<p>The magnetic properties of transition metal compounds have been studied using SQUID-magnetometry, magnetic force microscopy and Lorentz transmission electron microscopy. New magnetic materials have been found and their magnetic properties have been determined. How the magnetic properties of a material can be changed through e.g. chemical substitution of magnetic and nonmagnetic atoms and shape and size effects have also been studied. Three different sets of samples have been investigated: three new Mn-compounds, two substitution series of layered magnetic structures and ferromagnetic micronsized thin film elements.</p><p>The three Mn-compounds, Mn₃IrSi, IrMnSi and Mn₈Pd₁₅Si₇, show different magnetic ordering. Mn₃IrSi orders 'antiferromagnetically' at 210 K. IrMnSi forms a double cycloidal spin spiral below 460 K. Mn₈Pd₁₅Si₇ only shows short-range magnetic ordering.</p><p>Substituting Se with S in TlCo₂Se_2-xS_x changes the magnetic order from a spin spiral to a colinear ferromagnet for a composition of <i>x</i>=1.75. An intermediate region exists where the compound is neither a pure ferromagnet, nor purely a spin spiral, as evidenced by the magnetization versus field measurements for the <i>x</i>=1.3 and 1.5 samples. This is also seen in the temperature dependent susceptibility measurements. For the TlCu_2-xFe_xSe₂ compounds it was found that the ordering temperature and saturation magnetic moment per Fe-atom changed with composition <i>x</i>.</p><p>Ferromagnetic micronsized thin film elements in permalloy, Fe₂₀Ni₈₀, and epitaxial Fe/Co multilayers were studied. For the Fe/Co multilayer thin film elements it was found that it is possible to change the magnetization reversal process, by aligning the easy shape anisotropy axis with either the easy or the hard magnetocrystalline anisotropy axis. In the permalloy elements the effect of inter-elemental distance was found to determine the interval of fields where multidomain states were stable, so that for shorter inter-elemental distances multidomain states were stable for a shorter interval of fields. The domain structure of permalloy elements in rotating magnetic fields was also studied. Higher applied fields led to a broader interval of angles in which saturated states were stable.</p>

No description available.

The study involves the following stages: First, coupled-inductor and integrated magnetic structure used in Cuk converter circuit topologies are analyzed and the necessary information about these elements in circuit design is gathered. Also, benefits of using these magnetic elements are presented. Secondly / steady-state model, dynamic model and transfer functions of coupled-inductor Cuk converter topology are obtained via state-space averaging method. Third stage deals with determining the design criteria to be fulfilled by the implemented circuit. The selection of the circuit components and the design of the coupled-inductor providing ripple-free input current waveform are performed at this stage. Fourth stage introduces the experimental results of the implemented circuit operating in open loop mode. Besides, the controller design is carried out and the closed loop performance of the implemented circuit is presented in this stage.

The magnetic properties of transition metal compounds have been studied using SQUID-magnetometry, magnetic force microscopy and Lorentz transmission electron microscopy. New magnetic materials have been found and their magnetic properties have been determined. How the magnetic properties of a material can be changed through e.g. chemical substitution of magnetic and nonmagnetic atoms and shape and size effects have also been studied. Three different sets of samples have been investigated: three new Mn-compounds, two substitution series of layered magnetic structures and ferromagnetic micronsized thin film elements. The three Mn-compounds, Mn3IrSi, IrMnSi and Mn8Pd15Si7, show different magnetic ordering. Mn3IrSi orders 'antiferromagnetically' at 210 K. IrMnSi forms a double cycloidal spin spiral below 460 K. Mn8Pd15Si7 only shows short-range magnetic ordering. Substituting Se with S in TlCo2Se2-xSx changes the magnetic order from a spin spiral to a colinear ferromagnet for a composition of x=1.75. An intermediate region exists where the compound is neither a pure ferromagnet, nor purely a spin spiral, as evidenced by the magnetization versus field measurements for the x=1.3 and 1.5 samples. This is also seen in the temperature dependent susceptibility measurements. For the TlCu2-xFexSe2 compounds it was found that the ordering temperature and saturation magnetic moment per Fe-atom changed with composition x. Ferromagnetic micronsized thin film elements in permalloy, Fe20Ni80, and epitaxial Fe/Co multilayers were studied. For the Fe/Co multilayer thin film elements it was found that it is possible to change the magnetization reversal process, by aligning the easy shape anisotropy axis with either the easy or the hard magnetocrystalline anisotropy axis. In the permalloy elements the effect of inter-elemental distance was found to determine the interval of fields where multidomain states were stable, so that for shorter inter-elemental distances multidomain states were stable for a shorter interval of fields. The domain structure of permalloy elements in rotating magnetic fields was also studied. Higher applied fields led to a broader interval of angles in which saturated states were stable.

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.

In this thesis, results of the investigation of a new low-dimensional cobaltates Ba2-xSrxCoO4 are presented. The synthesis of both polycrystalline and single crystalline compounds using the methods of conventional solid state chemical reaction and floating-zone optical furnace is first introduced. Besides making polycrystalline powders, we successfully, for the first time, synthesized large single crystals of Ba2CoO4. Single crystals were also obtained for Sr doped Ba2-xSrxCoO4. Powder and single crystal x-ray diffraction results indicate that pure Ba2CoO4 has a monoclinic structure at room temperature. With Sr doping, the lattice structure changes to orthorhombic when x ≥ 0.5 and to tetragonal when x = 2.0. In addition, Ba2CoO4 and Sr2CoO4, have completely different basic building blocks in the structure. One is CoO4 tetrahedron and the later is CoO6 octahedron, respectively. Electronic and magnetic properties were characterized and discussed. The magnetic susceptibility, specific heat and thermal conductivity show that Ba2CoO4 has an antiferromagnetic (AF) ground state with an AF ordering temperature TN = 25 K. However, the magnitude of the Néel temperature TN is significantly lower than the Curie-Weiss temperature (|θ| ~ 110 K), suggesting either reduced-dimensional magnetic interactions and/or the existence of magnetic frustration. The AF interaction persists in all the samples with different doping concentrations. The Néel temperature doesn’t vary much in the monoclinic structure regime but decreases when the system enters orthorhombic. Magnetically, Ba2CoO4 has an AF insulating ground state while Sr2CoO4 has a ferromagnetic (FM) metallic ground state. Neutron powder refinement results indicate a magnetic structure with the spin mostly aligned along the a-axis. The result from a μ-spin rotation/relaxation (μ+SR) experiment agrees with our refinement. It confirms the AF order in the ab-plane. We also studied the spin dynamics and its anisotropy in the AF phase. The results from inelastic neutron scattering show that spin waves have a clear dispersion along a-axis but not along c-axis, indicating spin anisotropy. This work finds the strong spin-lattice coupling in this novel complex material. The interplay between the two degrees of freedom results an interesting phase diagram. Further research is needed when large single crystal samples are available.

Abstract not available

Au cours de cette étude, nous avons montré que dans les oxydes de fer type perovskite, les ions sulfates (SO42-) pouvaient être utilisés à escient pour apporter de l'anisotropie cristalline et électronique ou bien au contraire pour casser les mises en ordre à longue distance et provoquer l'isotropie du matériau. Ainsi, ce travail a permis d'isoler les composés bidimensionnels : Sr4Fe2.5-x□xO7.25-(3x/2)(SO4)0.5 (avec x=0, 0.25 et 0.5). Ils peuvent être décrits comme une intercroissance SrO/SrFeO2,5/SrFe0,5-x□xO1,25-(3x/2)(SO4)0,5/SrFeO2,5 et cristallisent dans une maille moyenne quadratique I4/mmm (a=ap et c≈29 Å). Les tétraèdres de sulfates non-pontés se présentent de façon transversale par rapport à c ⃗ pour x=0,5 et longitudinale pour x=0 et 0,25 parce que pontés aux pyramides de fer de la couche partagée. Les atomes de fer des couches non-mixtes SrFeO2,5 se situent eux en coordinence pyramidale pour x=0 et pyramidale et octaédrique pour x=0.5 et x=0.25 afin de respecter la trivalence du fer. Quel que soit x, les composés sont antiferromagnétiques de type G (les spins se situant dans le plan (a,b)). Cependant, pour x=0,5, deux configurations magnétiques sont observées, tandis que les composés x=0 et x=0.25 n'en montrent qu'une seule. Dans le composé Sr4Fe2.5□xO7.25(SO4)0.25(CO3)0.25, l'influence des carbonates (CO32-) se ressent directement sur le paramètre d'empilement, qui est plus petit. Cela n'entraîne cependant aucun changement sensible dans les propriétés physiques. Les composés sont des semi-conducteurs présentant de fortes valeurs de résistivité électronique (de l'ordre de 106Ω.cm) et ne sont pas conducteurs ioniques.La phase ordonnée "15R" SrFe0.6Cr0.4O2.8 se désordonne lorsque l'on substitue 10% du fer par des sulfates (SrFe0.5Cr0.4O2.1(SO4)0.1) et adopte une structure pseudo-cubique. Ses propriétés physiques sont alors bouleversées puisque l'on passe d'un comportement ferromagnétique à antiferromagnétique (TN=800K). Ce composé très lacunaire en oxygène montre des défauts structuraux plus ou moins étendus et un comportement de type semi-conducteur. Aucun phénomène de conduction ionique n'est observé. / In this study, we have shown that in perovskite-type iron oxides, sulfate ions (SO42-) can be used to bring structural and electronic anisotropy, or on the contrary to break long distance ordering and cause the isotropy of the material.Thus, this work made it possible to isolate the two-dimensional compounds: Sr4Fe2.5-x□x07.25-(3x/2)(SO4)0.5 (with x = 0.25 and 0.5). They can be described as an intergrowth SrO/SrFeO2,5/SrFe0,5-x□xO1,25-(3x/2)(SO4)0.5/SrFeO2,5 and crystallize in a quadratic mean cell I4/mmm (a=ap et c≈29 Å). The unbridged sulfates tetrahedra are oriented transversely with respect to c ⃗ for x=0.5 and longitudinal for x=0 and 0.25 because bridged to the iron pyramids of the shared layer. The iron atoms of the non-mixed layers SrFeO2,5 are in pyramidal coordination for x=0 and pyramidal and octahedral coordination for x=0.5 and x=0.25, in order to respect the trivalence of iron. However, for x=0.5, two magnetic configurations are observed whereas the compounds x=0 and x=0.25 show only one. In the compound Sr4Fe2.5□xO7.25(SO4)0.25(CO3)0.25, the influence of the carbonates (CO32-) is directly felt on the stacking parameter, which is smaller. The compounds are semiconductors with high electronic resistivity values (of the order of 106Ω.cm) and are not ionic conductors.The ordered phase "15R" SrFe0.6Cr0.4O2.8 becomes disordered when 10% of the iron is substituted with sulfates (SrFe0.5Cr0.4O2.1(SO4)0.1) and adopts a pseudo-cubic structure. Its physical properties are then modified because we switch from a ferromagnetic to an antiferromagnetic behavior (TN=800K). This oxygen-deficient compound shows more or less large structural defects and a semiconductor behavior. No ionic conduction phenomenon is observed.

Title: Magnetic properties of R2TIn8 and related tetragonal compounds Author: Petr Čermák Department / Institute: Department of Condensed Matter Physics Supervisor of the doctoral thesis: doc. Mgr. Pavel Javorský, Dr., Department of Condensed Matter Physics Abstract: Intermetallic compounds R2TIn8 (R = rare earth, T = transition metal), commonly called "218" because of stoichiometry, are structurally related to a class of well- known Ce-based heavy-fermions like CeCoIn5 or CeRhIn5. They are located between fully 3D cubic compound (e.g. CeIn3) and quasi-2D "115" superconductors, which makes them ideal candidates to study structural dimensionality effects on various properties. Recent developments in this field showed that it is possible to grow compounds with T = Pd or Pt with "218" stoichiometry. Therefore further study of "218" compounds is desired since much less is known about them compared to "115" compounds. We have focused mainly on the determination of magnetic structures and crystal field effects along the series of Rh based "218" compounds for various rare-earth elements. The single crystals of compounds with R = Nd, Tb, Dy, Ho, Er, Tm, La, Lu, Y were successfully grown. Results of bulk measurements (specific heat, susceptibility) together with magnetic structures determined from several neutron...

The local structure, atomic and magnetic structure correlated over a small length scale, of a material has a strong impact on material properties. Pair distribution function (PDF) analysis is a strong tool to investigate local structure and magnetism of this nature. This work outlines extensions to current one dimensional magnetic pair distribution functions (1D-mPDF) and the fitting of structures with symmetry breaking local atomic distortions. 1DmPDF analysis has been used to study local magnetic structure, but requires a rotational averaging of the correlations so directional information is lost, as in powder diffraction experiments. Three dimensional difference magnetic pair distribution function (3D-∆mPDF) analysis does not require this rotational averaging and preserves directional information. This is a useful tool in analyzing experimental data like single crystal neutron diffraction and studying locally anisoptropic magnetic structures. Here we present a technique and software tools to calculate the 3D-∆mPDF pattern of a given structure and give a brief analysis of the local magnetic structure of MnTe. Another problem in PDF analysis is the modeling of structures with symmetry breaking local atomic distortions. Symmetry-adapted distortion modes have been used for structural refinement in Rietveld refinement for at least 10 years; more more recently, this has also been applied to PDF data. We present here a detailed discussion of the use of symmetry-adapted modes for structural refinement using PDF data. We also outline new open-sourced software tools to apply this technique and show two analyses using symmetry-adapted structural modes.