Thesis advisor: Stephen Wilson / The work performed within this thesis is divided into two parts, each focusing primarily on the study of magnetic phase behavior using neutron scattering techniques. In first part, I present transport, magnetization, and neutron scattering studies of materials within the iridium oxide-based Ruddelsden-Popper series [Srn+1IrnO3n+1] compounds Sr3Ir2O7 (n=2) and Sr2IrO4 (n=1). This includes a comprehensive study of the doped bilayer system Sr3(Ir1-xRux )2O7. In second part, I present my studies of the effect of uniaxial pressure on magnetic and structural phase behavior of the iron-based high temperature superconductor Ba(Fe1-xCox)2As2. Iridium-based 5d transition metal oxides host rather unusual electronic/magnetic ground states due to strong interplay between electronic correlation, lattice structure and spin-orbit effects. Out of the many oxides containing iridium, the Ruddelsden-Popper series [Srn+1IrnO3n+1] oxides are some of the most interesting systems to study both from the point of view of physics as well as from potential applications. My work is focused on two members of this series Sr3Ir2O7 (n=2) and Sr2IrO4 (n=1). In particular, our combined transport, magnetization and neutron scattering studies of Sr3Ir2O7 (n=2) showed that this system exhibits a complex coupling between charge transport and magnetism. The spin magnetic moments form a G-type antiferromagnetic structure with moments oriented along the c-axis, with an ordered moment of 0.35±0.06 µB/Ir. I also performed experiments doping holes in this bilayer Sr3(Ir1-xRux)2O7 system in order to study the role of electronic correlation in these materials. Our results show that the ruthenium-doped holes remain localized within the Jeff=1/2 Mott insulating background of Sr3Ir2O7, suggestive of `Mott blocking' and the presence of strong electronic correlation in these materials. Antiferromagnetic order however survives deep into the metallic regime with the same ordering q-vector, suggesting an intricate interplay between residual AF correlations in the Jeff=1/2 state and metallic nanoscale hole regions. Our results lead us to propose an electronic/magnetic phase diagram for Sr3(Ir1-xRux)2O7 system showing how the system moves from Jeff=1/2 antiferromagnetic Mott insulator (Sr3Ir2O7) to paramagnetic Fermi liquid metal (Sr3Ru2O7). On the other hand, our neutron scattering measurements on Sr2IrO4 (n=1), a prototypical Jeff=1/2 Mott insulator, showed that the spins arranged antiferromagnetically in ab-plane with an ordered moment comparable to that of Sr3Ir2O7. The second part of my work is comprised of a neutron scattering-based study of the Ba(Fe1-xCox)2As2 system, a bilayer family of iron-based high temperature superconductors. Undoped, this system exhibits either simultaneous or nearly simultaneous magnetic and structural phase transitions from a high temperature paramagnetic tetragonal phase to low temperature orthorhombic antiferromagnetic phase. With the gradual suppression of these two temperatures, the superconducting phase appears with the highest TC obtained just beyond their complete suppression. It has been proposed that these coupled magnetostructural transitions are secondary manifestations which arise as a consequence of electronic nematic ordering that occurs at a temperature higher than either of them. My work is mainly focused on probing the spin behaviors coupling to this electronic nematic phase. I devised a small device to apply uniaxial pressure along an in-plane high symmetry axis and studied the magnetic and structural behavior in series of Ba(Fe1-xCox)2As2 compounds via neutron scattering in presence of uniaxial pressure. There is an upward thermal shift in the onset of structural and magnetic transition temperature caused by this uniaxial pressure which is surprisingly insensitive to cobalt concentration in the absolute scale. Furthermore, on the first order side of the phase diagram (below the tricritical point), the structural and magnetic transitions are decoupled with magnetic transition following structural distortion. This study suggests the importance of both spin-lattice and orbital-lattice interactions in these families of compounds. / Thesis (PhD) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

La structure du gap supraconducteur et sa modulation sont intimement liées au potentiel d'interaction responsable de l'appariement des électrons d'un supraconducteur. Ainsi, l'étude de la structure du gap-SC et de sa modulation permet de faire la lumière sur la nature du mécanisme d'appariement des électrons. À cet égard, les résultats expérimentaux des supraconducteurs à base de fer ne cadrent pas dans un seul ensemble, ce qui est en opposition au gap-SC universel des cuprates. Dans ce qui suit, nous présenterons une étude systématique du gap-SC pour plusieurs pnictides. En effet, en utilisant la conductivité thermique, une sonde directionnelle du gap-SC, nous avons été en mesure de révéler la structure du gap-SC pour les composés suivants : Ba[indice inférieur 1-x]K[indice inférieur x]Fe[indice inférieur 2]As[indice inférieur 2], Ba(Fe[indice inférieur 1-x]Co[indice inférieur x])[indice inférieur 2]As[indice inférieur 2], LiFeAs et Fe[indice inférieur 1-[delta]] Te[indice inférieur 1-x]Se[indice inférieur x]. L'étude de ces quatre composés, de trois différentes familles structurales, a pu établir un tableau partiel mais très exhaustif de la structure du gap-SC de pnictides. En effet, tel qu'illustré dans cette thèse, ces quatre composés ne possèdent aucun noeud dans leur structure du gap-SC à dopage optimal. Toutefois, à une concentration différente de celle optimale pour les composés K-Ba122 et Co-Ba122, des noeuds apparaissent sur la surface de Fermi, aux extrémités du dôme supraconducteur. Ceci suggère fortement que, pour ces composés, la présence de noeuds sur la surface de Fermi est nuisible à la phase supraconductrice.

La découverte récente de supraconductivité à relativement haute température (Tc,max=56K) dans les pnictures de Fer soulève des questions fondamentales sur l’origine et la nature de la supraconductivité : en particulier, la présence d’une phase antiferromagnétique à proximité de celle-ci dans leur diagramme de phase, comme dans d’autres supraconducteurs non conventionnels pose la question du lien entre magnétisme et supraconductivité.Nous nous sommes intéressés à la nature de l’état normal ainsi que des phases antiferromagnétique et supraconductrice d’un point de vue local grâce à la Résonance Magnétique Nucléaire (RMN) dans les pnictures de Fer. Nous avons pour cela étudié des pnictures de Fer de même composé parent BaFe2As2 pour des substitutions Co en site Fer de nature hétérovalente et réalisant un dopage électron ou bien Ru en site Fer de nature isovalente. L’état normal de ces matériaux présente des différences notables avec l’état normal des cuprates supraconducteurs : le désordre introduit par les substitutions au niveau intraplan est faible et on constate l’absence d’une phase de PseudoGap pour la susceptibilité de spin. Les diagrammes de phase sont similaires pour le Co et le Ru mais nos mesures montrent que la nature des phases antiferromagnétique et supraconductrice est en réalité qualitativement différente à l’échelle locale pour les deux types de substitution. Pour la substitution au Co réalisant un dopage électron, les phases électroniques sont homogènes et nous démontrons en particulier qu’à certains dopages, un ordre antiferromagnétique incommensurable coexiste avec la supraconductivité jusqu’à une échelle atomique, suggérant une nature itinérante du magnétisme et un état supraconducteur possédant une symétrie non conventionnelle. Pour la substitution isovalente au Ru, les phases électroniques sont inhomogènes à une échelle étonnamment faible, de l’ordre du nanomètre, mettant en jeu une coexistence entre magnétisme et supraconductivité très distribuée spatialement. Ce travail illustre la possibilité d’engendrer une phase supraconductrice non conventionnelle en déstabilisant une phase antiferromagnétique au moyen de mécanismes agissant soit dans l’espace réciproque (dopage électron), soit dans l’espace réel (substitution isovalente) et donnant lieu par ailleurs à une coexistence de ces phases de nature très différente dans les deux cas. / The recent discovery of superconductivity at a rather high temperature in the iron pnictides (Tc,max=56K) has revived some fundamental questions about the existence and the nature of the superconducting phase : in particular, the existence of an antiferromagnetic phase that is in vicinity of the superconducting phase in their phase diagram, as in other unconventional superconductors, raises questions about the link between magnetism and superconductivity. In this thesis, we studied the normal state as well as the antiferromagnetic and superconducting phases of the iron pnictides on a local scale with Nuclear Magnetic Resonance (NMR). Starting from the same parent compound BaFe2As2, we studied heterovalent Co substitution in Fe site realizing an electron doping and isovalent Ru substitution in Fe site. The normal state is shown to display important qualitative differences with the normal state of cuprates superconductors: disorder induced substitutions in electronically active layers is weak and we show the absence of a PseudoGap phase from spin susceptibility measurements. Whereas the phase diagram is similar for Co and Ru substitutions, we show that the nature of the antiferromagnetic and the superconducting phases is qualitatively different on a local scale in the two cases. For Co substitution leading to electron doping, the electronic phases are homogeneous and we demonstrate in particular the homogeneous coexistence of antiferromagnetism and superconductivity down to an atomic scale for some compositions: this suggests a magnetism of itinerant nature and an unconventional superconducting order parameter for the superconducting phase. For the isovalent Ru substitution, the electronic phases are inhomogeneous at a scale surprisingly low of the order of the nanometer scale, leading to a coexistence that is very distributed spatially. This works shows the possibility to induce an unconventional superconducting phase by the weakening of an antiferromagnetic phase made possible with very different means : either in reciprocal space with electron doping or in real space with isovalent substitution. Moreover, this is shown to lead to different kinds of coexistence between these phases in the two cases.

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2014-08-15T15:38:59Z No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Dissertacao Vanuildo Silva de Carvalho - Fisica.pdf: 2382600 bytes, checksum: 837960b5396c4b0abf08e52288edc3bf (MD5) / Made available in DSpace on 2014-08-15T15:38:59Z (GMT). No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Dissertacao Vanuildo Silva de Carvalho - Fisica.pdf: 2382600 bytes, checksum: 837960b5396c4b0abf08e52288edc3bf (MD5) Previous issue date: 2012-04-04 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / We study the low-energy properties of a two-band model by means of the eld-theoretical renormalization group (RG) up to two loops in order to describe some iron-based pnictide superconductors. Initially, we reproduce some known results of the RG up to one loop to show how one should proceed with the eld-theoretical RG. We then calculate the self-energy of the model and the irreducible four-point vertex functions ��(4) up to two loops. We derive the RG equations for the couplings and the quasiparticle weight of the model at this order of approximation. Following a similar procedure, we show how to determine the possible instabilities of the ground state of the model and the nature of the low-energy elementary excitations by calculating the RG ow equations up to two loops for the extended s-wave (s ) and conventional s-wave (s++) superconducting susceptibilities, charge and spin density wave susceptibilities and also the uniform charge and spin susceptibilities. The numerical analysis of these equations reveals that the couplings of the two-band model are divergent in the low-energy limit, but their ratio ows to a xed point. We obtain, by means of this analysis, that the antiferromagnetic instability is the dominant one, whereas the superconducting instability is the second most important instability. We show that the superconducting instability of s -type exceeds the s++-type when the Umklapp interaction is present and may overcome the antiferromagnetic instability for some doping value in the system. The numerical solution for the quasiparticle weight reveals that this quantity is always close to its initial value during the renormalization procedure of the model, while the uniform charge and spin susceptibilities remain nite for the energy scales where the RG approach up to two loops is valid. These results are consistent with the interpretation that the normal state of that model is, in fact, described by Landau's Fermi liquid theory with well-de ned quasiparticle excitations in the low-energy limit. / Estudamos as propriedades de baixa energia de um modelo de duas bandas por meio do grupo de renormalização (GR) de teoria de campos até dois loops para a descrição de alguns pnictídeos supercondutores baseados em ferro. Inicialmente, reproduzimos alguns resultados conhecidos do GR até um loop para mostrar como se deve proceder com o GR de teoria de campos. Calculamos, então, a auto-energia do modelo e as funções vértices irredutíveis de quatro pontos ��(4) até dois loops. Derivamos as equações do GR, nessa ordem de aproximação, para os acoplamentos e para o peso da quasipartícula do modelo. Seguindo um procedimento similar, mostramos como se determinar as possíveis instabilidades do estado fundamental do modelo e a natureza das excitações elementares de baixa energia por meio do cálculo das equações do GR até dois loops para as susceptibilidades supercondutoras do tipo onda-s estendida (s ) e onda-s convencional (s++), susceptibilidades de ondas de densidade de carga e spin e tamb em as susceptibilidades uniformes de carga e spin. A análise numérica dessas equações revela que os acoplamentos do modelo de duas bandas são divergentes no limite de baixa energia, mas a razão deles tende para um ponto fixo. Obtemos, por meio dessa análise, que a instabilidade antiferromagnética é a dominante, enquanto a instabilidade supercondutora é a segunda mais importante. Mostramos que a instabilidade supercondutora do tipo s se sobressai sobre a do tipo s++ quando a interação do tipo Umklapp está presente e, eventualmente, supera a instabilidade antiferromagnética para algum valor de dopagem no sistema. A solução numérica para o peso da quasipartícula revela que essa quantidade sempre fica próxima do seu valor inicial durante o processo de renormalização do modelo, enquanto que as susceptibilidades de carga e spin uniformes permanecem infinitas para escalas de energia onde o método do GR até dois loops é válido. Esses resultados s~ao consistentes com a interpretação de que o estado normal desse modelo é, de fato, descrito pela teoria do líquido de Fermi de Landau com excitações de quasipartículas bem definidas no limite de baixa energia.

Orientador: Sergio Gama / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-01T23:11:42Z (GMT). No. of bitstreams: 1 RamirezPlaza_EdisonJesus_D.pdf: 20488929 bytes, checksum: 440763eb44bb40a463607435ffc8cb2a (MD5) Previous issue date: 2002 / Resumo: Nos últimos anos tem surgido grande interesse da comunidade científica para entender os mecanismos físicos microscópicos que governam os efeitos magnetocalóricos, principalmente em compostos intermetálicos contendo terras raras (R). Em particular, tem-se estudado compostos do tipo RX com X = N, P, As, Sb e Bi, pois eles formam estruturas com alta simetria, facilitando a interpretação das observações experimentais. No presente trabalho são estudadas as propriedades magnéticas dos compostos RX, com R = Nd e Yb, dando ênfase na identificação da contribuição quadrupolar na região paramagnética. Para atingir nosso objetivo foram realizados programas computacionais tanto em Mathematica como em Fortran. O modelo de Hamiltoniano usado incorpora o campo cristalino, o termo quadrupolar e o campo molecular, incluindo o campo dinâmico. Foram realizados cálculos da variação da magnetização com a temperatura e o campo aplicado, da susceptibilidade, assim como da entropia magnética. Amostras policristalinas dos compostos RX foram preparadas e caracterizadas por nosso grupo. A análise dos resultados de medidas magnéticas mostram a contribuição quadrupolar e pelo confronto com os cálculos computacionais, são obtidos os parâmetros do campo molecular e quadrupolar. Suficientes evidências são apresentadas para reforçar os resultados obtidos. O entendimento da contribuição quadrupolar nestes compostos é o ponto central do presente estudo. O modelo desenvolvido pode ser utilizado para melhorar as previsões teóricas nos estudos do efeito magnetocalórico / Abstract: In the last years great interest from the scientific community has arisen for understand the microscopic physical mechanisms that govern the magnetocaloric effects, mainly in intermetalic compounds containing rare earths (R). In particular, ones have studied RX compounds with X = N, P, As, Sb and Bi, because they form structures with high symmetry, facilitating the interpretation of the experimental observations. In the present work are studied the magnetic properties of the RX compounds, with R = Nd and Yb, with emphasis in the identification of the quadrupolar contribution in the paramagnetic region. To reach our goals were developed computational algorithms in Mathematica and Fortran software. The model of Hamiltonian used incorporates the crystalline, quadrupolar and molecular field terms, including the dynamic field. Calculations were carried out of the magnetization variation with temperature and applied field, ofthe susceptibility, as well as ofthe magnetic entropy. Polycrystalline samples of the RX compounds were prepared and characterized by our group. Analysis of the magnetic measurements results show the quadrupolar contribution and from the confront with the calculations results the parameters of the molecular and quadrupolar field are obtained. Sufficient evidences are presented for reinforce the conclusions. The understanding of the quadrupolar contribution in these compounds is the main point of the present study. The developed model can be used to improve the theoretical calculations in magnetocaloric effect studies / Doutorado / Física / Doutor em Ciências

Thesis advisor: Hong Ding / Thesis advisor: Ziqiang Wang / The discovery of the high-T_c superconductivity in iron-arsenic materials in 2008 immediately became one of the hottest topics in the condensed matter physics. This dissertation presents a systematic study on the pairing symmetry and electronic structure on the hole doped materials of BaFe₂As₂ (so called &ldquo;122&rdquo;-system), by angle-resolved photoemission spectroscopy (ARPES). In the early ARPES studies on &ldquo;122&rdquo;-pnictides, we observed two hole-like Fermi surfaces (FSs) centered at the Brillouin zone (BZ) center, (&Gamma;), and two electron-like FSs centered at the zone corner (M), which is (&pi;, &pi;) in the BZ or (&pi;, 0) in the unfolded BZ. The size of these FS sheets can be changed by carrier doping, which causes change of the chemical potential. In the superconducting state, temperature (<italic>T</italic>) and momentum (<italic>k</italic>) dependence of ARPES measurements reveals the Fermi-surface-dependent nodeless superconducting gaps in this system and shows that an <italic>s</italic>-wave symmetry is the most natural interpretation for our findings in terms of the pairing order parameter. The ratio 2&Delta;/k_BT_c switches from weak to strong coupling on different FS sheets. Large superconducting gaps are observed with a strong coupling coefficient (2&Delta;/k_BT_c) on the near-nested FSs connected by the antiferromagnetic (AF) wave vector ((&pi;, &pi;) in the BZ or (&pi;, 0) in the unfolded BZ). When T_c is suppressed in the heavily overdoped materials, the near-nesting condition vanishes, or more precisely, the (&pi;, &pi;) inter-FS scattering disappears due to the absence of either the hole-like or the electron-like FS at the Fermi energy (E_F). We have also performed ARPES measurements on k_z-dependence of the superconducting gap and band structure of the optimally hole doped sample Ba_0.6K_0.4Fe₂As₂. By varying the photon energy, we can tune k_z continuously. While significant k_z dispersion of the superconducting gaps is observed on the hole-like bands, much weaker k_z dispersion of the superconducting gaps is observed on the electron-like bands. Remarkably, we find that a 3D gap function based on short-range pairing can fit the superconducting gaps on all the FS sheets. Moreover, an additional hole-like FS (referred as the &alpha;<super>&lsquo;</super> FS) predicted by local density approximation (LDA) calculations is observed around the Z point. The disappearance of intensity of the &alpha;<super>&lsquo;</super> band near E_F at k_z = &pi;/2 suggests that the &alpha;<super>&lsquo;</super> band could either sink below E_F or be degenerate with the inner hole (&alpha;) band. The studies on the &alpha;<super>&lsquo;</super> band in the superconducting state reveal a nearly isotropic superconducting gap on this FS sheet. Underdoped samples Ba_0.75K_0.25Fe₂As₂ are used to study how the AF fluctuations and superconductivity interplay in the underdoped regime that is closer to the AF phase. we observe that the superconducting gap of the underdoped pnictides scales linearly with T_c. A distinct pseudogap develops upon underdoping and coexists with the superconducting gap. Remarkably, this pseudogap occurs mainly on the FS sheets that are connected by the AF wave vector, where the superconducting pairing is stronger as well. This suggests that both the pseudogap and the superconducting gap are driven by the AF fluctuations, and the long-range AF ordering competes with the superconductivity. The observed dichotomic behaviour of the pseudogap and the SC gap on different FS sheets in the underdoped pnictides shares similarities with those observed in the underdoped copper oxide superconductors, providing a possible unifying picture for both families of high-temperature superconductors. / Thesis (PhD) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

The recent discovery of the iron-pnictide superconductors has generated tremendous excitement, in part because there are many tantalizing similarities to the cuprate superconductors. As with the cuprates, it is strongly suspected that the spins contribute to superconductivity. There seems to be a strong relationship between the lattice and magnetism in this system. Several authors have discussed the possibility of spin-phonon coupling, but direct experimental evidence has remained elusive. This work discusses the relationship between the spins and the lattice in the $BaFe_{2}As_{2}$ family. We demonstrate the presence of negative thermal expansion in these materials, which is a strong indicator of spin-lattice interaction. In addition, we have conducted inelastic neutron scattering experiments to examine the dynamical relationship between the spins and the lattice. In particular, we make use of the phenomenon known as magnetovibrational scattering to search for evidence of spin-phonon coupling. We believe that this is the first work to use magnetovibrational scattering in an antiferromagnetic system as a tool to study the spin-phonon interaction. Our results provide direct experimental evidence for the existence of spin-phonon coupling, with possible implications about the role of phonons in the superconductivity of iron pnictides.

In this work, a 3-terminal capacitance dilatometer was set up and used for measurements of the thermal expansion and magnetostriction of novel superconducting iron pinictides and related materials. In particular, \re~with R\,=\,La, Ce, Pr, Sm, Gd, \laf~and Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ have been investigated. The data on polycrystalline \laf~are the first published thermal expansion data on this material. The lattice effects at the structural and the magnetic phase transition have been investigated and the phase diagram upon F-doping has been studied. A main result is the observation of a previously unknown fluctuation regime for the doping level $\mathnormal{x}\leqslant$ 0.04 over a large $T$ range above the structural transition temperature \ts. The absence of any structural anomalies in the normal state of the superconducting \laf~samples with $\mathnormal{x}\geqslant$ 0.05 corroborates the discontinuous character of the phase boundary not only for the magnetism but also for the structural degrees of freedom. Similarly, the presence of high-temperature fluctuations is found for all \re~undoped materials under study. The discussion of the probable origin of the fluctuations as well as the definition of the structural transition temperature \ts~are done. The low temperature features shown by the thermal expansion data for \re~are caused by the onset of long range magnetic order of the $4f$-moments and their different configurations. In particular, \pr, which has a very pronounced anomaly associated with Pr-ordering exhibits a large magnetostriction at low temperatures. By discussing this effect along with the magnetization, resistivity and other measurements, it is found that this large magneto-elastic effect may originate from the correlations between the momentum from Fe$^{3+}$ and Pr$^{3+}$. Last, the thermal expansion of Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ 122 single crystals is investigated. Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ is one of the first materials where single crystals are available. The thermal expansion results on the undoped compound with $x=0$ show a large anomaly at the combined magnetic and structural transition which is far sharper than that for polycrystalline systems. Upon doping, both transitions are suppressed and their splitting is visible in the thermal expansion data. The high precision thermal expansion and magnetostriction results presented in this work are among the first data on the novel family of iron-based superconductors. A valuable insight in the respective ordering phenomena and the thermodynamic properties is provided.

La plupart des pnictures ternaires de formule générale MM'X (où M et M' sont des métaux de transition et X un élément p tel que P et As) cristallisent avec une structure dérivée de type Fe2P présentant un effet magnétocalorique (MCE) élevé avec une concentration en électrons d proche de celle de Fe. Au contraire, les systèmes polytypes MM'X de type Co2P conduisent à des performances modestes, même lorsque les éléments métalliques sont identiques, à savoir les phases MnFeP1-xAsx (hexagonale) et MnFe1-xCoxP (orthorhombique) présentes dans des diagrammes de phases magnétiques très similaires. Ainsi, le but de ce travail était tout d'abord de mieux comprendre l'évolution fondamentale des comportements structuraux et magnétiques de la série orthorhombique lorsqu'on effectue des substitutions partielles d'éléments choisis sur des sites spécifiques, tels que Mn par Cr, Ni par Fe ou Co, et P par Si ou Ge. D'autre part, il s'agissait d'optimiser la formulation et le procédé d'élaboration visant à concevoir des composés de type hexagonal dans lesquels As est entièrement remplacé par Si plus sûr, et présentant des performances MCE élevées, en vue d'une production pilote.Dans un premier temps, des efforts de synthèse suivis d'analyses DRX, de mesures d'aimantation en fonction du champ et de la température, d'analyses par diffraction de neutrons des structures magnétiques, de caractérisations calorimétriques et de mesures comparatives de MCE, ont été réalisées sur de nombreux échantillons formant un panel représentatif. De manière inattendue, Ni, le métal 3d qui porte le moment magnétique le plus faible, a conduit à des anomalies de volume de la maille en fonction du taux de substitution de Fe ou de Co, induisant une évolution non linéaire de l'aimantation à saturation. Après plusieurs tentatives, et en combinant des substitutions mixtes sur les sites métalliques et non métalliques (par exemple Ni à Co / Ge à P), la variation initiale de l'entropie magnétique à la transition (ΔSm) a été améliorée par environ un facteur 3, pour atteindre le niveau de référence du Gadolinium. En outre, des structures magnétiques non colinéaires et non commensurables aient été établies, en bon accord avec l'analyse Mössbauer 57Fe et les calculs de structure électronique.Le deuxième volet de ce travail comporte 3 parties. D'une part, des analyses de type XRD, MEB, aimantation, calorimétrie... ont été réalisées afin de caractériser les particularités des poudres magnétocaloriques Mn1-xFexP1-xSix de type Fe2P produites à grande échelle par atomisation sous jet de gaz. Le traitement de recuit après atomisation est apparu comme étant l'un des paramètres les plus importants pour accéder à des performances MCE élevées. Plusieurs essais ont permis de définir la meilleure gamme de température, le temps de recuit et la vitesse de refroidissement. Ainsi, la caractéristique ΔSm a été améliorée de 0,2 à 4 J/kg.K pour une variation de 0-2 T. Pour conforter les résultats ci-dessus, des travaux ont été focalisés sur une formule simple - MnFeP0.5Si0.5 - préparée à partir de précurseurs et en utilisant la fusion HF. L'objectif était de mieux contrôler l'équilibre de phases dans le système quaternaire. A partir de cette étape, des performances très intéressantes ont été atteintes, avec un ΔSm de 15 J/kg.K (0-2 T). Enfin, l'emploi de précurseurs spécifiques et de la fusion HF ont été appliqués pour produire des formules différentes (différents rapports Mn / Fe et P / Si), comprenant l'utilisation de poudres atomisées. Avec l'expérience acquise, des valeurs de 18 et de près de 24 J/kg.K (0-2 T) ont été atteintes.En conclusion, les paramètres chimiques et topologiques gouvernant les différences majeurs entre les pnictures MM'X de type Fe2P et de type Co2P ont été discutées. / Most of ternary pnictides with general formula MM’X (where M and M’ are transition metals and X being a p-element such as P and As) crystallize with a Fe2P type structure exhibiting high magnetocaloric effect (MCE) with d-electron concentration close to that of Fe. Surprisingly, polytype systems MM’X of Co2P-type lead to low performances, even when involving the same metal elements, i.e., MnFeP1-xAsx (hexagonal) and MnFe1-xCoxP (orthorhombic), which exhibit very similar magnetic phase diagrams. So the aim of the work was first to better understand the main crystalline and magnetic fundamental trends of the orthorhombic series when controlling substitutions of selected elements to specific sites e.g. Cr to Mn, Ni to Fe or Co, Si tor Ge to P. Then, the goal was to optimize formula and process aiming being able to design hexagonal type compounds where As is fully replaced by safer Si, for high MCE characteristics, in view of pilot production.At first, synthesis efforts followed by XRD analyzes, magnetization vs field and temperature, neutron scattering investigations of magnetic structures, calorimetry and comparative MCE measurements, were carried out on numerous samples forming a representative panel. Unexpectedly, the less magnetic 3d metal Ni caused cell volume anomalies vs substitution rate to Fe or Co leading the saturation magnetization to evolve non-linearly. From the several attempts, and combining mixed substitutions on metallic and non metallic sites (e.g. Ni to Co / Ge to P), the initial variation of the magnetic entropy at transition (ΔSm) was improved by about 3 times, to reach the reference level Gd. Besides non-collinear and non-commensurate magnetic structures were established, agreeing well with 57Fe Mössbauer analysis and electronic structure calculations.The second action of this work consists in 3 parts. First, analysis involving XRD, SEM, magnetization, calorimetry… was deployed to characterize well the peculiarities of MCE promising Mn1-xFexP1-xSix powders of Fe2P-type produced at a large scale by gas atomization. Of the most critical parameters was the post annealing treatment, which is expected delivering high MCE performances. Several attempts have progressively revealed the best range of temperature, annealing time and cooling down rate. Thus the ΔSm characteristic upgraded from 0.2 to 4 J/kg.K for a variation of 0-2 T. To confirm the above results, works focused to a simple formula MnFeP0.5Si0.5 prepared from precursors and using HF melting. The goal was to better control the phase equilibrium in the quaternary system. From this step, ΔSm up to 16 J/kg.K (0-2 T) was achieved. At the end, the method of dedicated precursor and HF melting was applied to produce different formula (various Mn/Fe and P/Si ratios), comprising the use of the atomized powders. With the gained experience 18 and then close to 24 J/kg.K (0-2 T) was reached.In a final conclusion, chemical and topology parameters driving the critical differences in between related to the Fe2P and Co2P types of MM’X pnictides were discussed.

The work in this thesis falls roughly into three parts, which I characterise loosely as a developmental stage, an exploratory stage, and an attempt to contribute to understanding of the field. In the developmental stage, I have worked to design a variety of methods to create high-quality samples of various Iron Pnictide superconductors, to dope them with various chemicals and to characterise the resulting crystalline samples. I discuss in depth the signature of good quality crystals and the various experiments that they have been used in by myself and my collaborators. These processes are ongoing and will hopefully continue to contribute to my research group's capabilities. My exploratory work involves a detailed survey of one particular family, Sr(Fe1-xCox)2As2, as the level of Cobalt is varied, and the mapping of the phase diagram for the system. I have also made a comparison to the better-measured Barium analogue, and discuss the reasons for the differences in character between the two, most notably the lack of a splitting of the structural and magnetic transitions in the first species. I also discuss the effect of pressure, which can lead to superconductivity in lightly doped samples for very modest pressures; and annealing, which increases transition temperatures within samples, on a limited quantity of crystals. Finally, I attempt to contribute to the understanding of the field via a series of Resonant Ultrasound Spectroscopic experiments conducted by a collaborator on my crystals and analysed by me. I see distinct first-order transitions in the parent compounds, characterisable above the high-T structural transition within a Ginzburg-Landau pseudoproper ferroelastic scheme for a transition coupling weakly to strain but driven by another order parameter. My observations allow several statements about the symmetry of the order parameter and are suggestive of a non-magnetically driven structural transition. In the case of doped samples a much richer behavior is seen, with a broad transition and simultaneous relaxation of all elastic peaks and a broad temperature range of significant dispersion. The effect of the softening is seen far above TN and lends strong support to the family of models predicting such high-T fluctuations.