This thesis gives a detailed overview about the electronic and magnetic structure of three different kinds of 3d-transition metal based materials with potential for possible future applications. The presented materials are a series of “high-k” rare-earth scandates, RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy), the muliferroic layered oxide LuFe2O4, and three iron-based magnetic polyoxometalates of the type {(M)M5}Fe30 (M =Mo,W). The samples are examined by several different x-ray spectroscopic techniques and complementary theoretical approaches, namely multiplet calculations and first principles electronic structure calculations, respectively. The occupied electronic states are determined by photoelectron spectroscopy (XPS) and x-ray emission spectroscopy (XES). The unoccupied states are investigated by x-ray absorption spectroscopy (XAS). X-ray magnetic circular dichroim (XMCD) is used to get the element specific magnetic moment of the materials, and these results are compared to SQUID measurements.

Ce travail de thèse expérimentale a été consacré à la synthèse par des méthodes de chimie par voie humide de nanoparticules à base du multiferroïque BiFeO3 et à leur caractérisation, avec comme objectif finale des applications photocatalytiques. Ce matériau présente une bande interdite, avec un gap de 2.6eV, qui permet la photo-génération de porteurs de charges dans le visible faisant ainsi de BiFeO3 un système intéressant pour des processus photo-induits. Ce travail s’est en particulier focalisé à caractériser les propriétés de nanoparticules à base de BiFeO3 en vue de comprendre l’effet de ses propriétés sur leur potentiel dans des applications liées à la photocatalyse. Tout d’abord, l’étude des effets de taille sur les propriétés structurales, de transitions de phase, et physico-chimiques des particules a été réalisée, en gardant comme principal objectif de découpler les propriétés liées à la surface de celles du massif/cœur de la particule. Pour cela, une maîtrise et une optimisation des procédés de synthèse de particules aux échelles nano- et micro-micrométriques de BiFeO3 a été nécessaire pour obtenir des composés de taille variable et de très bonne qualité cristalline. Malgré la diminution de la taille des particules, on constate que, grâce au contrôle de paramètres de synthèse, nos nanoparticules présentent des propriétés très proches à celles du massif de BiFeO3, gardant la structure rhomboédrique R3c avec des faibles effets de contrainte. Afin de contrôler indirectement par le dopage les propriétés optiques des composés à base de BiFeO3, on a réussi à réaliser un dopage très homogène en La3+, et un dopage partiel en Ca2+, sur le site de Bi3+. Les propriétés optiques des nanoparticules et leurs applications dans les premières expériences photocatalytiques sur la dégradation du colorant rhodamine B ont montré la complexité de la physico-chimie de leur surface et du processus d’interaction lumière-particule. Après analyse des données d’absorbance optique en fonction de la taille de nanoparticules, on observe que la bande interdite déduite pour ces différentes particules n’est pas le facteur prédominant sur les performances photocatalytiques. D’autres facteurs ont pu être identifiés comme étant à l’origine de la localisation de charges photo-générées, tels que des états de surface liés à une fine couche de peau ou skin layer sur les nanoparticules, présentant des défauts structuraux, une réduction de l’état d’oxydation du Fe3+ vers le Fe2+ et la stabilisation d’autres adsorbats, tels que FeOOH ; tous ces facteurs peuvent contribuer au changement dans les performances photocatalytiques. Les résultats photocatalytiques restent très encourageants pour poursuivre les études de nanoparticules à base de BiFeO3, montrant une dégradation de la rhodamine B à 50% au bout de 4h de réaction photocatalytique pour certaines des nanoparticules étudiées. / This experimental PhD work has been dedicated to the synthesis, by wet chemistry methods, and characterization of nanoparticles based on multiferroic BiFeO3, with the aim of using them for photocatalytic applications. This material presents a bandgap of 2.6eV, which allows the charge carrier photoexcitation in the visible range, making BiFeO3 a very interesting system for photoinduced processes. This thesis has been particularly focused on characterizing the properties of BiFeO3 nanoparticles in view of understanding the relationship of their properties on their potential use for photocatalytic applications. First of all, the topic of the size effect on the structural properties, phase transitions, and physics and chemistry of the particles has been developed, keeping as first aim to separate the properties related to the surface from those arising from the bulk/core of the particle. To do so, the mastering and optimization of the synthesis processes of BiFeO3 particles at the nano and microscale were needed, to finally obtain different size compounds with high crystalline quality. Despite the size reduction of the particles, we notice that, thanks to the control of the synthesis process, our BiFeO3 nanoparticles present properties very close to those of the bulk BiFeO3 material, keeping the rhombohedral structure R3c with weak strain effects. In order to indirectly tune the optical properties exploiting the doping, we have succeeded in realizing a homogenous La3+ doping, and a partial Ca2+ doping, on the Bi3+ site. The optical properties of the nanoparticles and their use on the first photocatalytic experiments for degrading rhodamine B dye have shown the complexity of the physics and chemistry phenomena at their surface and of the light-particle processes. After analyzing optical absorbance data as a function of the particle size, we observe that the deduced bandgap for different particles is not the main parameter directing the photocatalytic performances. Other factors have been identified to be at the origin of the localization of the photoexcited charges, as the surface states linked to the skin layer of the nanoparticles, depicting structural defects, a reduction of the oxidation state of Fe3+ towards Fe2+ and the stabilization of other adsorbates, such as FeOOH; all these parameters may contribute to the change on the photocatalytic performances. The photocatalytic results are very encouraging, motivating to continue the study of BiFeO3 based nanoparticles, though depicting a 50% rhodamine B degradation after 4h of photocatalytic reaction using some of the present nanoparticles.

Les films magnétiques ultra-minces font partie intégrante des technologies d'aujourd'hui, comme l'illustre leur omniprésence dans de nombreuses applications courantes telles que les disques durs. A cause de leurs dimensions réduites, les propriétés magnétiques spécifiques à ces échelles conduisent à la formation de structures de spin exotiques et de taille nanométrique. Pour explorer ces matériaux en détail, nous utilisons un magnétomètre à balayage développé dans notre laboratoire et qui est basé sur un défaut de spin unique dans le diamant. Ce capteur non-invasif peut mesurer à l'échelle nanométrique à la fois le champ magnétique et la topographie, et fonctionne aux conditions ambiantes. En développant une méthode d'évaluation originale du champ magnétique, nous déterminons la structure interne de parois de domaines ferromagnétiques et quantifions la force de l'interaction Dzyaloshinskii-Moriya dans des hétérostructures à couches minces. Ensuite, nous mettons en évidence le rôle clé du désordre et de l'histoire magnétique sur la stabilisation des skyrmions dans un échantillon de bicouche magnétique. Enfin, nous visualisons dans l'espace réel une spirale de spin de 70 nm de période dans le matériau multiferroïque BiFeO3 et nous manipulons sa direction de propagation avec des champs électriques. Les connaissances tirées de ces études aideront à exploiter au maximum les capacités des matériaux magnétiques à couche ultra-mince et à les mettre en œuvre dans de nouveaux dispositifs de spintronique. / Thin film magnetic materials are an integral part of today’s technology and widespread applications like the magnetic hard drive disk mirror their potential. Due to their reduced dimensions, size-specific magnetic properties induce the formation of nanoscale, exotic spin structures. To explore such materials in detail, we utilize a home-built nitrogen vacancy scanning magnetometer, based on a single defect in diamond. This non-perturbative probe combines nanoscale magnetic field- and spatial resolution and works under ambient conditions. We develop a new way to determine the inner structure of magnetic domain walls and quantify the strength of the Dzyaloshinskii-Moriya interaction in thin film heterostructures. We reveal the key role of disorder and magnetic history on the stabilization of skyrmions in a magnetic bilayer sample. Finally, we reveal the 70nm-pitch spin spiral in the multiferroic bismuth ferrite in real space and manipulate its propagation direction with electric fields. The insight gained from these studies will help to exploit the full capacity of thin film magnetic materials for spintronic application.

We present a detailed study of the local atomic and magnetic structure of the type-I multiferroic perovskite system (Sr,Ba)(Mn,Ti)O3 using x-ray and neutron pair distribution function (PDF) analysis, polarized neutron scattering, and muon spin relaxation (μSR) techniques. The atomic PDF analysis reveals widespread nanoscale tetragonal distortions of the crystal structure even in the paraelectric phase with average cubic symmetry, corresponding to incipient ferroelectricity in the local structure. Magnetic PDF analysis, polarized neutron scattering, and μSR likewise confirm the presence of short-range antiferromagnetic correlations in the paramagnetic state, which grow in magnitude as the temperature approaches the magnetic transition. We show that these short-range magnetic correlations coincide with a reduction of the tetragonal (i.e. ferroelectric) distortion in the average structure, suggesting that short-range magnetism can play an important role in magnetoelectric and/or magnetostructural phenomena even without genuine long-range magnetic order. The reduction of the tetragonal distortion scales linearly with the local magnetic order parameter. These findings provide greater insight into the multiferroic properties of (Sr,Ba)(Mn,Ti)O3 and demonstrate the importance of investigating the local atomic and magnetic structure to gain a deeper understanding of the intertwined degrees of freedom in multiferroics.

No description available.

Une sous-classe intéressante de matériaux multiferroïques est celle des composés multiferroïques magnétoélectriques, dans lesquels il existe un couplage entre les paramètres d’ordres ferroïques (magnétique et électrique). De ce point de vue, la classe des matériaux Ga₂₋ₓFeₓO₃ a attiré l’attention des chercheurs. Ces composés sont actuellement connus pour leur température de transition élevée ainsi que pour l’interaction possible entre leurs propriétés ferrimagnétiques et piézoélectriques. Leur structure cristallographique et magnétique est assez complexe, du fait du désordre de substitution interne Fe/Ga. Les oxydes M₂Ga₂Fe₂O₉ (M=In, Sc) appartiennent à cette même famille de matériaux et ont été synthétisés pour la première fois afin d’obtenir une structure cristallographique ordonnée de GaFeO₃. Afin d’étudier les propriétés physiques de ces différents composés, nous avons synthétisé par la méthode de la zone flottante (au four à image), en utilisant différentes conditions de croissance, des monocristaux de composition Ga₂₋ₓFeₓO₃ (x=0.90, 1.00 et 1.10). Nous avons également élaboré des échantillons polycristallins de composés GaFeO₃ faiblement dopés en indium ainsi que le composé M₂Ga₂Fe₂O₉ (M=In, Sc). Nous avons enfin préparé de monocristaux de composition In₂Ga₂Fe₂O₉ par la méthode de croissance en flux. L’affinement Rietveld des diffractogrammes des rayons X et des neutrons nous a permis de montrer que les céramiques de GaFeO₃ faiblement dopées en indium et les monocristaux de Ga₂₋ₓFeₓO₃ cristallisent dans le groupe d’espace Pc2₁n. Les paramètres cristallins et la température de Néel caractéristiques pour les monocristaux de Ga₂₋ₓFeₓO₃ varient de manière linéaire avec la teneur en fer. Les affinements nous ont permis de conclure que la structure de ces composés est caractérisée par un désordre élevée (25% de la quantité du fer se trouve sur les sites natifs du gallium). L’incorporation graduelle de l’indium s’accompagne d’une augmentation du volume de la maille ainsi qu’à une diminution de la température de transition magnétique. Le spectre d’excitations magnétiques mesuré pour les cristaux de Ga₂₋ₓFeₓO₃ nous a permis de mettre en évidence une coexistence de l’ordre ferrimagnétique à longue portée et d’un signal de diffusion diffuse en-dessous de la température de Néel. Ce signal diffus suggère l’existence d’une composante de type verre de spin du fait du désordre interne des sites. L'étude de la variation thermique de la constante diélectrique sur un cristal de GaFeO₃ révèle l’absence d’un couplage magnétoélectrique au sein de ces matériaux. L’affinement Rietveld des diagrammes de diffraction des rayons X et des neutrons mesurés sur les poudres de M₂Ga₂Fe₂O₉ (M=In, Sc) révèle une structure orthorhombique de type Pba2 fortement désordonnée, avec quatre sites cationiques d’occupation mixte. Les données de susceptibilité DC et AC couplées avec les mesures de chaleur spécifique et les spectres Mössbauer indiquent, en-dessous d’une température de Tg ≈ 19 K, l’existence d’un état fondamental de type verre de spin dans ce système. Les mesures du spectre d’excitations magnétiques ont mis en évidence l’absence d’ordre magnétique à longue portée et confirment l’existence d’une transition d’un état paramagnétique vers un état verre de spins. L’existence d’un comportement de type verre de spin dans les systèmes Ga₂₋ₓFeₓO₃ et M₂Ga₂Fe₂O₉ (M=In, Sc) souligne l’importance du désordre interne pour la caractérisation de l'état fondamental magnétique. / An interesting sub-class of the multiferroic materials are the multiferroic magnetoelectrics, in which exists a coupling between the two ferroic order parameters (magnetic and electric). From this viewpoint, the case of the (Ga₂₋ₓFeₓO₃)-class of materials has retained special attention. These compounds are now well-known for their high transition temperature as well as for the potential interaction between their ferrimagnetic and piezoelectric properties. Their crystallographic and magnetic structure are quite complicated, due to the existence of an internal site disorder. M₂Ga₂Fe₂O₉ (M=In, Sc) belongs to the (Ga₂₋ₓFeₓO₃)-class of materials and it was first synthesized in an attempt to obtain an ordered GaFeO₃ crystallographic structure. In order to study the physical properties of these compounds, Ga₂₋ₓFeₓO₃ (x=0.90, 1.00 and 1.10) single crystals have been synthesized by the floating zone method in an infrared image furnace, using different growth conditions. Indium doped GaFeO₃ (up to 10% indium content amount) and M₂Ga₂Fe₂O₉ (M=In, Sc) polycrystalline materials have been prepared by solid state reaction. Also, In₂Ga₂Fe₂O₉ single crystals were prepared by the flux method. The indium doped GaFeO₃ and Ga₂₋ₓFeₓO₃ samples crystallize in the Pc2₁n space group as determined from Rietveld refinement of the X-ray and neutron single crystals and powder patterns. The cell parameters and the Néel temperature (TN) of the Ga₂₋ₓFeₓO₃ single crystals varies linearly with the iron content amount. The occupation factors were calculated by refinement and the results showed a disordered structure (25% of the iron amount is found on the native gallium sites). The gradual incorporation of indium is accompanied by an increase of the cell volume and a decrease of the magnetic transition temperature. The magnetic excitations spectra measured for the Ga₂₋ₓFeₓO₃ single crystals revealed a coexistence of a ferrimagnetic ordering and a diffuse scattering signal below the Néel temperature. The diffuse signal suggests the existence of a spin glass like component due to the internal site disorder. Dielectric investigations showed no temperature dependent anomaly of the dielectric constant for the GaFeO₃ single crystal, suggesting a lack of a magnetoelectric coupling signal in this system. The Rietveld refinement of the X-ray and neutron powder patterns for the M₂Ga₂Fe₂O₉ (M=In, Sc) revealed a highly disordered orthorhombic Pba2 structure, offering four mixed cationic crystallographic sites. DC and AC susceptibility data in conjunction with the heat capacity data and Mössbauer measurements indicated a spin-glass-like behavior in this system, with a freezing temperature near Tf ≈ 19 K. The absence of an long range magnetic ordering and the existence of a transition from a paramagnetic state to a «frozen» state were showcased by the spin dynamics spectra measurements. The evidence for glassy behavior in the Ga₂₋ₓFeₓO₃ and M₂Ga₂Fe₂O₉ (M=In, Sc) systems highlights the importance of site disorder in determining the ground state magnetic properties.

In this thesis, we have performed systematical study of anomalous electric and magnetic behaviors of the 3d transition metal oxides; colossal magnetoresistance (La1−xRxMnO3 where R is a divalent alkaline earth ion) and Multiferroic (Ho1-xLaxMn2O5) systems by X-ray and Neutron scattering techniques. In our study, the enhancement of the transfer temperature for La0.8Ba0.2MnO3 under strain effect from the SrTiO3 substrate could be possible due to two reasons which one is Sr diffusion from SrTiO3 substructure, and other one is the octahedral MnO6 high symmetry are increasing. We focus the intrinsic strain effect on La0.67Ca0.33MnO3 and La0.8Ba0.2MnO3 films, and findings show that due to the different ionic sizes of doped Ca or Ba ions, the strain effect acts differently in the way it deforms. The interfacial strain effect produces opposite influences on the lattice symmetry, the average Mn¡VO bond lengths, the average oxygen disorders, the coupling symmetries inside and in the vicinity of the MnO6 octahedrons, as well as producing an opposing trend in metal-insulator and magnetic transition temperatures of the strained La0.67Ca0.33MnO3 and La0.8Ba0.2MnO3 films. The strain effects on the electronic structures of La0.67Ca0.33MnO3 and La0.8Ba0.2MnO3 thin films have been studied by O K-edge x-ray absorption near edge structure (XANES) spectroscopy. For La0.67Ca0.33MnO3, the first-principles calculations reveal that the features in the XANES spectra are associated with hybridized states between O 2p and Mn minority-spin 3d t2g and eg, La 5d/Ca 3d, and Mn 4s/Ca 4p states. An analysis of these features shows that the tensile strain decreases substantially La¡VO and Ca¡VO hybridization and TC for La0.67Ca0.33MnO3. For La0.8Ba0.2MnO3, the small compressive strain enhances slightly La¡VO and Ba¡VO hybridization and TC. In this thesis, the influence of the local structure distortion on the magnetic transition in La doped HoMn2O5 Multiferroics has been investigated systematically. The orthorhombic crystal structure of Ho1−xLaxMn2O5 is maintained up to x¡Ø0.2 but decomposed into multiphase for x¡Ù0.25. By doping La ions to a concentration of 0.1¡Øx¡Ø0.2, the formation of the RMnO3 1(13) phase can be suppressed and single-phase Ho1−xLaxMn2O5 (0.1¡Øx¡Ø0.2) compounds can be formed under 1 atm flowing oxygen. For x=0.2, a ferromagnetic FM transition at 150 K is superimposed on the paramagnetic background, which implies that the compound undergoes a ferromagnetic to antiferromagnetic (AFM) transition. This unique FM to AFM transition is observed for the first time. The FM transition is attributed to the formation of magnetic clusters in a host paramagnetic matrix. The anomalous magnetic clusters phenomena observed in Ho0.8La0.2Mn2O5 can be directly attributed to the different properties between Ho and La ions, and the differences of Ho and La ions are not only in the ionic radius but also in the electron negativity. During 90~150K, X-ray scattering diffraction presented the new addition peaks indicates the new electric density distribution, and the Neutron powder scattering diffraction (NPD) refining results show that the local structure of R-O (R: La, Ho) is un-symmetry which is conflict to the La Extended X-ray absorption fine structure (EXAFS) (which shows that the local structure of La-O becomes more symmetry than H-O. Since the refining values of the NPD are an average of entire crystal, such that it cannot tell the local changes. X-ray absorption spectrum (XAS) and EXAFS, in contrarily, can provide the local information. They implies that the temperature evolutions of the coupling strength with O 2p or unoccupied density state are opposite for the Ho and La ions in our Ho0.8La0.2Mn2O5 sample. Therefore, local change of ions position and charge redistribution happens in this specific temperature range.

Es werden die Verbindungen YMn2O5 und YFeMnO5 diskutiert. Die erstere zeigt unterhalb von TN = 45 K Ferromagnetismus und in der magnetischen Phase schwache Ferroelektrizität. Die elektrische Polarisation wird mit Symmetriebrechung durch die Spinstruktur erklärt, die zur Aufhebung der Inversionssymmetrie führt (sog. unechtes Ferroelektrikum). Isostrukturelle Ersetzung von Mn durch Fe führt zu YFeMnO5, einer Verbindung, die bei T<165 K ferrimagnetisch, jedoch nicht ferroelektrisch ist. Die Spin-Strukturen beider Verbindungen sind bereits eingehend untersucht und zeigen charakteristische Unterschiede. Für Verbindungen der Zusammensetzung YFe(x)Mn(2-x)O5 wurden Röntgenbeugungs-und Absorptionsfeinstruktur-Experimente zur Bestimmung der Kristallstrukur in Abhängigkeit vom Fe-Anteil x durchgeführt und ausgewertet. Die Ergebnisse zeigen, daß die Strukturparameter einen nahezu linearen Verlauf zwischen den aus der Literatur bekannten Grenzfällen YFeMnO5 und YMn2O5 nehmen. Fe ersetzt dabei das Mn auf der fünffach koordinierten Lage innerhalb einer Sauerstoff-Pyramide. Besonders markant ist die unterschiedliche Position von Mn bzw. Fe in dieser Umgebung. Mit Hilfe der Strukturdaten wurden kollineare DFT-Rechnungen im LSDA+U-Formalismus in skalar-relativistischer Näherung durchgeführt. Für YFeMnO5 konnte der experimentell bekannte Grundzustand im Rahmen der Näherung reproduziert werden, obgleich eine Bandlücke nur in Abhängigkeit von der U-Korrektur auftritt. Der berechnete Grundzustand von YMn2O5 gibt die komplizierte magnetische Struktur dieser Verbindung nicht wieder, weil die gewählte Elementarzelle des Gitters dafür zu klein ist. Statt dessen ist der berechnete Grundzustand hier sehr ähnlich zu dem von YFeMnO5. Eine ausführliche Untersuchung der projizierten Zustandsdichten der Metallatome ermöglicht die Diskussion der Kristallfeldaufspaltung im Zusammenhang mit deren Position innerhalb der Sauerstoffpolyeder. Durch Berechnung mehrerer Spinstrukturen in einer kristallographischen Elementarzelle mit erniedrigter Symmetrie konnten die Austauschparameter eines Heisenberg-Modells zwischen den lokalisierten Spins der Metallatome berechnet werden. Die Größenverhältisse dieser Parameter können mit den aus der Literatur bekannten Spinstrukturen in Einklang gebracht werden. Die Wechselwirkungen sind überwiegend antiferromagnetisch, in Übereinstimmung mit den GKA-Regeln für den Superaustausch. Bei YMn2O5 wird insbesondere eine der schwächeren Kopplungen in der magnetischen Struktur periodisch frustriert. Man geht davon aus, daß dies eine mögliche Ursache für das Auftreten von Ferroelektrizität in der magnetischen Phase ist. Bei YFeMnO5 ist der berechnete Wert dieser Kopplung wesentlich größer und die magnetische Struktur beinhaltet keine Frustration. Dies ist eine mögliche Erklärung für die Abwesenheit der magnetisch induzierten Ferroelektrizität in YFeMnO5. Im zweiten Teil stehen das in Perowskitstruktur kristallisierende SrTiO3 und die durch Hinzufügen von SrO daraus hervorgehenden Kristallstrukturen der sog. Ruddlesden-Popper-Phasen (RP) im Mittelpunkt. Die Daten von Nahkanten-Elektronenenergieverlustspektren (ELNES) an der Sauerstoff K-Kante in SrTiO3, SrO und einer RP-Phase wurden ausgewertet und mit DFT-berechneten projizierten Zustandsdichten (PDOS) der 2p-Orbitale der O-Atome in diesen Verbindungen verglichen. Bei ELNES-Nahkantenspektren ist ein solcher Vergleich mit Experimenten im Bereich hoher Elektronenenergien möglich, weil die Auswahlregel auch für die inelastischen Elektronenstöße zutrifft. Die Spektren zeigen für jede Verbindung charakteristische Maxima, deren Ursache die unterschiedliche nähere Umgebung der Sauerstoffatome ist. Weiterhin wurden Experimente an SrTiO3-Einkristallen unter Einfluß elektrischer Gleichströme und -felder durchgeführt. Bei Experimenten an Einkristall-Wafern waren Hinweise auf lokale Veränderungen der Kristallstruktur unter diesen Bedingungen gefunden worden. Ergänzend dazu wurden mikroskopische einkristalline Proben untersucht. Bei geringen Stromstärken zeigte sich dabei das bereits bekannte Degradationsverhalten des elektrischen Widerstands. Bei hohen Stromstärken kommt es zum elektrischen Durchbruch und dauerhafter Erniedrigung des Widerstands. Röntgenbeugungsmessungen ergaben keine Hinweise auf Veränderungen an der Kristallstruktur oder in Form von Zwillings-oder Bruchstückbildung. Im dritten Teil werden Röntgenbeugungsmessungen an CeCu2Si2-Einkristallen diskutiert. Bei der Auswertung älterer Messungen fielen nach der Strukturbestimmung charakteristische Maxima der Restelektronendichte auf, deren Ursprung nicht erklärt werden konnte und die bei mehreren Kristallen beobachtet wurden. Mit erneuten Messungen und Simulationen konnte nun gezeigt werden, daß diese Maxima von einer fehlerhaften Auswertungsmethode verursacht wurden.

Made available in DSpace on 2016-06-02T20:15:32Z (GMT). No. of bitstreams: 1 5887.pdf: 17427536 bytes, checksum: 86a72410b26eb5474b5d0a693e193449 (MD5) Previous issue date: 2013-12-19 / Financiadora de Estudos e Projetos / Multiferroic are materials which have at least two of the three ferroics orders: ferromagnetism, ferroelectricity and/or ferroelasticity. The coupling between these properties, make multiferroic materials of great technological and scientific interest, mainly in the design of new devices such as sensors and spintronic devices. The lead iron tungstate Pb(Fe2=3W1=3)O3 (PFW) is a relaxor multiferroic with ferroelectric transition TC ~ 180K and antiferromagnetic TN ~ 340K, it is a member of the relaxor ferroelectrics family with perovskite structure, in which the two types of cations (Fe3+ e W6+) are randomly distributed in the B site, causing the formation of polar nanoregions (or clusters) of order/disorder at microscopic scale, which would be the origin of the relaxor behavior. Lead iron niobate Pb(Fe1=2Nb1=2)O3 (PFN) is a ferroelectric with diffuse phase transition (DPT) around TC ~ 380K and antiferromagnetic, with the G-type ordering below the Néel temperature, reported in TN ~ 143K. This material has a high dielectric constant, and excellent ferroelectric properties. The Pb2+, at site A, and Nb5+, at site B are responsible for the ferroelectric order, as the Fe3+ at site B is responsible to provide the necessary magnetic moment for the magnetic ordering. In this work, the electrical conduction and relaxation mechanism of multiferroics PFW and PFN ceramics were investigated. The PFW and PFN samples were prepared by solid state reaction method through two stages. This method was effective to obtain samples with majoritary perovskite phase (95,6% and 95,7%), respectively. After the densification process through sintering methods used in this work (conventional sintering, hot pressing and spark plasma), the perovskite phase was increased, being almost 100 %. The samples of PFW and PFN obtained by the different densification techniques, were dense and practically free of pores. For the analysis of the dielectric response of the samples, it was proposed in this work, an analysis protocol, which was effective to find the responsible mechanisms for the dielectric response of the studied materials. Two relaxation processes were identified for each studied sample, which were labeled as: PR-1, PR-2 or PR *. These processes can be attributed to the interfacial polarization and polarization by hopping, occurring due to the presence of defects in the lattice, such as the oxygen vacancies and lead vacancies. From the dependence of the DC electrical conductivity versus temperature, it was possible to identify regions with different mechanisms of electrical conduction for the samples of PFW and PFN. These mechanisms are, thermally activated at high temperatures, hopping of small polarons at intermediate temperatures and variable range hopping at low temperatures. / Multiferróicos são materiais que têm pelo menos duas das três ordens ferróicas: ferromagnetismo, ferroeletricidade e/ou ferroelasticidade. O acoplamento entre essas propriedades, faz com que os materiais multiferróicos despertem um grande interesse científico e tecnológico, principalmente na concepção de novos dispositivos, como sensores e dispositivos de spintrônica. O tungstanato de ferro e chumbo Pb(Fe2=3W1=3)O3 (PFW) é um multiferróico relaxor com transição ferroelétrica TC ~ 180K e antiferromagnética TN ~ 340K, ele é membro da família dos ferroelétricos relaxores com uma estrutura perovskita, em que os dois tipos de cátions (Fe3+ e W6+) são aleatoriamente distribuídos no sítio B, fazendo com que em escala microscópica existam nanoregiões polares (ou clusters) de ordem/desordem neste sítio, que seriam a origem do comportamento relaxor. O niobato de ferro e chumbo Pb(Fe1=2Nb1=2)O3 (PFN) é um ferroelétrico com transição de fase difusa (TFD) ao redor de TC ~ 380K, e antiferromagnético, com ordenamento do tipo-G abaixo da temperatura de Néel reportada em TN ~ 143K. Ele ainda apresenta uma alta constante dielétrica, além de excelentes propriedades ferroelétricas. Neste material o Pb2+, no sítio A, e Nb5+, no sítio B são responsáveis pelo ordenamento ferroelétrico, enquanto o Fe3+ no sítio B é responsável em fornecer o momento magnético necessário para o ordenamento magnético. Neste trabalho, os mecanismos de condução e relaxação elétrica das cerâmicas multiferróicas de PFW e PFN foram investigados. As amostras de PFW e PFN foram preparadas por reação de estado sólido através do método de dois estágios, que se mostrou eficaz na obtenção de amostras com fase perovskita majoritaria (95,6% e 95,7%), respectivamente. De modo geral, essas amostras ao passar pelo processo de densificação através dos métodos de sinterização utilizados nesse trabalho (sinterização convencional, sinterização por prensagem uniaxial a quente e sinterização por spark plasma), tiveram suas fases perovskitas majoritarias aumentadas, chegando à praticamente 100%. As amostras de PFW e PFN, obtidas pelas diferentes técnicas de densificação, mostraram-se densas e praticamente livres de poros. Para análise da resposta dielétrica apresentada por essas amostras, foi proposto neste trabalho um protocolo de análise, que se mostrou eficaz em descrever o comportamento da resposta dielétrica apresentado pelas mesmas. Foram identificados dois processos de relaxação para cada amostra em estudo, os quais foram identificados por: PR-1, PR-2 ou PR*. Esses processos podem ser atribuídos à polarização interfacial e à polarização por salto, os quais ocorrem pela presença de defeitos na rede, tais como, as vacâncias de oxigênio e as vacâncias de chumbo. Através da dependência da condutividade elétrica DC em função da temperatura, foi possível identificar regiões com diferentes mecanismos de condução para as amostras de PFW e PFN. Dentre eles estão, ativação térmica em altas temperaturas, hopping de pequenos polarons em temperaturas intermediárias e hopping de alcance variável em baixas temperaturas.

Submitted by Aelson Maciera (aelsoncm@terra.com.br) on 2017-05-17T17:35:18Z No. of bitstreams: 1 teseDSFV.pdf: 5669668 bytes, checksum: 165ca1333f2345e5e1406b40a2c0548c (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-05-22T13:43:35Z (GMT) No. of bitstreams: 1 teseDSFV.pdf: 5669668 bytes, checksum: 165ca1333f2345e5e1406b40a2c0548c (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-05-22T13:43:44Z (GMT) No. of bitstreams: 1 teseDSFV.pdf: 5669668 bytes, checksum: 165ca1333f2345e5e1406b40a2c0548c (MD5) / Made available in DSpace on 2017-05-22T13:49:08Z (GMT). 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.