This thesis uses ferromagnetic resonance to explore the current-induced torques (CITs) in two different systems, namely YIG/heavy metal bilayers and bulk NiMnSb, at room temperature. We apply a microwave current to the sample while sweeping the external magnetic field, and measure the longitudinal DC voltage. From a symmetry analysis of the ferromagnetic resonance lineshape, the amplitudes and directions of the CITs parameterised by an effective magnetic field are accurately estimated. In Chapter 4, YIG samples of different thickness, capped by either Pt or Ta, are studied. The resonance is driven by both spin-transfer torque and Oersted field, and the DC voltage is attributed to both spin rectification and spin pumping. The CITs can be well analysed from the lineshape of the voltage and its dependence on YIG thickness, from which we deduce that the Oersted field dominates over the spin-transfer torque in driving magnetization dynamics. In Chapter 5, we characterise the CITs in bulk NiMnSb induced by the relativistic spin-orbit coupling effect. Both field-like and antidamping-like spin-orbit torques are observed and analysed in detail. At the end of this chapter, we study the spin-wave resonance driven by the CITs, from which the exchange stiffness of NiMnSb is determined. In Chapter 6, we extrapolate a new form of magnetoresistance in NiMnSb: unidirectional spin-orbit magnetoresistance (USOMR). USOMR scales linearly with the current and has opposite sign when the magnetization is reversed. Similar to the giant magnetoresistance in magnetic multilayers, USOMR can be used to distinguish between two opposite magnetization directions directly in the bulk of the ferromagnet.

Les alliages NiMnSb, matériaux multifonctionnels nouveaux, ont attiré une attention en raison de leurs multiples propriétés, telles que l'effet de mémoire de forme, magnétocalorique, de biais d'échange, de magnétorésistance. Jusqu'à présent, de nombreux aspects des NiMnSb, tels que structure cristalline, microstructure, propriétés magnétiques et mécaniques ont été étudiés. Cependant, de nombreuses questions fondamentales de ces matériaux n'ont pas été entièrement révélées, ce qui limite leur développement. Une étude a été menée sur les alliages ternaires NiMnSb en termes de structures cristallines de l'austénite et de la martensite; Caractéristiques microstructurales et cristallographiques de la martensite; La relation d'orientation (OR) de transformation martensitique et sa corrélation avec l'organisation des variantes; Les caractéristiques de déformation de la transformation et l'autoaccommodation de la déformation de transformation. Le travail a confirmé que l'austénite possède une structure cristalline L21 cubique, groupe spatial Fm3m (No. 225). La martensite a une structure orthorhombique modulée (4O) à quatre couches, groupe spatial Pmma (No. 051). Les constantes de réseau de martensite de Ni50Mn37Sb13 et Ni50Mn38Sb12 sont aM = 8.5830 Å, bM = 5.6533 Å et cM = 4.3501 Å, et aM = 8.5788 Å, bM = 5.6443 Å et cM = 4.3479 Å. La microstructure de la martensite 4O NiMnSb modulée possède une caractéristique d'organisation hiérarchique. Les lamelles fines de martensite sont d'abord organisées en larges plaques. Chaque plaque possède 4 variantes apparentées aux jumeaux A, B, C et D formant des jumeaux de type I (A et C, B et D), de type II (A et B, C et D) et des macles composées (A et D; B et C). Les interfaces des variantes sont définies par les plans de maclage correspondants. Les éléments de maclage sont entièrement déterminés pour chaque relation de maclage. Les plaques sont ensuite organisées en sous-colonies et les sous-colonies en colonies de plaques. Les plaques voisines d'une sous-colonie et d'une colonie de plaques partagent une interface de plaque commune. Des colonies de plaques avec différentes interfaces de plaques ayant différentes orientations occupent finalement l'ensemble du grain d'austénite original. La OR de Pitsch, spécifiée comme {011}A // {221}M et <011>A // <122>M, est l'OR effective entre l'austénite cubique et la martensite 4O modulée. Sous cette OR, un maximum de 24 variantes distinctes peut être produit. Les 24 variantes sont organisées en 6 colonies de variantes distinctes, 12 sous-colonies distinctes et enfin 6 colonies de plaques distinctes. Le plan de maclage de type I et les interfaces intra-plaques correspondent tous à la même famille de plans {011}A de austénite. La formation des colonies de variantes martensitiques peut être à la fois intragranulaire et intergranulaire pendant la transformation de phase La colonie de variantes structurée en sandwich est l'unité micro-structurale de base de la martensite. Cette structure est composée de variantes de relation macles et possède des interfaces de variantes internes totalement compatibles et les plans d'habitat invariants. Les caractéristiques de déformation des variants en relation de macles conduisent à la fraction de volume élevée de macles de type II et affecte la morphologie des colonies en sandwich. La structure en forme de coin est composée de deux sandwichs compatibles et reliés par un plan de nervure médiane avec une petite incompatibilité atomique. Tous ces résultats indiquent que la transformation martensitique est autoaccommodée et la microstructure est déterminée par l'auto-accommodation des constituants microstructuraux. Ce travail vise à fournir des informations cristallographiques et micro-structurales fondamentales des alliages NiMnSb pour l'interprétation de leurs caractéristiques magnétiques et mécaniques associées à la transformation martensitique et des recherches complémentaires sur l'optimisation des propriétés / NiMnSb based Heusler type alloys, as a novel multi-functional material has attracted considerable attention due to their multiple properties, such as magnetic shape memory effect, magnetocaloric effect, exchange bias effect, magnetoresistance effect. To date, many aspects of the NiMnSb alloys, such as crystal structure, microstructure, magnetic properties and mechanical properties etc., have been widely investigated. However, many fundamental issues of this family of materials have not been fully revealed, which largely restricts the development of this new kind of multi-functional materials. In the present work, a thorough investigation has been conducted on ternary NiMnSb alloys in terms of crystal structures of austenite and martensite; microstructural and crystallographic features of martensite; martensitic transformation orientation relationship (OR) and its correlation with variant organization; transformation deformation characteristics and self-accommodation of transformation strain. The work confirmed that the austenite of NiMnSb alloys possesses a cubic L21 crystal structure belonging to the space group Fm3m (No. 225). The martensite has a four-layered orthorhombic (4O) structure with space group Pmma (No. 051). The lattice constants of the Ni50Mn37Sb13 and Ni50Mn38Sb12 martensite are aM = 8.5830 Å, bM = 5.6533 Å and cM = 4.3501Å, and aM = 8.5788 Å, bM = 5.6443 Å and cM = 4.3479 Å, respectively. The microstructure of the 4O NiMnSb modulated martensite possesses a hierarchical organization feature. Martensite fine lamellae are first organized into broad plates. Each plate possesses 4 distinct twin related variants A, B, C and D forming type I twins (A and C; B and D), type II twins (A and B; C and D) and compound twins (A and D; B and C). The variant interfaces are defined by the corresponding twinning planes. The complete twinning elements for each twin relation are fully determined. The plates are further organized into sub-colonies and sub-colonies into plate colonies. The neighboring plates in one sub-colony and plate colony share one common plate interface orientation. Plate colonies with different oriented plate interfaces finally take the whole original austenite grain. The Pitsch OR, specified as {011}A // {221}M and <011>A // <122>M, is the effective OR between the cubic austenite and the 4O modulated martensite. Under this OR, a maximum of 24 distinct variants can be produced. The 24 variants are organized into 6 distinct variant colonies, 12 distinct sub-colonies and finally 6 distinct plate colonies. The twinning plane of type I twin and the intra-plate plate interfaces all correspond to the same family of {011}A planes of austenite. The formation of martensite variant colonies can be both form intragranular and intergranular during the phase transformation. The sandwich structured variant colony is the basic microstructural unit of the martensite. This structure is composed of twin related variants and possesses the full compatible inner variants interfaces and invariant habit planes. The deformation manner of the twin related variants result in the high occurrence frequency of the type II twins and affects the morphology of the sandwich colonies. The wedge-shaped structure is composed of two compatible sandwiches and conjoined by a midrib plane with a small atomic misfit. All these results indicate that the martensitic transformation is self-accommodated and the microstructure is determined by the self-accommodation of the microstructural constituents. The aim of this work is to provide fundamental crystallographic and microstructural information of NiMnSb alloys for interpreting their magnetic and mechanical characteristics associated with the martensitic transformation and further investigations on property optimization

This thesis is a theoretical study of configurational and magnetic interactions in multicomponent solids. These interactions are the projections onto the configurational and magnetic degrees of freedom of the underlying electronic quantum mechanical system, and can be used to model, explain and predict the properties of materials. For example, the interactions govern temperature induced configurational and magnetic order-disorder transitions in Heusler alloys and ternary nitrides. In particular three perspectives are studied. The first is how the interactions can be derived from first-principles calculations at relevant physical conditions. The second is their consequences, like the critical temperatures for disordering, obtained with e.g. Monte Carlo simulations. The third is their origin in terms of the underlying electronic structure of the materials. Intrinsic defects in the half-Heusler system NiMnSb are studied and it is found that low-energy defects do not destroy the important half-metallic property at low concentrations. Deliberate doping of NiMnSb with 3d-metals is considered and it is found that replacing some Ni with extra Mn or Cr creates new strong magnetic interactions which could be beneficial for applications at elevated temperature. A self-consistent scheme to include the effects of thermal expansion and one-electron excitations in the calculation of the magnetic critical temperature is introduced and applied to a study of Ni1−xCuxMnSb. A supercell implementation of the disordered local moments approach is suggested and benchmarked for the treatment of paramagnetic CrN as a disordered magnetic phase. It is found that the orthorhombic-to-cubic phase transition in this nitride can be understood as a first-order magnetic order-disorder transition. The ferromagnetism in Ti1−xCrxN solid solutions, an unusual property in nitrides, is explained in terms of a charge transfer induced change in the Cr-Cr magnetic interactions. Cubic Ti1−xAlxN solid solutions displays a complex and concentration dependent phase separation tendency. A unified cluster expansion method is presented that can be used to simulate the configurational thermodynamics of this system. It is shown that short range clustering do influence the free energy of mixing but only slightly change the isostructural phase diagram as compared to mean-field estimates.

Cette thèse présente une étude des interfaces métal de transition ferromagnétique/oxyde et leur importance dans les mécanismes de transport tunnel polarisé en spin. Le travail a porté sur des bicouches ultraminces NiMnSb/MgO(001), Fe/ MgO(001), Co/ MgO(001) et Mn/ MgO(001) élaborées par épitaxie par jets moléculaires. L'originalité de ce travail réside dans l'étude approfondie des propriétés électroniques du matériau magnétique en contact avec la barrière d'oxyde : hybridation, polarisation et magnétisme à l'interface ont été étudiés en utilisant les techniques de laboratoire ainsi que le rayonnement synchrotron. Parallèlement, l'étude des effets magnétorésistifs dans des jonctions tunnel totalement épitaxiées Fe/MgO/Fe(001) a apporté des preuves du filtrage par la barrière MgO des ondes de Bloch en fonction de leur symétrie. Enfin, nous avons mis en évidence l'influence de la qualité structurale et chimique de l'interface Fe/MgO et des électrodes sur ces mécanismes.