFe/LBFO depositadas em diferentes substratos apresentam acoplamento interfacial entre ordenamento ferromagnético e antiferromagnético com momento ferromagnético de rede. / For the last few decades, the consumption of electronic devices and the high demand for data storage have shown great opportunities to create modern technologies that assure the worldwide needs in computing and development. Some multiferroic materials have been extensively studied and BiFeO3, considered the only multiferroic material at room temperature, has received attention as a candidate to produce logic and memory devices. The use of growth techniques such as pulsed laser deposition allowed the production of thin films of BiFeO3 with high quality control. Multiferroic film heterostructures of BiFeO3 and LaBiFeO3 were grown with different thicknesses on SrTiO3 (100), DyScO3 (110) and SrTiO3/Si (100) substrates to evaluate and test their electrical and magnetic properties. The allow Co0.9Fe0.1 ferromagnetic films were deposited by sputtering on the multiferroic films to evaluate the interfacial interaction between magnetic ordering. Techniques such as photolithography were used to pattern microdevices on the samples. Both the BiFeO3 and LaBiFeO3 thin films were grown epitaxially on the substrates already covered with a SrRuO3 buffer layer used as the lower electrical contact. The rhombohedral crystalline structure of the bismuth ferrites was confirmed by the X-ray diffraction data as well as the strain maintenance caused by the crystal lattice of the substrate for 20 nm samples. The coefficient values of the piezoelectric tensor d33 were around 0.15 V (∼ 60 kV.cm-2) for 20 nm thick samples whereas the coercive voltage values for the electrical hysteresis analyzes were about 0.5 V for the same samples. The relation between electric coercivity and the thickness corresponds to the profile found in the literature by the relation E≈d-2/3. The samples of CoFe/BFO and CoFe/LBFO deposited in different substrates present interfacial coupling between ferromagnetic and antiferromagnetic arrangement with net ferromagnetic moment.

Nas últimas décadas, o consumo de dispositivos eletrônicos e a alta demanda por armazenamento de dados tem mostrado grandes oportunidades para a criação de novas tecnologias que garantam as necessidades mundiais na área de computação e desenvolvimento. Alguns materiais multiferroicos tem sido amplamente estudados e o BiFeO3, considerado o único material multiferroico em temperatura ambiente, ganhou destaque como candidato para produção de dispositivos lógicos e de memória. O uso de técnicas de crescimento como a deposição por laser pulsado permitiu a produção de filmes finos de BiFeO3 com elevado controle de qualidade. Heteroestruturas de filmes multiferroicos de BiFeO3 e LaBiFeO3 foram crescidas com diferentes espessuras sobre substratos de SrTiO3(100), DyScO3(110) e SrTiO3/Si(100) para avaliação e teste de suas propriedades elétricas e magnéticas. Filmes ferromagnéticos de Co0,9Fe0,1 foram depositados por sputtering sobre os filmes multiferroicos para avaliação da interação interfacial entre ordenamentos magnéticos. Técnicas como fotolitografia foram utilizadas para padronização de microdispositivos gravados sobre as amostras. Tanto os filmes finos de BiFeO3 como os de LaBiFeO3 foram crescidos epitaxialmente sobre os substratos já cobertos com uma camada buffer de SrRuO3 usado como contato elétrico inferior. A estrutura cristalina romboédrica das ferritas de bismuto foi confirmada pelos dados de difração de raios X, bem como a manutenção de tensão estrutural causada pela rede cristalina do substrato para amostras de 20 nm. Os valores de coeficiente do tensor piezelétrico d33 foram da ordem de 0,15 V (∼ 60 kV.cm-2) para amostras com 20 nm de espessura enquanto que os valores de voltagem coerciva para as análises de histerese elétrica foram da ordem de 0,5 V para as mesmas amostras. A relação de coercividade elétrica com a espessura corresponde ao perfil encontrado na literatura pela relação E≈d-2/3. As amostras de CoFe/BFO e CoFe/LBFO depositadas em diferentes substratos apresentam acoplamento interfacial entre ordenamento ferromagnético e antiferromagnético com momento ferromagnético de rede. / For the last few decades, the consumption of electronic devices and the high demand for data storage have shown great opportunities to create modern technologies that assure the worldwide needs in computing and development. Some multiferroic materials have been extensively studied and BiFeO3, considered the only multiferroic material at room temperature, has received attention as a candidate to produce logic and memory devices. The use of growth techniques such as pulsed laser deposition allowed the production of thin films of BiFeO3 with high quality control. Multiferroic film heterostructures of BiFeO3 and LaBiFeO3 were grown with different thicknesses on SrTiO3 (100), DyScO3 (110) and SrTiO3/Si (100) substrates to evaluate and test their electrical and magnetic properties. The allow Co0.9Fe0.1 ferromagnetic films were deposited by sputtering on the multiferroic films to evaluate the interfacial interaction between magnetic ordering. Techniques such as photolithography were used to pattern microdevices on the samples. Both the BiFeO3 and LaBiFeO3 thin films were grown epitaxially on the substrates already covered with a SrRuO3 buffer layer used as the lower electrical contact. The rhombohedral crystalline structure of the bismuth ferrites was confirmed by the X-ray diffraction data as well as the strain maintenance caused by the crystal lattice of the substrate for 20 nm samples. The coefficient values of the piezoelectric tensor d33 were around 0.15 V (∼ 60 kV.cm-2) for 20 nm thick samples whereas the coercive voltage values for the electrical hysteresis analyzes were about 0.5 V for the same samples. The relation between electric coercivity and the thickness corresponds to the profile found in the literature by the relation E≈d-2/3. The samples of CoFe/BFO and CoFe/LBFO deposited in different substrates present interfacial coupling between ferromagnetic and antiferromagnetic arrangement with net ferromagnetic moment.

Avec le développement des nouvelles technologies de l'information et de la communication (NTIC), la consommation énergétique des systèmes de traitement et de stockage de données est devenue un problème majeur. Les limites des systèmes actuels à cet égard impliquent le besoin de technologies de rupture ultra-basse consommation.Cette thèse propose une approche originale de cette problématique, basée sur l'utilisation d'un élément magnétoélectrique composite (piézoélectrique/magnétostrictif) bistable et commandable de façon univoque, baptisé MELRAM.L'étude énergétique statique montre que la combinaison d'une anisotropie uni-axiale et d'un champ de polarisation magnétique statique définit deux positions d'équilibre stables perpendiculaires pour l'aimantation dans la partie magnétostrictive. L'application de contraintes piézoélectriques sur celle-ci permet de contrôler électriquement la position de l'aimantation. L'étude énergétique du système permet également de montrer la stabilité du système à long terme (10 ans), dans une large gamme de températures autour de l'ambiante, avec une barrière énergétique de 60kBT. L'étude dynamique, utilisant le modèle du macrospin, permet quant à elle d'exhiber un temps de réponse inférieur à 1ns. L'énergie dissipée lors de l'écriture, d'origine électrique et magnétique, est évaluée à 261kBT (1,1aJ), soit quatre ordres de grandeur en dessous de l'état de l'art.Plusieurs stratégies de lecture par vanne de spin et jonction tunnel magnétique sont proposées et commentées. Les premières réalisations d'éléments nanométriques magnétostrictifs sont présentées ainsi qu'une solution de polarisation magnétique intégrée par aimant permanent / As new information and communication technologies boom, the energy consumption of data processing and storage systems has become a major issue. The limits of state of the art systems regarding this gives rise to the need for ground-breaking ultra-low power technologies.This PhD thesis suggests an original approach of this issue, based on a bistable composite magnetoelectric element (piezoelectric/magnetostrictive) which can be controlled unequivocally, named MELRAM.The static energetic study shows that the combination of an uniaxial anisotropy and a static magnetic bias field defines two stable and perpendicular equilibrium positions for magnetization in the magnetostrictive part. The application of piezoelectric stress allows the electric control of the magnetization position.The energetic study also shows the long term (10 years) stability of the system, in a large temperature range around room temperature, with an energy barrier of 60kBT. The dynamic study, using the macrospin model, gives a response time less than 1ns. The dissipated energy during writing, of electric and magnetic origin, is estimated at 261kBT (1.1aJ), that is to say four orders of magnitude below the state of the art.Several reading strategies using spin valves and magnetic tunnel junction are proposed and commented. First realization of nanometer-sized magnetostrictive elements are presented as well as an integrated polarization solution, using permanent magnets

Multiferroic materials are those materials in which more than one ferroic order coexist. The most technologically appealing multiferroic materials are those showing ferromagnetism and ferroelectricity. Coupling between the mentioned ferroic orders, called magnetoelectric coupling, can yield to new interesting functional applications. In spintronics this coupling would result in the possibility of building magnetic memories controlled by electric field, or transistors where charge is contact-less controlled by a magnetic field. The ultimate goal of the present thesis is to explore the control of the ferroelectric polarization and dielectric properties by magnetic field in thin films. To that purpose dielectric, ferroelectric and magnetoelectric characterization methods of different multiferroic materials have been developed and used. Two big groups of multiferroic materials can be found. On one hand, single-phase multiferroics are those that intrinsically display multiferroicity. On the other hand, multiferroic composites are those where multiferroicity results from the mixture of two different materials that display ferroelectric and ferromagnetic order separately. Single-phase multiferroics can be divided in two subgroups: those where ferroic orders have different sources and those, called magnetic ferroelectrics, where magnetic order induces ferroelectricity and, consequently, larger magnetoelectric coupling is expected. The single-phase multiferroic material studied in the present thesis is YMnO3 in its orthorhombic phase, and it belongs to the magnetic ferroelectrics family. Even though it shows collinear magnetic order in bulk, we will show that cycloidal order in thin film form can be stabilized, giving rise to the capability of controlling the ferroelectric polarization by magnetic field in a reversible manner. Multiferroic composite thin films can be built mainly in two different architectures: vertical (ferromagnetic/ferroelectric columns embedded in a ferroelectric/ferromagnetic matrix) and horizontal (multilayered structures alternating ferromagnetic and ferroelectric materials). Here we compare both, using a ferroelectric perovskite (BaTiO3) and a ferromagnetic spinel (CoFe2O4). We will show that horizontal heterostructures display better ferroelectric properties and larger magnetoelectric coupling, compared to vertical heterostructures, where leakage current is a limiting parameter. The control of dielectric/ferroelectric properties under appropriate heterostructure configuration (in horizontal heterostructures) or deposition conditions (in vertical heterostructures) has been also achieved. / Els materials multiferroics són aquells materials en què coexisteix més d'un ordre ferroic. D'aquests els més interessants són els que presenten ferromagnetisme i ferroelectricitat. La presencia d'acoblament entre aquests dos ordres ferroics, anomenat acoblament magnetoelèctric, obre un nou camp d'aplicacions. En spintrònica, aquest acoblament significaria poder construir memòries magnètiques controlades mitjançant camp elèctric, o transistors on la càrrega es controlaria mitjançant camp magnètic. L'objectiu final d'aquesta tesi és explorar el control de la polarització ferroelèctrica mitjançant camp magnètic en capes fines. Amb aquesta finalitat, s'han utilitzat mètodes de caracterització dielèctrica, ferroelèctrica i magnetoelèctrica en diferents materials multiferroics en capa fina. Existeixen dos grans grups de materials multiferroics. D'una banda, els materials de fase única són aquells que presenten multiferroïcitat de manera intrínseca. D'altra banda, els multiferroics de fase mixta són aquells en els quals la multiferroïcitat és resultat de la barreja de dos materials diferents que presenten ordre ferroelèctric i ferromagnétic per separat. Els materials de fase única es poden dividir en dos subgrups: aquells en què els ordres ferroics tenen diferent origen i aquells, anomenats ferroelèctrics magnètics, en què l'ordre magnètic indueix ferroelectricitat i, de manera conseqüent, s'espera un major acoblament magnetoelèctric. El material multiferroic de fase única estudiat en la present tesi és la o-YMnO(3) en la seva fase ortoròmbica que pertany a la família dels ferroelèctrics magnètics. Tot i que presenta ordre magnètic col.lineal en forma màssica, mostrarem que es pot estabilitzar l'odre cicloïdal en capa fina, permetent el control de la polarització ferroelèctrica mitjançant camp magnètic de manera reversible. Els multiferroics de fase mixta en capa fina es poden créixer utilitzant principalment dues arquitectures diferents: vertical (les columnes ferromagnètiques/ferroelèctriques en una matriu ferroelèctrica/ferromagnètica) i horitzontal (estructures multicapa alternant materials ferromagnètics i ferroelèctrics). Aquí comparem aquestes dues arquitectures, utilitzant una perovskita ferroelèctrica (BaTiO(3)) i una espinela ferromagnètica (CoFe(2)O(4)). Demostrarem que les heteroestructures horitzontals presenten millors propietats ferroelèctriques i un major acoblament magnetoelèctric comparades amb les heteroestructures verticals, en les quals el corrent de pèrdues sembla ser un paràmetre limitant. També s'han aconseguit controlar les propietats dielèctriques/ferroelèctriques mitjançant la modificació de la configuració en les heteroestructures horitzontals o mitjançant la modificació de les condicions de dipòsit en heteroestructures verticals. / Los materiales multiferroicos son aquellos en los que coexiste más de un orden ferroico. DE estos los más interesantes son los que presentan ferromagnetismo y ferroelectricidad. Su acoplamiento, llamado acoplamiento magnetoeléctrico, puede permitir la aplicación de nuevas funcionalidades en el campo de la tecnología. En espintrónica, este acoplamiento significará poder construir memorias magnéticas controladas mediante campo eléctrico, o transistores donde la carga se controlará mediante campo magnético. El objetivo final de esta tesis es explorar el control de la polarización ferroeléctrica mediante campo magnético en capas finas. Con este fin, se han utilizado métodos de caracterización dieléctrica, ferroeléctrica y magnetoeléctrica en diferentes materiales multiferroicos en capa fina. Existen dos grandes grupos de materiales multiferroicos. Por un lado, los materiales de fase única son aquellos que presentan multiferroicidad de forma intrínseca. Por otro lado, los multiferroicos de fase mixta son aquellos en los cuales la multiferroicidad es el resultado de la mezcla de dos materiales diferentes que presentan orden ferroeléctrico y ferromagnético por separado. Los materiales de fase única se pueden dividir en dos subgrupos: aquellos en los que los órdenes ferroicos tienen diferente origen y aquellos llamados ferroeléctricos magnéticos en los que el orden magnético induce ferroelectricidad y, por consiguiente, se espera un mayor acoplamiento magnetoeléctrico. El material multiferroico de fase única que se ha estudiado en esta tesis es la o-YMnO(3) en su fase ortorrómbica y pertenece a la familia de los ferroeléctricos magnéticos. Aunque presenta orden magnético colineal en forma másica, mostraremos que se puede estabilizar el orden cicloidal en capa fina, permitiendo el control de la polarización ferroeléctrica mediante campo magnético de forma reversible. Los multiferroicos de fase mixta en capa fina se pueden crecer utilizando principalmente dos arquitecturas diferentes: vertical (las columnas ferromagnéticas/ferroeléctricas en una matriz ferroeléctrica/ferromagnética) y horizontal (estructuras multicapa alternando materiales ferromagnéticos y ferroeléctricos). Aquí comparamos ambas, utilizando una perovskita ferroeléctrica (BaTiO(3)) y una espinela ferromagnética (CoFe(2)O(4)). Demostraremos que las heteroestructuras horizontales presentan mejores propiedades ferroeléctricas y un mayor acoplamiento magnetoeléctrico comparadas con las heteroestructuras verticales, en las cuales la corriente de pérdidas parece ser un parámetro limitante. También se han conseguido controlar las propiedades dieléctricas/ferroeléctricas mediante el cambio de configuración en heteroestructuras horizontales o mediante el cambio de las condiciones de depósito en heteroestructuras verticales.

Les dernières années les composés multiferroïques ont attiré un grand intérêt en raison de leurs propriétés uniques qui rendent ces matériaux très intéressants pour des applications réelles, par exemple des condensateurs, des détecteurs et des actuateurs ou des dispositifs de mémoire d'ordinateur. Dans la recherche de multiferroïques, PbVO3 est un matériel très prometteur. PbVO3 est isostructural avec PbTiO3 qui est un matériel ferroélectrique très bien connu et étudié et il contient également des ions de vanadium qui portent un spin 1/2, donc, un certain type d'ordre magnétique peut être attendu. Cependant, aucun ordre magnétique n'a pu être observé ce qui pose les deux questions suivantes:1) pourquoi PbVO3 n'est pas magnétique ?2) est-ce que PbVO3 peut être réglé de sorte qu'il devienne magnétique ?L'objectif du projet était de fournir une réponse aux questions ci-dessus par l'étude de la synthèse et des propriétés physiques des oxydes potentiellement multiferroïques de type Pb(V1-xMx)O3 où M = Ti, Fe.Nous avons préparé deux types d'échantillons, échantillons polycristallins et monocristaux. Le batch polycristallin est plus grand et il contient deux séries: la série de titane et de la série de fer avec les compositions chimiques suivantes:PbV1-xTixO3 où x = 0, 0.1, 0.25, 0.5, 0.6, 0.75, 0.8, 1PbV1-xFexO3 où x = 0.1, 0.25, 0.3, 0.4, 0.5, 0.55, 0.6 0.65, 0.75.Des monocristaux ont été préparés uniquement sous la forme de PbVO3 dans lequel aucune substitution n'a été essayée.La structure et les propriétés physiques des échantillons ont été étudiés et on a tenté de corréler les résultats et de formuler un modèle qui pourrait expliquer les comportements fascinants et souvent apparemment contradictoires de nos composés.Le document est structuré en cinq chapitres. Le premier chapitre passe en revue les concepts fondamentaux de la physique de derrière les composés multiferroïques, mettant l'accent sur les propriétés / phénomènes qui sont connectés à nos composés. Le premier chapitre s'occupe également des données déjà publiées dans la littérature pour PbVO3 et les composés substitués.Le deuxième chapitre décrit les méthodes expérimentales et les techniques de mesure utilisées au cours de l'étude.Le troisième chapitre présente les méthodes de préparation, les équipements de haute pression - hautes températures employées et les conditions de réaction nécessaires pour la synthèse de PbVO3 (et ses homologues substitués). La discussion est continuée avec les premières investigations et les résultats qui traitent de la pureté de phase.Le quatrième chapitre traite avec plus de détails les propriétés structurales des échantillons et le cinquième chapitre traite les propriétés magnétiques et diélectriques des composés potentiellement multiferroïques, Pb(V1-xMx)O3.Comme les propriétés physiques de ces matériaux sont fortement dépendantes de la structure des échantillons, les chapitres s'entrecroisent parfois, dans chaque chapitre, des résultats des autres chapitres sont mentionnés et une certaine redondance est donc inévitable. Les mesures d'absorption des rayons X forment le noyau de l'étude parce que ces mesures ont confirmé les états d'oxydation des cations du site B (où la substitution prend place). A partir de ces observations, presque toutes les propriétés structurales et physiques peuvent être expliquées. La substitution avec du titane, est donc révélée isovalente ce qui conduit à la formation de la solution solide PbVO3-PbTiO3 et à la dilution du réseau magnétique de PbVO3. D'autre part, la substitution avec du fer n'est pas isovalente et donc la solution solide PbVO3-PbFeO3 (ce dernier composé n'a jamais été signalé) s'arrête à x = 0.5. Le désordre introduit par la substitution avec du fer conduit à la formation d'un état magnétique similaire à un verre de spin et d'un comportement diélectrique typique pour un relaxeur ferroélectrique. / Recently, multiferroic compounds attracted huge interest due to their unique properties which make such materials very interesting for real life applications, from capacitors, sensors and actuators to computer memory devices. PbVO3 is a very promising material. It is isostructural with PbTiO3 which is a very well known and studied ferroelectric material and it also contains vanadium ions which carry a 1/2 spin so magnetic ordering can be expected. However, no sign of magnetic ordering could be observed which raises the following questions:1) why is PbVO3 not magnetic? and2) could it be tuned so that it becomes magnetic?The objective of the project was to provide some answer to the above questions by studying the synthesis and investigating of the physical properties of the Pb(V,M)O3, M = Ti, Fe potentially multiferroic oxides.We prepared two kinds of samples, polycrystalline and single crystals. The polycrystalline batch is larger containing two series, the titanium series and the iron series with the chemical compositions as follows.PbV1-xTixO3 with x = 0, 0.1, 0.25, 0.5 0.6, 0.75, 0.8, 1PbV1-xFexO3 with x = 0.1, 0.25, 0.3, 0.4, 0.5, 0.55, 0.6 0.65, 0.75Single crystals were prepared only in the form of clean PbVO3 in which no substitution was attempted.The structure and the physical properties of the samples were studied and an attempt was made to correlate the results and try to formulate a model which could explain the intriguing and often apparently contradicting behaviours of our compounds. The present work discusses the results obtained during the study and attempts to shed some light on the subject, without asserting that it reaches a final and definitive conclusion.The work is structured on five chapters.The first chapter reviews the fundamental concepts of the physics behind the multiferroic compounds, emphasizing the exotic properties / phenomena that are connected to our compounds. The first chapter also deals with the data already published in literature for PbVO3 and some substitution compounds.The second chapter describes the experimental methods and investigation techniques used during the study.The third chapter presents the preparation methods, the high pressure - high temperature equipments employed and the reaction conditions required for the synthesis of PbVO3 (and the substitution counterparts). The discussion continues with the first investigations and results which are concerned with phase purity.The fourth chapter discusses with somewhat greater details the structural properties of the samples.The fifth chapter deals with the magnetic and dielectric properties of the potentially multiferroic Pb(V1-xMx)O3 compounds.Since the physical properties of these materials are strongly dependent on the structure of the samples, the chapters intertwine at times, in each chapter results from the other chapters being mentioned and some redundancy is thus unavoidable. The measurements of X-Ray absorption form the core of the work as these measurements confirmed the oxidation states of the B site cations (where the substitution takes places). From these observations almost the entire set of structural and physical properties can be explained. The substitution with titanium is thus shown to be isovalent which leads to the formation of the PbVO3-PbTiO3 solid solution and the dilution of the magnetic network of PbVO3. On the other hand the substitution with iron is not isovalent and therefore the solid solution PbVO3-PbFeO3 (the latter compound was never reported) stops at x = 0.5. The disorder caused by the iron substitution leads to the formation of spin glass like magnetic states and ferroelectric relaxor states on these compounds.