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Multiferroic materials exhibit simultaneously, magnetic and electric order. In a magnetoelectric composite structure, a coupling is induced via an interfacial elastic interaction between magnetostrictive and piezoelectric materials enabling the control of the magnetisation by applying an electric field and vice versa. However, despite the potential of such coupling, experimental limits of theoretical models were observed. This work sheds some light on these limits by focusing the research on the chemistry of nanocomposite CoFe2O4 and BaTiO3, particularly at the interfaces where the coupling predominates. A comparison of the most common conductive oxides, Nb doped SrTiO3 and SrRuO3, was made for the bottom electrode application. The variation of conductive properties in Nb-SrTiO3 thin films at high temperature has been quantified when artificially strained and 60 nm SrRuO3 film was found to be the best bottom electrode choice for room temperature use. Epitaxial growth of magnetic CoFe2O4 was achieved on various metal oxide substrates despite large lattice mismatches. Crystallographic properties and strain evaluation were investigated and a Stranski-Krastanov growth mechanism, arising from the PLD deposition, was predominant. A notable drop of magnetisation was observed depending on the growth template, particularly on BaTiO3 substrates, the piezoelectric counterpart of the magnetoelectric structures. However, an encouraging magnetoelectric coupling induced by thermal phase transition of BaTiO3 was revealed. For BaTiO3, a control of the growth direction was realised by varying the deposition pressure, and the existence of both 180° and 90° ferroelectric domains was observed for films up to 300 nm in thickness. However, both the ferroelectric and piezoelectric properties were reduced in the thin films due to the clamping effect of the substrate. Finally, highly crystalline multilayers of CoFe2O4 and BaTiO3 were prepared on SrRuO3 buffered SrTiO3 substrates. It was found that the degradation of both magnetic and ferroelectric properties was proportional to the increase in the number of interfaces. A thorough microscopic study revealed interdiffusion and chemical instability occurring between CoFe2O4 and BaTiO3 at the interface. This undesired effect was partially recovered by the insertion of an ultra thin layer of SrTiO3, acting as a barrier layer at every interface. This research shows how interfacial chemistry need to be understood to achieve high magnetoelectric coupling in these types of epitaxial engineered structures.

Atomic scale computer simulation techniques are used to predict the thermal conductivity and defect properties in Li2O and Bi4Ti3O12 ceramics. Li2O represents a simple model system while Bi4Ti3O12 is a complex layered oxide ceramic with potential engineering applications due to its highly anisotropic thermal conductivity. The efficacy of the available pair potential models for Li2O are evaluated by comparing with experimental properties such as lattice parameters, elastic constants, thermal expansion and bulk modulus. Other properties, such as the Li ion superionic transition temperature, activation energy and diffusion coefficients for lithium diffusion are also compared with experimental data. The empirical potential set of Chroneos et al. [1] was found to best replicate the experimental diffusion data; therefore this potential was used to calculate the reaction energies of the intrinsic disorder properties and then used in the investigation of the thermal properties of Li2O. The thermal conductivity of Li2O was calculated using the velocity exchange methodology within molecular dynamic simulations. The calculated thermal conductivity was compared to experimental data. A number of corrections have been suggested to improve the agreement between the simulated thermal conductivity and the equivalent experimental value. These corrections and the underlying physics are also discussed within the context of Li2O. The extent of the isotope effect on thermal conductivity was investigated by comparing a supercell, in which there is a random distribution of 6Li and 7Li atoms, to a supercell where all the atoms are given a fractional average mass. The results show that the effect of a non-homogeneous distribution of Li mass can lead to a significant decrease in the thermal conductivity at low temperatures but this effect gradually decreased up to the superionic transition temperatures, where it disappears altogether. Two different structures have been proposed for low temperature Bi4Ti3O12, one with a monoclinic space group (B1a1 ) and the other with a very similar structure but an more symmetric orthorhombic space group (B2cb). A combination of empirical pair potential and first principles density functional simulations are employed to compare the two structures. Both techniques suggest the monoclinic structure has the lower energy of the two candidate structures. The enthalpy of formation of Bi4Ti3O12 is calculated using first principles simulation. The macroscopic properties of a material are determined by the concentration and behaviour of point defects such as vacancies and interstitials. Therefore, density functional theory (DFT) simulations were performed to investigate the formation energies and defect volume of vacancy defect in Bi4Ti3O12. The pair potential set of Islam et al. [2] and Snedden et al. [3] which replicate the structural properties of Bi4Ti3O12 were used to predict the anisotropic thermal conductivities of Bi4Ti3O12 in x, y and z direction. The sound velocity and mean free paths were calculated and used to explain the temperature independent thermal conductivity in the z direction and the anisotropic thermal conductivity. Finally, the Debye temperature as a function of temperature was computed.

Les oxydes de basse dimensionnalité présentent un intérêt scientifique croissant de par leurs propriétés magnétiques étonnantes : transition métamagnétique, aimantation en plateau…. Outre la compréhension de la nature de ces phénomènes, l’intérêt qui émerge concerne les propriétés indirectement associées à l’aimantation particulière de ces systèmes qui peuvent s’inscrire dans le champ de la « spintronique » ou du stockage. Nos travaux, en amont, concernent la synthèse et l’étude de nouveaux composés, essentiellement des oxydes de basses dimensions, qui amènent un regard nouveau sur le contexte de ce magnétisme particulier. L’objectif de cette thèse a donc été de préparer et de caractériser de nouveaux composés inédits, mais également de résoudre les relations structure-propriétés parfois complexes. Les résultats obtenus via un travail de synthèse réfléchi, ont permis d’identifier des systèmes chimiques favorables à la réalisation de matériaux de basse dimensionnalité magnétique. Plusieurs phases inédites aux propriétés remarquables ont pu être identifiées. Il est clair que l’ensemble des comportements observés, 2D-Ising-FM, SCM, plateaux d’aimantation, extrusion réversible, sont tous relativement originaux et nous ont conduit à des avancées importante dans la compréhension du magnétisme de sous-unités en faible interaction. / Low-dimensional oxides with disconnected magnetic units are of increasing interest due the peculiar properties and the versatile interplay between individual magnetic moments into an external magnetic field: metamagnetic transition, magnetization steps…. In addition to understanding these phenomena, the importance of this type of compounds is also emerging properties indirectly related to the specific magnetization of these systems that can be part of the field of "spintronic". Our work, upstream, is the synthesis and study of new compounds, mainly low dimensional oxides, which bring a new vision on such properties, due to their particular magnetic behaviors. The aim of this thesis was therefore to prepare and characterize novel compounds, but also to solve the structure-properties relationship. The results obtained through a work of reflected synthesis, have identified chemical systems favorable to the realization of low-dimensional magnetic materials. Several new phases with specific properties have been identified. It is clear that all the observed behaviors: 2D Ising-FM, SCM, magnetization step, reversible extrusion are relatively original and have led to important advances in the understanding of magnetism of weakly interacting subunits.

Όταν ένα εναλλασσόμενο ηλεκτρικό πεδίο, χαμηλής συχνότητας, εφαρμόζεται κάθετα σε ένα στρώμα νηματικού υγρού κρυστάλλου, με πάχος της τάξης από 10 μm έως 100 μm, παρατηρείται ένας αναπροσανατολισμός του πεδίου του κατευθυντή. Πρόκειται για την περίπτωση της μονοδιάστατης μετάβασης Freedericsz, όπως επίσης και για τη δισδιάστατη ή τρισδιάστατη περίπτωση της ηλεκτροϋδροδυναμικής αστάθειας. Λόγω της διπλοθλαστικότητας των νηματικών υγρών κρυστάλλων, η έναρξη της αστάθειας, και στις δύο περιπτώσεις, προκαλεί σε μονοχρωματική φωτεινή δέσμη, που προσπίπτει στο νηματικό στρώμα, αλλαγή στην ένταση ή/και στη φάση της, με επακόλουθο ιδιαίτερα οπτικά αποτελέσματα. Οι αστάθειες παρατηρούνται όταν η εξωτερικά εφαρμοζόμενη εναλλασσόμενη τάση είναι υψηλότερη από μία τιμή κατωφλίου, ο προσδιορισμός της οποίας έχει ιδιαίτερη σημασία σε πιθανές ηλεκτροοπτικές εφαρμογές των νηματικών υγρών κρυστάλλων. Το κατώφλι της αστάθειας καθορίζεται μέσω της παρατήρησης του οπτικού αποτελέσματος, καθώς η φωτεινή δέσμη προσπίπτει στο νηματικό στρώμα. Όταν η εξωτερικά εφαρμοζόμενη τάση είναι χαμηλότερη ή ίση με την τιμή κατωφλίου, το οπτικό αποτέλεσμα είναι μηδενικό. Καθώς η τιμή της τάσης αυξάνει, σταδιακά εμφανίζεται, όλο και πιο έντονα, το ιδιαίτερο οπτικό αποτέλεσμα της αστάθειας. Έχοντας υπόψη την παραπάνω πειραματική συμπεριφορά, μπορεί κανείς να υπολογίσει την τάση κατωφλίου αρκεί να καθορίσει, μέσω οπτικής παρατήρησης, το σημείο έναρξης της αστάθειας. Στην παρούσα εργασία παρουσιάζεται μία αντικειμενική μέθοδος πειραματικού προσδιορισμού της ηλεκτροϋδροδυναμικής αστάθειας, βασισμένη στην εξάρτηση του χρόνου αποκατάστασης του κατευθυντή, Τ, από την τιμή του εξωτερικά εφαρμοζόμενου ηλεκτρικού πεδίου. Όταν η εφαρμοζόμενη τάση τείνει στην τιμή κατωφλίου, ο ρυθμός εξασθένισης, 1/Τ, του πεδίου του κατευθυντή τείνει στο μηδέν. Επομένως, η γραφική παράσταση του 1/Τ σε συνάρτηση με την εξωτερικά εφαρμοζόμενη τάση καθιστά δυνατό τον προσδιορισμό της τάσης κατωφλίου της αστάθειας. / When an AC electric field in the acoustic frequency range is applied perpendicularly to a nematic liquid crystal layer, with a thickness of the order of 10μm to 100μm, a reorientation of the director of the nematic layer is observed. This is the case of the one dimensional Freedericsz instability, as well as of the two or three dimensional electrohydrodynamic instability. Due to the birefringence of any nematic material, the onset of the instability in both cases causes a monochromatic light beam illuminating the nematic layer, to change its intensity and/or its phase, resulting in spectacular optical effects. On the other hand, the instabilities are observed when the applied AC voltage is larger than a threshold value, the determination of which is of major importance for the potential electro-optical applications of nematic liquid crystals. As a rule, the instability threshold is determined by observing its optical effect on the incident of the monochromatic light beam : When the applied voltage is lower than or equal to its threshold value, the optical effect is zero. Upon further increasing of the applied voltage, the optical effect increases gradually. The measurement of the threshold voltage, based on the above experimental behavior, leads to a subjective estimation of the experimental result. One has to decide which optical result has to be considered to mark the onset of the instability. In what follows, we present a completely objective method for the experimental determination of the instability threshold, based on the dependence of the decay time, T, of the director field on the value of the electric field applied across the nematic layer. When the applied voltage tends to its threshold value, the decay rate, 1/T, of the director field tends to zero. Thus, plotting 1/T as a function of the applied voltage, enables us to graphically determine the threshold value of the latter.

The aim of the current study is to develop an efficient model which can predict low-energy compatible microstructures in ferroelectric bulks and film devices and their dynamic behaviour. The results are expected to assist in the interpretation of microstructure observations and provide a knowledge of the possible domain arrangements that can be used to design future materials with optimum performance. Several recent models of ferroelectric crystals assume low energy domain configurations. They are mainly based on the idea of fine phase mixtures and average compatibility, and can require intensive computation resulting in complex domain configurations which rarely occur in nature. In this research, criteria for the exact compatibility of domain structure in the form of a periodic multi-rank laminate are developed. Exactly compatible structure is expected to be energetically favourable and does not require the concept of a fine mixture to eliminate incompatibilities. The resulting method is a rapid and systematic procedure for finding exactly compatible microstructures. This is then used to explore minimum rank compatible microstructure in various crystal systems and devices. The results reveal routes in polarization and strain spaces along which microstructure can continuously evolve, including poling paths for ferro- electric single crystals. Also, the method is capable to generate all possible exactly compatible laminate configurations for given boundary conditions. It is found that simple configurations are often energetically favourable in conditions where previous approaches would predict more complex domain patterns. Laminate domain patterns in ferroelectrics are classified and corre- lated with observations of domains in single crystals, showing good agreement. The evolution of microstructures under applied mechanical and electrical loads is studied. A variational method, which minimises the overall energy of the crystal is developed. A new concept of transitional “pivot states” is introduced which allows the model to capture the feature that the microstructure in ferroelectric crystal switches between possible domain patterns that are energetically favourable, rather than assuming one particular domain pattern throughout. This model is applied to study the hysteresis responses of barium titanate (BaTiO3) single crystals subjected to a variety of loads. The results have good agreement with experimental data in the literature. The relationship between domain patterns and ferroelectric hysteresis responses is discussed.

The aim of this thesis was to improve the understanding of microstructure in single crystal ferroelectrics. This was achieved through macroscopic testing of Lead Magnesium Niobate – Lead Titanate (PMN-PT) and microscopic observations of Barium Titanate (BT) single crystals. Multi-axial polarization rotation tests on PMN-PT showed a gradual increase in the change in dielectric displacement due to ferroelectric switching as the electric field is applied at increasing angles to the initial polarization direction. A relatively high remnant polarization for loading angle near to 90° suggested that PMN-PT is more polarizable in certain directions. Strains measured in two directions, parallel to the electric field and perpendicular to the electric field, showed a noticeable variation on two opposite faces of the specimen suggesting an effect of local domain configurations on macroscopic behaviour. A micromechanical model gave an insight into the switching systems operating in the crystal during the polarization rotation test. Domain structure in BT was mapped using synchrotron X-ray reflection topography. By making use of the angular separation of the diffracted reflections and specimen rocking, different domain types could be unambiguously identified, along with the relative tilts between adjacent domains. Fine needle domains (width ≈ 10μm) were successfully mapped providing a composite topograph directly comparable with optical micrograph. The domain structure was confirmed using other techniques such as piezoresponse force microscopy and atomic force microscopy/scanning electron microscopy and optical observations on the etched crystal. Results show that combined use of multiple techniques is necessary to gain a consistent interpretation of the microstructure. Finally, domain evolution in BT under compressive mechanical loading was observed in-situ using optical and X-ray diffraction techniques providing a series of images that show ferroelastic transition. The domain configurations influence the switching behaviour and constitutive models that can account for such effects need to be developed. Quantitative and qualitative data presented in this thesis can assist model development and validation.