This thesis is focused on two open problems of antiferromagnetic (AFM) spintronics: manipulation of AFM coupled moments and development of new materials combining AFM and semiconductor properties. We present three particular methods enabling AFM moments manipulation. The rst method, based on the exchange spring effect in an AFM/FM double layer, strongly de- pends on the AFM layer thickness and temperature. We systematically vary these two parameters and identify the conditions when AFM moments can be manip- ulated. By the second method, cooling an AFM in a magnetic eld through the critical temperature, we prove the concept of a fully AFM-based (containing no FM) spintronic device. The last studied method is based on current induced effects in nanostructures containing an AFM. By systematic study of samples with and without AFM we demonstrate the ability of AFM moments to absorb a current induced torque. Relying neither on a FM nor on cooling in magnetic eld, this method represents an elegant way of AFM moments manipulation. In the second experimental part new materials for AFM spintronics are discussed, and one representative example, CuMnAs, is studied in detail. Characterization of bulk and epitaxial CuMnAs is presented and rst spintronic functionality is shown. Powered by TCPDF (www.tcpdf.org)

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A combinação das funções de Green para operadores de criação e destruição de bosons com as transformações de Holstein e Primakoff é utilizada na análise de sistemas antiferromagnéticos, isolantes e anisotrópicos, na região de baixas temperaturas. Os limites de estabilidade das fases antiferromagnética e \"spin-flop\", assim como a transição \"spin-flop\"-paramagnética são determinados em função da temperatura. A Hamiltoniana modelo considerada leva em conta as interações de intercâmbio anisotrópicas entre primeiros e segundos dos vizinhos numa rede cúbica simples e anisotropias de íon único, dos tipos uniaxial e ortorrômbico. Em particular, as fronteiras de fase \"spin-flop\"-paramagnéticas dos antiferromagnetos NiCl2.6H2O, CoCl2. 6H2O, NiCl2.4H2O e MnCl2.4H2O são determinadas e comparadas com os dados experimentais. As interações dipolares são consideradas explicitamente no circulo da fronteira de fase \"spin-flop\"-paramagnética do EuTe, obtendo-se boa concordância com a experiência. / The combination of Greens functions for boson creation and destruction operators with the Holstein - Primakoff transformations is employed in the analysis of insulating and anisotropic antiferromagnetic systems at very low temperatures. The stability limits of antiferromagnetic and spin- flop phases, as well as the spin-flop-paramagnetic transition are determined as a function of temperature. The model Hamiltonian considered takes account of the anisotropic exchange interactions between first and second neighbors for a simple cubic lattice and of single ion anisotropies, of the uniaxial and orthorhombic types. Particularly, the spin-flop-paramagnetic phase boundaries of the antiferromagnetics NiCl2.6H2O, CoCl2. 6H2O, NiCl2.4H2O and MnCl2.4H2O are determined and compared with experimental values. The dipolar interactions are explicitly considered in the calculation of the spin-flop-paramagnetic phase boundary of antiferromagnetic EuTe, and we obtain good agreement with the experiments.

A combinação das funções de Green para operadores de criação e destruição de bosons com as transformações de Holstein e Primakoff é utilizada na análise de sistemas antiferromagnéticos, isolantes e anisotrópicos, na região de baixas temperaturas. Os limites de estabilidade das fases antiferromagnética e \"spin-flop\", assim como a transição \"spin-flop\"-paramagnética são determinados em função da temperatura. A Hamiltoniana modelo considerada leva em conta as interações de intercâmbio anisotrópicas entre primeiros e segundos dos vizinhos numa rede cúbica simples e anisotropias de íon único, dos tipos uniaxial e ortorrômbico. Em particular, as fronteiras de fase \"spin-flop\"-paramagnéticas dos antiferromagnetos NiCl2.6H2O, CoCl2. 6H2O, NiCl2.4H2O e MnCl2.4H2O são determinadas e comparadas com os dados experimentais. As interações dipolares são consideradas explicitamente no circulo da fronteira de fase \"spin-flop\"-paramagnética do EuTe, obtendo-se boa concordância com a experiência. / The combination of Greens functions for boson creation and destruction operators with the Holstein - Primakoff transformations is employed in the analysis of insulating and anisotropic antiferromagnetic systems at very low temperatures. The stability limits of antiferromagnetic and spin- flop phases, as well as the spin-flop-paramagnetic transition are determined as a function of temperature. The model Hamiltonian considered takes account of the anisotropic exchange interactions between first and second neighbors for a simple cubic lattice and of single ion anisotropies, of the uniaxial and orthorhombic types. Particularly, the spin-flop-paramagnetic phase boundaries of the antiferromagnetics NiCl2.6H2O, CoCl2. 6H2O, NiCl2.4H2O and MnCl2.4H2O are determined and compared with experimental values. The dipolar interactions are explicitly considered in the calculation of the spin-flop-paramagnetic phase boundary of antiferromagnetic EuTe, and we obtain good agreement with the experiments.

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La synthèse et la caractérisation de nouveaux matériaux sont des enjeux majeurs en chimie et en physique. En particulier, les réseaux organométalliques (metal –oganic frameworks : MOFs) avec deux ou plusieurs fonctionnalités couplées, encore rares, sont très attractifs en raison de leur grande variété de propriétés et de leurs applications prometteuses connectant de nombreuses disciplines.Le développement de nouveaux matériaux fascinants peut avoir un impact considérable sur notre vie quotidienne, ce qui explique en partie l'intensification de la recherche dans le cadre de la science des matériaux et de la matière condensée.Cette thèse se concentre sur la synthèse et la caractérisation des propriétés physiques de structures magnétiques basées sur le formiate de métal en utilisant la combinaison de diffraction par neutrons et par rayons X ainsi que d'autres techniques.Les réseaux de métal-formiate sont un sous-groupe spécifique des réseaux organométalliques, typiquement synthétisés en faisant réagir un ligand organique avec un sel métallique sous des conditions solvo-thermiques ou par des techniques d'évaporation ou de diffusion lente. Les centres métalliques sont liés par des molécules de formiate formant un cadre anionique qui peut être équilibré électrochimiquement par des amines protonées.Dans ce travail, j’ai étudié la structure cristalline, les transitions de phases et les propriétés magnétiques de deux familles de composés qui sont représentés par les composés hétéro-métalliques ou à valence mixtes, adoptant une structure de type niccolite et des composés homo-métalliques adoptant une structure pérovskite (ABX3). Les composés suivants ont été synthétisés et caractérisés: [(CH3)2NH3][FeIIIMII(HCOO)6] (M = Mg, Mn, Fe, Co, Ni), [(CH3)2NH3][FeIIIFeII(HCOO)6], [(CH3NH3)[M(HCOO)3] (M = Co, Mn, Fe, Ni, Cu), and [(NH4)[Mn(HCOO)3].Le choix d'utiliser des ions métalliques spécifiques a été motivé par leur configuration électronique différente et, par conséquent, par les différents comportements physiques, c'est-à-dire une grande différence bien connue dans l'anisotropie magnétique des différents ions divalents utilisés dans cette étude. Outre les effets sur les propriétés lorsque différents ions métalliques divalents sont introduits dans la structure, un autre aspect intéressant est l’effet produit en changeant la nature des contre-ions. Même s'il n'y a pas de corrélation claire entre le contre-ion sélectionné et le changement de comportement magnétique, la diffraction des neutrons permet d'élucider les différences dans la structure nucléaire et dans la structure magnétique lorsque différents contre-ions sont utilisés. Par ailleurs, ce travail, nous aide à comparer nos résultats neutroniques avec les mesures complémentaires de magnétométrie. Une variété de phénomènes magnétiques tels que le comportement ferromagnétique, l'ordre antiferromagnétique, l’angle de basculement spin ont été observés dans les composés étudiés. De plus, du point de vue de la structure nucléaire, différentes transitions de phases ont été détectées impliquant par exemple l'ordre-désordre du contre-ion employé (dans [(CH3)2NH3][FeIIIMII(HCOO)6] par exemple), ou le passage d'une phase commensureable à une phase incommensurable donnant lieu à une modulation de la structure (dans [(CH3NH3)[Co(HCOO)3] par exemple). / The synthesis and the characterization of new materials are key challenges in chemistry and physics. In particular, metal–organic frameworks (MOFs) with two or more coupled functionalities are still rare and very attractive candidates because of their wide variety of properties, and promising applications interesting many disciplines. The impact of the development of new fascinating materials on our day life might be considerable. This is also one of the reason explaining the intense increasing of the research in material science and condensed matter.This thesis is focused on the synthesis and the physical characterization of magnetic metal formate frameworks using the combination of neutron and synchrotron X-Ray diffraction as well as other techniques. Metal-formate frameworks are a specific subgroup of metal-organic frameworks, where the metal centres are linked by the formate molecules to form an anionic framework. The negative charge of the framework is balanced by a counter-cation inside the frameworks’ cavities, that can be for example protonated amines.Typically, these compounds are synthesized by reacting formate or formic acid with a metal salt under solvo-thermal conditions or by slow evaporation or diffusion techniques.In this work, I investigated the crystal structure, phase transitions and magnetic properties of two families of metal formate frameworks, which are represented by the hetero-metallic or mixed valance compounds adopting a niccolite-like structure, and the homo-metallic compounds adopting a perovskite-like (ABX3) structure.Altogether, the following compounds were synthesized and characterized: [(CH3)2NH3][FeIIIMII(HCOO)6] (M = Mg, Mn, Fe, Co, Ni), [(CH3)2NH3][FeIIIFeII(HCOO)6], [(CH3NH3)[M(HCOO)3] (M = Co, Mn, Fe, Ni, Cu), and [(NH4)[Mn(HCOO)3].The choice of using specific metal ions has been motivated by their different electronic configuration and therefore different physical behaviours, i.e. a large difference in the magnetic anisotropy is well known among the different divalent ions used in this study. Beside the effects on the properties when different divalent metal ions were introduced within the framework, the effects of the nature of the counterions was investigated.Even though there is not a clear correlation between the selected counterion and the change of magnetic behaviour, neutron diffraction allows elucidating the differences in the nuclear and in the magnetic structure when different counterions are used. Moreover, these works help us to compare our neutron results with the magnetometry measurements, which is a complementary technique.A variety of magnetic phenomena such as ferromagnetic behaviour, antiferromagnetic ordering, spin canting have been observed in the compounds studied. Furthermore, from the nuclear structure point of view many different kind of phase transitions were detected involving for instance, the order-disorder of the counter ion employed (in [(CH3)2NH3][FeIIIMII(HCOO)6] for example), or the transition from a commensurate to incommensurate phase giving rise to a modulation of the structure (in [(CH3NH3)[Co(HCOO)3] for example).