SENA FILHO, Roberto Ferreira. Teoria microscópica de ondas de spin em nanofios magnéticos. 2007. 74 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2007. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-05-22T19:45:31Z No. of bitstreams: 1 2007_dis_rfsenafilho.pdf: 3612063 bytes, checksum: 5cbe75be9b4bf3c77cd9b1136fad2a2a (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-05-22T19:46:04Z (GMT) No. of bitstreams: 1 2007_dis_rfsenafilho.pdf: 3612063 bytes, checksum: 5cbe75be9b4bf3c77cd9b1136fad2a2a (MD5) / Made available in DSpace on 2015-05-22T19:46:04Z (GMT). No. of bitstreams: 1 2007_dis_rfsenafilho.pdf: 3612063 bytes, checksum: 5cbe75be9b4bf3c77cd9b1136fad2a2a (MD5) Previous issue date: 2007 / The dynamical behavior of spins in magnetic materials is affected by its geometry and dimensionality. One can find several new results in the literature exploiting the magnetic properties of low dimension systems with different geometries, since the development of new devices such as: nanosensors, high density magnetic storage, etc., is closely related to new geometries. In this piece of work, we study the propagation of spin waves on cylindrical magnetic nanowires described by a microscopic theory through the Heisenberg Hamiltonian, where we consider the spins fixed at the sites lattice and the transversal section of the wire is hexagonal. Our model takes into account the exchange interaction between the spins that can be ferromagnetic or antifferomagnetic, the interaction of an external field with the spins (Zeeman interaction), anisotropic interactions due to a preferred direction of magnetization, and finally dipole-dipole interactions. The spins are described by boson operators through Holstein-Primakoff representation. The equations of motion for the spins are written in terms of these operators and translational symmetry in a preferential direction allows us to calculate several excitations spectra. / O comportamento dinâmico de spins em materiais magnéticos é influenciado pela geometria que eles apresentam. Além disso, outro aspecto relevante é a dimensionalidade do sistema. Trabalhos recentes comprovam o interesse do estudo das propriedades magnéticas em sistemas de baixa dimensionalidade, que é devido em grande parte as aplicações tecnológicas, tais como: nanosensores, gravadores magnéticos de alta densidade, dispositivos magneto-eletrônicos, etc. Neste trabalho estudamos a propagação de ondas de spin em nanofios magnéticos cilíndricos, onde a abordagem é feita utilizando teoria microscópica, através do Hamiltoniano de Heisenberg, em que os spins são considerados fixos nos sítios da rede e cuja geometria da seção transversal dos cilindros é hexagonal. Entre as interações magnéticas estudadas consideramos: a interação de troca que pode ser ferromagnética se os primeiros vizinhos dos spins estão numa configuração paralela, ou antiferromagnética se estiverem antiparelelos; a interação Zeeman que é devido ao campo magnético externo aplicado ao sistema; a interação de Anisotropia, esta sendo responsável pela direção de magnetização preferida que diversos sistemas magnéticos reais apresentam e a interação dipolar de natureza magnetostática, presente em todos os materiais. O formalismo leva em consideração a dependência espacial dos spins no sistema, onde os operadores de spin do hamiltoniano são escritos em termos de operadores bosônicos de criação e aniquilação através da Representação de Holstein-Primakoff. Em seguida, aproveitando-se da simetria translacional em uma direção devido a periodicidade da rede, realizamos a transformada de Fourier para estes operadores fornecendo um sistema de equações matriciais no espaço dos vetores de onda. A partir desse sistema de equações obtemos vários espectros de excitação como: a relação de dispersão para as ondas de spin, que é o gráfico onde mostra como a frequência de ondas de spin varia em função do vetor de onda e a variação da energia do sistema com o campo aplicado.

L’objectif central de cette thèse est d’étudier des phases ordonnées en compétition dans des matériaux magnétiques présentant une structure cristalline tétragonale centrée.Ce travail est divisé en deux parties principales. Dans la première, nous présentons les résultats de notre étude de la compétition entre des états ordonnés antiferromagnétiques et des phases liquides de spin. Nous montrons comment ces dernières peuvent être stabilisées par la frustration géométrique et par une généralisation de la symétrie de spinau groupe SU(n). Les états antiferromagnétiques sont décrits par une théorie d’onde despin et l’analyse de liquide de spin est effectuée par une représentation fermionique des opérateurs de spin. Dans la deuxième partie, nous décrivons une théorie effective pour dércrire des expériences de diffusion Raman. Nous fournissons un aperçu de la phase d’ordre caché affichée par le composé de fermions lourds URu2Si2. / The main objective of this thesis is to investigate the competing ordered phases in the metallic heavy fermion compound URu2Si2, which displays a body-centered tetragonallattice. We first provide a study case of the competition between antiferromagnetic(AF) and spin liquid phases. The antiferromagnetic state is study with spin-wave theory. Whereas the spin liquid analysis has been carried out in an algebraic spin liquid representation.In the second part, we describe an effective theory for Raman scattering experiments at these particular phases. We provide insight about the hidden order phase displayed by the heavy fermion compound URu2Si2.

Orientador: Bernardo Laks / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-11T12:54:31Z (GMT). No. of bitstreams: 1 Araujo_AlexandreAbdalla_D.pdf: 3128592 bytes, checksum: cb2ac303a68b8fb439a89a0ee1627986 (MD5) Previous issue date: 2008 / Resumo: A Física de superfícies, interfaces e filmes finos vem se desenvolvendo muito rapidamente nas últimas décadas com o aparecimento de inúmeras técnicas experimentais para estudo das propriedades de superfície. Por outro lado, tem ocorrido um grande avanço dos equipamentos de informática e dos métodos computacionais, com o desenvolvimento de novos algoritmos, os quais já permitem o estudo de sistemas mais complexos como interfaces, defeitos, filmes-finos e nanofios, contendo um número cada vez maior de átomos. Um considerável interesse em superfícies e na deposição de filmes finos sobre superfícies, envolvendo metais, tem sido motivado pela possibilidade de se conseguir novas propriedades magnéticas e eletrônicas, incluindo temperaturas acima da temperatura ambiente, visando avanços tecnológicos em dispositivos eletrônicos. Nosso trabalho representa uma estratégia bastante promissora nessa área, pois nele identificamos claramente a possibilidade de produção de filmes finos com caráter ferromagnético half-metallic (isto é, com um canal de condução eletrônico semicondutor e outro metálico). Conforme pudemos mostrar, este caráter foi atingido a partir de pequenas variações de parâmetro de rede, de espessura de filme e de composição atômica. As propriedades observadas em nossos resultados teóricos sinalizam a importância de aplicação de diferentes materiais tais como CrAs, CrTe, CrAs(1-x)Sex, CrAs(1-x)T e x, CrSe(1-x) Tex, objetivando suas utilizações em Spintrônica. Desta forma, realizamos um estudo sistemático desses materiais, verificando suas propriedades eletrônicas e magnéticas e suas viabilidades de aplicações em novos dispositivos. Dois métodos de cálculo de estrutura eletrônica: o RS-LMTO-ASA (Real-Space ¿ Linear Muffin-Tin ¿ Atomic Sphere Approximation) e o FLAPW (Full Potential - Linearized Augmented Plane wave), assim como o método da Matriz Transferência foram utilizados em nossas investigações. Em primeiro lugar, apresentamos estudos teóricos sobre as fases estruturais e magnéticas observadas nas primeiras camadas de filmes finos de CrAs, crescidos sobre substratos de GaAs(001). Esses estudos englobaram processos de otimização de geometria, realizados através do método FLAPW, baseados em cálculos autoconsistentes de primeiros princípios, levando em consideração a polarização de spin. Em segundo lugar, estudamos as propriedades eletrônicas e magnéticas das superfícies CrAs(001) através do RS-LMO-ASA e determinamos as dispersões dos estados eletrônicos de superfície segundo direções de alta simetria na zona de Brillouin bidimensional. A seguir, como os resultados apontaram a possibilidade de obtermos mais materiais com comportamento ferromagnético half-metallic, passamos a investigar toda uma classe de materiais com estruturas volumétricas ou de filmes finos envolvendo os elementos Cr, As, Te, e Se, arranjados em ligas binárias (CrAs, CrSe, CrTe) e ternárias (CrAs(1-x)Sex, CrAs(1-x)T ex, CrSe(1-x)Tex), em diferentes concentrações e diferentes regiões superficiais. Como conseqüência, um amplo conjunto de resultados interessantes foi conseguido, confirmando nossas expectativas de que pequenas variações de parâmetro de rede, de espessura e de composição atômica são ingredientes fundamentais a serem considerados para se atingir uma transição do regime ferromagnético metálico para half-metallic e que isto representa uma área bastante promissora, que deverá estimular novos experimentos, com a produção de novos tipos de filmes finos, com espessura e composição controladas. Por último, apresentamos um estudo teórico do composto Fe2CoAl, no qual a precisão de nossos cálculos é comparada a medidas experimentais / Abstract: In the last decades, the Physics of Surfaces, Thin Films and Interfaces has motivated a great advance of the experimental techniques applied to study surface properties. In addition, a fast progress in the computational area has also occurred, with the development of powered computers, new methods of calculations, and new algorithms, which already allow the description of more complex systems, such as interfaces, defects, thin films and nanowires. A considerable interest in the deposition of thin films on surfaces, involving metals, has been motivated by the possibility of producing new devices using the fascinating electronic and magnetic properties, in order to produce technological advances in electronic devices. This work represents a promising strategy in this area, because we identify, clearly, the possibility of producing thin films with half-metallic character (that is, with a semiconductor electronic spin channel and a metallic spin channel, simultaneously). As we showed, this character was attained from small variations of lattice parameter, film thickness or atomic composition. The results of our theoretical calculations have pointed the importance of some materials such as CrAs, CrTe, CrAs(1-x)S ex, CrAs(1-x)Tex, CrS e(1-x)Tex to be used in the Spintronic branch. So, we carry out a systematic analysis of these new materials, emphasizing its structural, electronic and magnetic properties and the viability of using these materials in new electronic devices. Two different methods of electronic structure calculations: the RS-LMTO-ASA (Real-space - Linear Muffin-Tin - Atomic Sphere Approximation) and the LAPW (Linearized Augmented Plane-Wave), as well as the Matrix Transfer method have been used in our studies. Initially, we present the theoretical results of the structural and magnetic phases, observed in the first layers of thin films of orthorhombic CrAs, grown on a GaAs(001) substrate. Two geometry optimization processes have done with the Full-Potential Linearized Augmented Plane-Wave (FLAPW) method, based on first principles, self-consistent calculations, taking in account the spin polarization, at the scalar relativistic level. Secondly, we study the electronic and magnetic properties of the CrAs(001) surfaces, via the RS-LMTO-ASA, and determined the energy dispersion of the electronic surface states along two highly symmetric directions in the two-dimensional Brillouin zone. Then, as the results suggested the possibility of obtaining new thin films, with ferromagnetic half-metallic behavior, we started to investigate a large class of materials, with volumetric and thin films structures, of binary (CrAs, CrSe, CrTe) and ternary (CrAs(1-x)Sex, CrAs(1-x) Te, CrSe(1-x)Tex) systems, in different atomic concentrations and with different superficial regions. Consequently, a large quantity of interesting results was obtained for these ferromagnetic materials, confirming that small variations of lattice parameters, film thickness and atomic composition are the fundamental ingredients to be considered, in order to reach the transition from metallic regime to ferromagnetic half-metallic regime and that our results can stimulate new experiments with the aim of producing new thin films, with controlled thicknesses and atomic compositions. Finally, we present a theoretical study of the inter-metallic compound Fe2CoAl, by comparing the precision of our calculations with experimental measurements / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / O comportamento dinÃmico de spins em materiais magnÃticos à influenciado pela geometria que eles apresentam. AlÃm disso, outro aspecto relevante à a dimensionalidade do sistema. Trabalhos recentes comprovam o interesse do estudo das propriedades magnÃticas em sistemas de baixa dimensionalidade, que à devido em grande parte as aplicaÃÃes tecnolÃgicas, tais como: nanosensores, gravadores magnÃticos de alta densidade, dispositivos magneto-eletrÃnicos, etc. Neste trabalho estudamos a propagaÃÃo de ondas de spin em nanofios magnÃticos cilÃndricos, onde a abordagem à feita utilizando teoria microscÃpica, atravÃs do Hamiltoniano de Heisenberg, em que os spins sÃo considerados fixos nos sÃtios da rede e cuja geometria da seÃÃo transversal dos cilindros à hexagonal. Entre as interaÃÃes magnÃticas estudadas consideramos: a interaÃÃo de troca que pode ser ferromagnÃtica se os primeiros vizinhos dos spins estÃo numa configuraÃÃo paralela, ou antiferromagnÃtica se estiverem antiparelelos; a interaÃÃo Zeeman que à devido ao campo magnÃtico externo aplicado ao sistema; a interaÃÃo de Anisotropia, esta sendo responsÃvel pela direÃÃo de magnetizaÃÃo preferida que diversos sistemas magnÃticos reais apresentam e a interaÃÃo dipolar de natureza magnetostÃtica, presente em todos os materiais. O formalismo leva em consideraÃÃo a dependÃncia espacial dos spins no sistema, onde os operadores de spin do hamiltoniano sÃo escritos em termos de operadores bosÃnicos de criaÃÃo e aniquilaÃÃo atravÃs da RepresentaÃÃo de Holstein-Primakoff. Em seguida, aproveitando-se da simetria translacional em uma direÃÃo devido a periodicidade da rede, realizamos a transformada de Fourier para estes operadores fornecendo um sistema de equaÃÃes matriciais no espaÃo dos vetores de onda. A partir desse sistema de equaÃÃes obtemos vÃrios espectros de excitaÃÃo como: a relaÃÃo de dispersÃo para as ondas de spin, que à o grÃfico onde mostra como a frequÃncia de ondas de spin varia em funÃÃo do vetor de onda e a variaÃÃo da energia do sistema com o campo aplicado. / The dynamical behavior of spins in magnetic materials is affected by its geometry and dimensionality. One can find several new results in the literature exploiting the magnetic properties of low dimension systems with different geometries, since the development of new devices such as: nanosensors, high density magnetic storage, etc., is closely related to new geometries. In this piece of work, we study the propagation of spin waves on cylindrical magnetic nanowires described by a microscopic theory through the Heisenberg Hamiltonian, where we consider the spins fixed at the sites lattice and the transversal section of the wire is hexagonal. Our model takes into account the exchange interaction between the spins that can be ferromagnetic or antifferomagnetic, the interaction of an external field with the spins (Zeeman interaction), anisotropic interactions due to a preferred direction of magnetization, and finally dipole-dipole interactions. The spins are described by boson operators through Holstein-Primakoff representation. The equations of motion for the spins are written in terms of these operators and translational symmetry in a preferential direction allows us to calculate several excitations spectra.

Orientador: Eduardo Miranda / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-09-01T08:41:01Z (GMT). No. of bitstreams: 1 Almeida_DalsonEloy_D.pdf: 2470369 bytes, checksum: 93d3b945f62f374cfd686217575dda95 (MD5) Previous issue date: 2017 / Resumo: Nesta tese estudamos a relação entre antiferromagnetismo e supercondutividade em pnictídeos à base de ferro. Este estudo será feito através da análise de uma energia livre de Ginzburg-Landau de parâmetros de ordem acoplados que será derivada de um modelo microscópico. Em particular, estamos interessados em saber se a transição entre os estados ordenados é de primeira ordem ou se as duas ordens podem coexistir. Para o caso de supercondutividade convencional as duas fases puras nunca coexistem. Entretanto, quando a supercondutividade é não-convencional e a condição de nesting perfeito não é satisfeita, pode haver um regime intermediário de coexistência microscópica das duas ordens. Nesta nova fase termodinâmica, as simetrias de rotação no espaço de spins, de reversão temporal e U(1) são quebradas simultânea e localmente. Logo, os canais de supercondutividade singleto e tripleto se misturam quanticamente. Em outras palavras, uma componente tripleto secundária do estado supercondutor é gerada. Os diagramas de fases do sistema são apresentados e analisamos também como flutuações magnéticas, acima da temperatura de Néel pura, afetam a temperatura de transição tripleto. Investigamos também o efeito da magnetização alternada no efeito Josephson, i.e., na supercorrente que flui através de uma junção entre dois supercondutores na fase de coexistência. Por fim, mas não menos importante, estudamos o efeito de proximidade em uma interface entre um supercondutor e um antiferromagneto. Veremos que os pares de Cooper podem penetrar a região magnética e em consequência, uma componente tripleto é induzida próximo da interface / Abstract: In this thesis, we study the interplay between antiferromagnetism and superconductivity in iron pnictides. This study will be done analyzing a free energy of coupled order parameters which will be derived from a microscopic model. In particular, we are interested if the phase transition between the ordered states is first order or if the two orders can coexist. For the case of conventional superconductivity, the two phases cannot coexist. However, when superconductivity is unconventional and the perfect nesting condition is not satisfied, there can exist an intermediary state of microscopic coexistence of the two orders. In this new thermodynamic phase, spin rotation, time reversal and U(1) symmetries are simultaneously and locally broken. Therefore, the singlet and triplet superconductivity channels are quantum mechanically mixed. In other words, a secondary triplet component is generated. The phase diagrams of the system are presented and we also analyze the effect of magnetic fluctuations above the pure Néel temperature on the triplet temperature transition. We also investigate the effects of the staggered magnetization on the Josephson effect, i.e., on the supercurrent that flows through a junction of two superconductors in the coexistence phase. Last, but not least, we study the proximity effect at an interface between a superconductor and an antiferromagnet. We will see that the Cooper pairs can penetrate the magnetic region and consequently a triplet component is induced near the interface / Doutorado / Física / Doutor em Ciências / 140834/2013-3 / 2342/15-4 / CNPQ / CAPES / BEX

No description available.

This thesis presents the NMR measurements on the single crystals LiMnPO4 and Li0.9FeAs. Therefore, the thesis is divided into two separated sections. The first part reports on the competitive next generation cathode material LiMnPO4 with a stable reversible capacity up to 145 mAh/g and a rather flat discharge voltage curve at 4.1 V. For the basic understanding of the material the magnetic properties have been investigated by a Li and P NMR study in the paramagnetic and antiferromagnetic phase. LiMnPO4 shows a strong anisotropy of the dipolar hyperfine coupling due to the strong local magnetic moments at the Mn site. The corresponding dipole tensor of the Li- and P-nuclei is fully determined by orientation and temperature dependent NMR experiments and compared to the calculated values from crystal structure data. Deviations of the experimentally determined values from the theoretical ones are discussed in terms of Mn disorder which could have an impact on the mobility of the Li ions. The disorder is corroborated by diffuse x-ray diffraction experiments which indicate a shift of the heavy elements in the lattice, namely the Mn atoms. Furthermore, the spin arrangement in the relative strong field of 7.0494 T in the antiferromagnetic state is understood by the NMR measurements. In order to obtain parameters of the Li ion diffusion in LiMnPO4 measurements of the spin lattice relaxation rate were performed. Due to the strong dipolar coupling between the Li-nuclei and the magnetic moments at the Mn site it is difficult to extract parameters which can characterize the diffusive behavior of the Li ions. The second section reports on the AC/DC susceptibility and NMR/NQR studies on Li deficit samples labeled as Li0.9FeAs. LiFeAs belongs to the family of the superconducting Pnictides which are discovered in 2008 by H. Hosono et al. In recent studies the stoichiometric compound reveals triplet superconductivity below Tc ∼ 18 K which demands ferromagnetic coupling of the electrons in the Cooper pairs. In Li0.9FeAs the Li deficit acts like hole doping which suppresses the superconductivity. Then ferromagnetism can arise which is very interesting because of the vicinity to the triplet superconductivity. With the microscopic methods NMR/NQR on the Li and As nuclei, it was investigated where the ferromagnetism can be located in Li0.9FeAs. Recent susceptibility, ESR and µSR studies reveal an internal field due to the ferromagnetism. In contrast, the internal field could not be used to perform zero field NMR measurements. Possible reasons for this discrepancy are discussed. In addition, the automatic insitu AC susceptibility technique by using the NMR radio frequency circuit has been tested by a reference compound Co2TiGa which shows itinerant ferromagnetism. Similar curves are observed for Li0.9FeAs which indicate the existence of itinerant magnetic moments in Li0.9FeAs. Furthermore, in order to determine the size of the dipolar contribution from the magnetic moments of the Fe the dipolar hyperfine coupling tensor was calculated from the crystal structure data. The comparison of the experimental and calculated hyperfine coupling elements reveals transferred hyperfine fields in LiFeAs.

The antiferromagnetic semiconductor MnTe has recently attracted significant interest as a potential high-performance thermoelectric material. Its promising thermoelectric properties are due in large part to short-range magnetic correlations in the paramagnetic state, which enhance the thermopower through the paramagnon drag effect. Using magnetic pair distribution function (mPDF) analysis of neutron total scattering data, we present a detailed, real-space picture of the short-range magnetic correlation in MnTe, offering a deeper view into the paramagnon drag effect and the nature of the correlated paramagnetic state. We confirm the presence of nanometer-scale antiferromagnetic correlations far into the paramagnetic state, show the evolution of the local magnetic order parameter across the N\'eel temperature T_N=307 K, and discover a spatially anisotropic magnetic correlation length. By combing our mPDF analysis with traditional atomic PDF analysis, we also gain detailed knowledge of the magnetostructural response in MnTe. We observed a spontaneous volume contraction of nearly 1\%, the largest spontaneous magnetovolume effect reported so far for any antiferromagnetic system. The lattice strain scales linearly with the local magnetic order parameter, in contrast to the quadratic scaling observed for the conventional magnetostriction properties of this technologically relevant material. Using neutron and X-ray PDF analysis, we also investigated the local distortion on MnTe and Mn-based systems, MnS and MnO as a function of temperature. Such local distortion on MnTe increases with the rise in temperature and becomes more pronounced at 500 K.

This thesis contains the results of the extensive study into the synthesis of nickel niobate (NiNb2O6) including the formation of what was a previously unreported polymorph of the material, as well as the magnetic properties of both cobalt niobate (CoNb2O6) and nickel niobate using techniques including SQUID magnetometry, powder and single crystal x-ray scattering, powder and single crystal neutron scattering and muon spin rotation/relaxation. In cobalt niobate we found extremely long relaxation times in the heat capacity which showed up strongly in muon spin rotation experiments but not in neutron measurements. Additionally, with field applied to the system we see the emergence of spin-wave like structures in the neutron scattering data. Within cobalt niobate the strongest interaction is ferromagnetic and along the chain. The chains themselves are laid out on a triangular fashion and interact, although far more weakly, in an antiferromagnetic manner. This triangular patterning as well as an antiferromagnetic interaction results in interchain frustration, which protects the quasi-1D nature of the system due to the difficulty generated in creating 3D order. In nickel niobate we found that growth conditions caused highly variable changes, and we were able to create two different polymorphs. One polymorph was in the same space group as cobalt niobate, which gave us an opportunity to explore the magnetic difference between a spin-½ and spin-1 magnetic system and in nickel niobate in the new space group we performed an ab initio characterization solving the unit cell structure, the magnetic structure with neutron scattering as well as a magnetic characterization with SQUID magnetometry and muon spin rotation, allowing us to contrast the significant crystallographic differences. For the new polymorph we were able to determine its magnetic structure, characterized by Ising-like spins arranged in frustrated tetrahedra with three of the four points lying in the same plane as the spin, and for both materials we were able to use zero-field μSR data to estimate behaviour near the critical point and determine a critical exponent near the magnetic transitions. In both polymorphs there is evidence of constrained magnetism or reduced dimensionality, although the evidence for low dimensionality is much stronger in the columbite polymorph. / Thesis / Doctor of Philosophy (PhD) / This thesis examines two different niobium-based compounds: cobalt niobate (CoNb2O6) and nickel niobate (NiNb2O6). In these systems the cobalt and nickel atoms provide interesting magnetic properties. Within a magnetic material, the magnetic atoms tend to have their spins align in certain ways. The atoms themselves are fixed to particular sites by the way the material is assembled; an atomic framework. In the case of cobalt niobate, the magnetic atoms are arranged in well-separated chains so that a magnetic atom interacts strongly with its magnetic neighbours within a chain, and weakly with ones that are further away. This is an example of a material that is called `low dimensional'. The chains themselves form triangular patterns, and the interactions between chains are both weaker and antialigned, which creates a frustrated competition between the chains, protecting the low dimensional state by creating conditions where it is hard for all the spins in the material to order. For nickel niobate, the magnetic moments all want to anti-align, or be pointing in the opposite direction as its nearest neighbour. The magnetism is `frustrated' because each magnetic atom is tetrahedrally connected to three other atoms, so it cannot meet that condition. This can be visualized by drawing a triangle and trying to make each corner have an arrow pointing up or down. The third corner of the triangle cannot satisfy this requirement for its neighbours (one up and one down arrow). Both decreased dimensionality and frustration can lead to the emergence of novel quantum states of matter at low temperature. This thesis explores these materials with that in mind.

Ce mémoire traite des propriétés du La2CuO4 dopé en trous, le premier supraconducteur à haute température critique ayant été découvert. Les différentes phases électroniques du cristal y seront présentées, ainsi que le diagramme de phases en dopage de ce matériau. Les trois structures dans lesquelles on peut retrouver ce cristal seront décrites en détail, et leurs liens présumés avec les phases électroniques seront présentés. Il s’en suivra une étude utilisant la théorie de la fonctionnelle de la densité combinée au modèle de Hubbard (DFT+U) des différentes phases structurales, en plus des phases antiferromagnétiques et paramagnétiques. L’effet de la corrélation électronique sur la structure cristalline sera également étudié par l’intermédiaire du paramètre de Hubbard. Le but sera de vérifier si la DFT+U reproduit bien le diagramme de phases expérimentales, et sous quelles conditions. Une étude des effets de l’inclinaison des octaèdres d’oxygène sur la structure électronique sera également présentée. / Presented here is a study on the hole doped La2CuO4, the first discovered high-­‐Tc superconductor of the cuprate family. The different electronic phases of this crystal are briefly reviewed. The three crystal structures present in this material are described, and the link between those phases and the electronic structure are discussed. The relationship of those structural phases with the magnetic phases is investigated with the help of calculations based on the density functional theory where an additional Hubbard term has been added (DFT+U). With the help of the Hubbard parameter, the effect of the electronic correlation’s strength on the structural parameters of the crystal is also studied. The idea here is to verify how well the DFT+U is able to reproduce the experimental phase diagram of this material. The effect of the tilting of the oxygen octahedras on the electronic structure is also addressed.