Dans le système YbMn6Ge6-xSnx, l’ytterbium de valence intermédiaire (v.i.) s’ordonne magnétiquement à des températures étonnamment élevées (jusqu’à TYb~125 K). Ce travail contribue à une meilleure compréhension des propriétés physiques de ces alliages grâce à la stabilisation de nouveaux composés dans la gamme de concentration 4,6<x<5,3. Des expériences extrêmement sensibles utilisant le rayonnement synchrotron (XANES, XMCD) ont permis de situer l’instabilité magnétique d’Yb vers x~5 ,2. Le diagramme de phase magnétique (x,T) a pu être complété. Il montre une étroite ressemblance avec le diagramme de Doniach. Dans les systèmes classiques à base d’Yb v.i., les propriétés physiques sont interprétées dans le cadre d’une compétition entre effet Kondo et interactions RKKY. La particularité du système YbMn6Ge6-xSnx est la présence d’un sous-réseau magnétique de Mn qui est très probablement à l’origine des propriétés singulières d’Yb : TYb élevées et existence de mYb jusqu’à des valeurs de valence plus basse qu’à l’accoutumée (u>~2,8). Des expériences sous pression ont souligné l’analogie entre les effets de pression mécanique et chimique. L’augmentation de pression favorise le caractère trivalent et le magnétisme d’Yb. La 2nde partie de ce travail constitue une ouverture vers les matériaux pour les applications magnétocaloriques haute température telles que les pompes à chaleur ou la conversion d’énergie. Elle concerne l’étude des composés Mn4-xFexGa2Sn et Fe3Sn2 qui possèdent des températures de travail supérieures à la température ambiante. Leur EMC est modéré mais comparable à celui d’autres matériaux à transition de 2nd ordre fonctionnant dans la même gamme de température / In the YbMn6Ge6-xSnx system, intermediate valent Yb magnetically orders at an astonishingly high temperature (up to TYb~125 K). This work aims to improve the understanding of the physical properties of these compounds, with an emphasis on the concentration range 4.6<x<5.3. Some results have been obtained thanks to highly sensitive experimental techniques using synchrotron radiation (XANES, XMCD), and allowed us to locate the Yb magnetic instability near x~5.2. The magnetic phase diagram (x,T) has been completed and shows some similarities with the Doniach diagram. In conventional intermediate valent Yb systems, there is a competition between the Kondo screening and the magnetic exchange interactions of RKKY-type. The particularity of YbMn6Ge6-xSnx compounds is related to the strong 3d Mn-5d Yb exchange interaction and to the nonzero exchange field at the Yb site generated by the magnetic Mn sublattice. It is most likely responsible of very unusual Yb magnetic properties, in particular the astonishingly high magnetic ordering temperatures of low valence Yb (u>~2,8). External pressure experiments showed an analogy between mechanical and chemical pressure effects. Increasing the pressure favors trivalent state and Yb magnetism. In a second part, we investigated the magnetic and magnetocaloric properties of Mn4-xFexGa2Sn and Fe3Sn2. High temperature magnetocaloric applications, such as heat pump or heat conversion, need materials with high Curie temperature. These two compounds have working temperatures above 330 K. The magnetocaloric effect has been evaluated : the magnitude is moderate but close to other compounds with a second order transition near this temperature region.

Depuis quelques dizaines d’années, l’étude de composés intermétalliques à base des métaux de transition 3d, et d’éléments de terres rares 4f, présente un vif intérêt tant d’un point de vue fondamental qu’appliqué. Les propriétés remarquables de ces matériaux magnétiques proviennent de la présence, dans le même composé, de métaux de transition 3d, caractérisés par un magnétisme itinérant donné par les électrons de la bande externe 3d, et de métaux de terres-rares 4f qui, eux, présentent un magnétisme localisé dû aux électrons de la couche interne 4f. La recherche présentée ici se concentrera sur deux diagrammes des phases ternaires alliant samarium, fer et nickel et l’yttrium, fer et Gallium dans le deuxième système. Ces types d’intermétalliques sont aussi potentiellement caractérisés par un effet magnétocalorique (EMC) défini par le réchauffement ou le refroidissement de ces matériaux magnétiques sous l’application ou la suppression d’un champ magnétique extérieur.Le but de la thèse est la construction des deux diagrammes ternaires qui n’ont jamais été publiés et étudier les propriétés physicochimiques dans les systèmes (Sm,Y)-Fe-(Ni,Ga). Cette recherche aboutira à la détermination des diagrammes ternaires Sm-Fe-Ni et Y-Fe-Ga expérimentales (section isotherme à 800°C) et d’étudier les propriétés structurales de ces composés intermétalliques. Les propriétés magnétiques et magnétocaloriques ont également été étudiées en couplant les analyses magnétiques avec les mesures par diffraction des rayons X et par spectroscopie Mössbauer. Ces travaux ont mis en évidence l’influence importante de la nature et du taux de fer substitué au nickel et au gallium dans les deux systèmes sur les propriétés magnétiques / In recent decades, the study of intermetallic compounds containing 3d transition metals and 4f rare earth elements presents great interest both from a fundamental point of view and in its various applications. The remarkable properties of these magnetic materials come from the presence, in the same compound, of 3d transition metal, characterized by an itinerant magnetism given by the electrons in the 3d external band, and 4f rare-earth which themselves have a localized magnetism due to the electrons of the 4f inner layer. The research presented here will focus on the construction of two ternary phase diagrams combining [Sm-Fe-Ni] in the first system and [Y-Fe-Ga] in the second one. These types of intermetallics are also characterized by a magnetocaloric effect (EMC) defined by the heating or cooling of these magnetic materials under the application or removal of an external magnetic field.The aims of the thesis are the construction of two ternary phase diagrams that have never been published before and the study of the physicochemical properties in the (Sm, Y) -Fe- (Ni, Ga) systems. This research will lead to the determination of experimental ternary phase diagrams Sm-Fe-Ni and Y-Fe-Ga (isothermal section at 800°C) and to study the structural properties of some intermetallic compounds.The magnetic and magnetocaloric properties were also studied by coupling magnetic analysis with the X-ray diffraction and Mössbauer spectroscopy measurements. This work has highlighted the important influence of the nature and rate of iron substituted by nickel and gallium in both systems on the magnetic properties

La technologie de la réfrigération magnétique présentée dans ce manuscrit constitue une alternative prometteuse à la technologie de production de froid. Le travail effectué a porté tout d’abord sur une étude de la production de froid magnétique et un état de l’art de cette technologie émergente. Nous nous sommes fixés pour objectif de dimensionner et réaliser un réfrigérateur magnétique qui se présente sous la forme d’un actionneur intégré. Le dimensionnement a été réalisé par le biais d’un modèle à éléments finis. Il s’agit d’un dispositif présentant deux entrefers, dont un suffisamment large pour pouvoir accueillir un régénérateur magnétocalorique au sein duquel la valeur de l’induction est la plus élevée possible tout en offrant un profil d’induction de forme trapézoïdale. La réalisation du démonstrateur sur la base des étudeseffectuées par éléments finis constitue la première étape vers la réalisation d’un réfrigérateur magnétique intégré pouvant être logé dans une automobile. Pour finir, et afin d’améliorer encore les performances de notre dispositif, une optimisation de l’inducteur électromagnétique a été effectuée par le biais d’un modèle à éléments finis couplé à un algorithme d’optimisation. / The magnetic refrigeration technology is a promising alternative technology to the production of cold. The work carried out focuses on the technology of magnetic refrigeration, a state of the art of this emerging technology has also been done. A magnetocaloric regenerator is placed in the air gap of the motor. It is necessary to design a motor with an air gap wide enough and where the induction will be as high as possible in order to insert the magnetocaloric regenerator. The regenerator is a hollow cylinder whose dimensions are adapted to the air gap of the synchronous machine. It is intended to contain the magnetocaloric material. The design of the electric motor is made in order to obtain a maximal variation of induction ΔB in the air gap and a temperature difference ΔT as large as possible, improving the magnetocaloric performance of the prototype. The profile of induction should also be as close as possible to a rectangular signal. The realization of the demonstrator based on the finite element studies is the first step towards the realization of integrated magnetic refrigerator which can be housed in an automobile. Finally, and in order to improve the performance of our device, an optimization of the electromagnetic inductor was carried out with a finite element model coupled to an optimizationalgorithm.

Die magnetische Kühlung ist eine etablierte Technologie im Bereich der Tieftemperaturphysik. Allerdings bieten die Skalierbarkeit des magnetokalorischen Effektes und die Möglichkeit zur kompakten Bauweise auch ein breites Anwendungsspektrum für den Einsatz bei Raumtemperatur. Besonders hervorzuheben ist die Möglichkeit zur Anpassung der magnetostrukturellen Umwandlungstemperatur in La(Fe, Si)13-basierten Materialien an die Arbeitstemperatur einer Kühleinheit. Die Herstellung von Ausgangsmaterial über das Schmelzspinnen, ist von hoher technologischer Relevanz, da im Vergleich zu konventionell gegossenem Massivmaterial die anschließende Glühdauer drastisch reduziert werden kann [1]. In der vorliegenden Arbeit wird zunächst auf die optimalen Glühbedingungen in rasch-erstarrtem Bandmaterial für die Bildung der relevanten magnetokalorischen Phase eingegangen. Durch Variation der Glühtemperatur wird der Einfluss von Sekundärphasen auf den magnetokalorischen Effekt bewertet. Darüber hinaus können bei optimaler Wahl der Legierungszusammensetzung ein großer magnetokalorischer Effekt und der gewünschte Arbeitstemperaturbereich eingestellt werden. Besonderes Augenmerk wird auf die Verknüpfung des Substitutionseffektes (hier: Si für Fe) und der Aufweitung des Gitters durch Hydrierung mit dem resultierenden magnetokalorischen Effekt gelegt. Ein weiterer Punkt, sind die Untersuchungen zur Langzeitstabilität der Eigenschaften von hydriertem Band- und Massivmaterial. Grundlegende und umfassende Untersuchungen zur Substitution von Eisen durch Mangan und zum daraus folgenden Einfluss auf Phasenbildung, Umwandlungstemperatur sowie auf den magnetokalorischen Effekt, insbesondere nach der Hydrierung, werden ebenfalls dargestellt. Die Ergebnisse der vorliegenden Arbeit erlauben damit die Bewertung verschiedener Strategien zur Optimierung der magnetokalorischen Eigenschaften von La(Fe, Si)13.

No description available.

Ever since the discovery of the giant magnetocaloric effect (GMCE) in the Gd5(Si,Ge)4 phases, magnetic cooling has gained significant interest because of its potential environmental benefits and increased efficiency compared to vapour-based refrigeration. This current work is focused on one of the most promising GMCE systems, the (Mn,Fe)2(Si,P) materials. An alternative synthetic route has been explored for the Mn2-xFexSi0.5P0.5 and MnFeSiyP1-y series which is capable of producing phase-pure samples. The new preparation technique eliminates common impurities that arise from established methods thus providing a more accurate description of the structural and physical properties. The low cost, non-toxicity, abundance of starting materials and easy tuning of the magnetic properties make these materials desirable for potential applications. Phase-pure magnetocaloric Mn2-xFexSi0.5P0.5 materials (x = 0.6, 0.7, 0.8, 0.9) were synthesized through arc-melting followed by high temperature sintering. Structural features of samples with x = 0.6, 0.9 were studied through temperature dependent synchrotron powder x-ray diffraction. Magnetic measurements established the Curie temperature, thermal hysteresis, and magnetic entropy change of this system. According to the diffraction and magnetization data, all of the samples were shown to have a first-order magnetostructural transition which becomes less pronounced for Mn-richer samples. The MnFeSixP1-x phases (x = 0.30, 0.35, 0.40, 0.48, 0.52, 0.54, 0.56) have also been synthesized by the same method. For the first time, single crystals of x = 0.30, 0.40 were successfully grown. Variable temperature x-ray diffraction experiments for x = 0.30 were completed which show the structural changes across the phase transition. This structural data was complemented with magnetization data providing Curie temperatures and thermal hysteresis. / Thesis / Master of Science (MSc)

Par une approche supramoléculaire, des architectures radiales hétéro-poly-métalliques ont été réalisées pour des applications en photosynthèse artificielle et en magnétisme moléculaire. Dans une première partie, la synthèse et la caractérisation (spectroscopie UV-vis, émission, électrochimique, DRX) de complexes de ruthénium(II), possédant une gamme de ligands polypyridines, ont été réalisées. Les calculs théoriques ont été effectués afin de soutenir l’interprétation des propriétés photophysiques. Ces complexes, présentant un certain nombre de pyridines externes, ont servi de cœur à des architectures à base de rhénium tris-carbonyles (pour les effets d’antenne), et de cobaloximes (pour les propriétés catalytiques). Les nucléarités obtenues varient de 2 à 7 selon le cœur utilisé. Ces systèmes ont été engagés dans des cycles de photo-production de dihydrogène, démontrant une meilleure efficacité que la référence du domaine, le [Ru(bpy)3]2+. La seconde partie concerne l’étude de couples de métaux de transition, construits à partir de briques polycyanométallates, ou de lanthanides pontés par des ligands oxamides. Ces approches « complexes comme ligand » puis « assemblages comme ligand » permettent d’obtenir des systèmes de haute nucléarité, présentant des propriétés de molécule-aimant ou des effets magnéto-caloriques (à base de CrNi, GdCu, DyCu). Des propriétés photomagnétiques ont été observées sur les couples RuCu et MoCu, pouvant servir de commutateurs moléculaires dans des systèmes complexes. Enfin, une structure hétéro-tétra-métallique trifonctionnelle a été obtenue contenant à la fois un commutateur MoCu, une entité molécule-aimant CuTb et un complexe de ruthénium. / By a supramolecular strategy, radial hetero-poly-metallic architectures were obtained for applications in artificial photosynthesis and molecular magnetism. The first part is devoted to the synthesis, as well as the photophysical characterization (UV-vis absorption, emission, electrochemistry, X-ray diffraction) of ruthenium(II) complexes bearing a wide range of polypyridine ligands. Theoretical calculations were performed to support the interpretation of the photophysical properties. Through their pendant pyridine moieties, these complexes were used as core of architectures, bearing rhenium tris-carbonyl (for antenna effects), and cobaloximes (for catalytic properties) complexes. The nuclearities obtained vary from two to seven according to the core involved. These systems were engaged in photo-production of hydrogen, demonstrating more efficient systems than the reference in the field, the archetypal [Ru(bpy)3]2+. The second part concerns the study of transition metal couples, built on polycyanometallate cores, or oxamide-bridged lanthanide-based assemblies. These “complex as ligand” and “assemblies as ligand” approaches allow us to obtain high nuclearity systems on which we seek single molecule magnet (SMM) properties or good magnetocaloric effect (based on CrNi, GdCu, DyCu). Photo-magnetic properties have been studied on the RuCu and MoCu couples, which can serve as molecular switches in complex systems. Finally, a tri-functional hetero-tetra-metallic architecture was obtained containing a MoCu switch, a CuTb SMM entity, and a ruthenium complex.

In this thesis, a numerical study of simulating and computing the magnetocaloric properties of magnetic materials is presented. The main objective was to deduce the optimal procedure to obtain the isothermal change in entropy of magnetic systems, by evaluating two different formulas of entropy extraction, one relying on the magnetization of the material and the other on the magnet's heat capacity. The magnetic systems were simulated using two different Monte Carlo algorithms, the Metropolis and Wang-Landau procedures. The two entropy methods proved to be comparably similar to one another. Both approaches produced reliable and consistent results, though finite size effects could occur if the simulated system became too small. Erroneous fluctuations that invalidated the results did not seem stem from discrepancies between the entropy methods but mainly from the computation of the heat capacity itself. Accurate determination of the heat capacity via an internal energy derivative generated excellent results, while a heat capacity obtained from a variance formula of the internal energy rendered the extracted entropy unusable. The results acquired from the Metropolis algorithm were consistent, accurate and dependable, while all of those produced via the Wang-Landau method exhibited intrinsic fluctuations of varying severity. The Wang-Landau method also proved to be computationally ineffective compared to the Metropolis algorithm, rendering the method not suitable for magnetic simulations of this type.

La réfrigération magnétique attire beaucoup d’attention ces dernières années parce qu’elle est considérée comme une technologie respectueuse de l’environnement et énergétiquement économique. Aujourd’hui, cette technologie avancée est encore en phase de recherche que des dispositifs de réfrigérations magnétiques soient déjà opérationnels. Ce travail de thèse consiste à étudier la potentialité de résistance à la corrosion de différents types de matériaux magnétocaloriques (Gd6Co1.67Si3, Ni2Mn0.75Cu0.25Ga et Pr0.66Sr0.34MnO3) en contact avec un fluide caloporteur. Afin de comprendre les propriétés magnétocaloriques des matériaux, nos recherches se sont aussi focalisées sur les relations entre la transition magnéto-structurales d’alliages Heusler Ni2Mn0.75Cu0.25Ga et (i) la distribution cationique au sein de la structure cristalline et/ou (ii) la microstructure. Finalement, le diagramme de phase magnétique et nucléaire en lien avec les effets magnétocalorique obtenu grâce à la diffraction de neutrons et de pérovskite Pr1-xSrxMnO3 (0.25≤x≤0.45) est également présenté. / Magnetic refrigeration has gained lot of importance and attention as they are highlighted to be environmental friendly, energy efficient. Presently, though at research stage, the magnetic refrigerators are pushed towards realization in domestic application with extensive work on materials and with few working models. One critical issue, the potential resistance to corrosion in case of different class of magnetocaloric materials (Gd6Co1.67Si3, Ni2Mn0.75Cu0.25Ga and Pr0.66Sr0.34MnO3) against the heat transport fluid is addressed. To better understand and improve the observed magnetocaloric properties in Heulser alloys Ni2Mn0.75Cu0.25Ga and to elaborate the same with the magneto-structural relation, studies on (i) cation distribution with in crystal structure and/or (ii) microstructural dependence are presented. Nuclear and magnetic phase diagram based on detailed neutron diffraction and magnetism studies for magnetocaloric perovskite oxide Pr1-xSrxMnO3 (0.25≤x≤0.45) is also presented

Orientador: Mario Antonio Bica de Moraes / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-11T18:58:49Z (GMT). No. of bitstreams: 1 Rouxinol_FranciscoPauloMarques_D.pdf: 6433084 bytes, checksum: 84d703ca8da3620b077c3b1335c59965 (MD5) Previous issue date: 2008 / Resumo: Técnicas de condensação de vapor são úteis na preparação de ligas magnéticas cujos componentes têm pouca, o mesmo nenhuma, solubilidade em condições de equilíbrio Neste trabalho, uma dessas técnicas ¿ sputtering ¿ foi empregada para fabricar ligas metaestáveis de GdXCr1-X, cujas propriedades magnéticas foram investigadas em função da concentração de Gd, x. Difratometria de raios-X de baixo ângulo (GAXRD) e espectroscopia de retroespalhamento Rutherford foram utilizados para determinar a estrutura do filme e sua composição elementar, respectivamente. As análises de GAXRD mostraram que a estrutura da fase de Gd, é hcp quando x ³ 0,88; e amorfa quando 0,16 £ 0,76. Uma estrutura bcc , para a fase de Cr, foi observada nos difratogramas quando x < 0,16, e amorfa quando x ³ 0,16 Para investigar as propriedades magnéticas utilizamos um magnetometro SQUID e m PPMS. O primeiro foi utilizado para as medidas de momento magnético em função do campo estático e temperatura. O PPMS foi tilizado nas investigações de susceptibilidade-AC em função da freqüência de oscilação do campo, temperatura e campo estático. A complexa natureza magnética dos filmes de Gd-Cr foi observada através das isotermas MxH, que não apresentaram saturação em baixas temperaturas, nem comportamento linear em altas temperaturas. Pela análise dos dados magnéticos, observamos que as amostras admitem um comportamento ferromagnético para x ³ 0,5 e paramagnético para as outras concentrações de Gd. A temperatura de Curie (TC) apresenta um aumento monotônico de 170 para 290 K quando x aumenta de 0,5 para 1,0. A temperatura de Curie-Weiss (q C) mostra um aumento monotônico com x. A partir das isotermas MxH a 2 K, o momento de saturação foi calculado, sendo independente de x e aproximadamente constante com um valor médio de 7,3 µB. Medidas de susceptibilidade em campos estáticos e dinâmicos revelaram a existência de comportamentos de vidros magnéticos em todas as amostras abaixo da temperat ra de freezing (Tf). Observamos, nas ligas com altas concentrações de Gd, a presença de comportamentos ferromagnéticos e cluster-glass em baixas temperaturas. Concluímos que a interação de troca entre os átomos de Gd dentro dos clusters de Gd não é do tipo RKKY, e sim do tipo supertroca. O efeito magnetocalórico (MCE) foi investigado através da variação de entropia magnética ( D SM) em função da temperatura, para a remoção de um campo de 50 kOe Curva de D SMxT para as amostras com x < 0,2 apresentaram um formato típico de superferromagneto, consistente com a existência de clusters Gd nos filmes. Nas outras amostras em que o EMC foi analisado, a presença de clusters é observada pelo comportamento dessas curvas a baixa temperatura; a altas temperaturas o comportamento de D SM com T indica fortemente a presença de mais fases magnéticas no filme. O diagrama de fase baseado em TC e Tf, e sua dependência com x é apresentado / Abstract: Vapor condensation techniques are very useful for preparation of alloys whose components have no mutual solubility under equilibrium conditions In this work, one of these techniques ¿ sputtering ¿ has been used to fabricate metastable GdXCr1-X alloys whose magnetic properties were investigated as a function of the Gd concentration, x. Grazing incidence angle X-ray diffraction (GAXRD) and Rutherford backscattering spectroscopy were employed to characterize the film structure and elemental composition, respectively. The GAXRD measurements revealed, for the Gd fraction, a hcp structure for x ³ 0,88; for 0,16 £ x £ 0,76 the Gd fraction was amorphous. The existence of a bcc structure for x < 0,16 was observed in the diffractograms for the Cr phase, which was amorphous for x ³ 0,16. To investigate the magnetic properties, a SQUID magnetometer and a PPMS were used. The former was employed for magnetic moment measurements as a function of applied static field and temperature. The PPMS was used for ac-susceptibility determinations as a function of the frequency of the ac driving field, temperature, and applied static field. The complex magnetic nature of the Gd-Cr films was revealed from the MxH isotherms which did not show saturation even at the lowest temperatures, and did not exhibit a linear behavior at higher temperatures. Processing of the magnetic data has shown that the films exhibit a ferromagnetic behavior for x ³ 0,5 and paramagnetic one for all other Gd concentrations. The Curie temperature (TC) increased monotonically from 170 to 290 K as x increased from 0,5 to 1,0. A monotonical increase in the Curie-Weiss temperature ( q C ) with x was also observed for all films. From the extrapolated MxH isotherm at 2 K (saturation magnetization), the saturation moments were calculated and found to be nearly constant at about 7.3 µB. Both static and dynamic susceptibility measurements revealed the existence of a magnetic glassy behavior in all alloys, occurring below a freezing temperature Tf . For the higher concentration alloys, the ferromagnetic and the cluster glass state were observed at low temperatures. It was thus concluded that the exchange interactions within Gd atoms in the clusters were not of the RKKY but of the superexchange type. The magnetocaloric effect (MCE) was investigated from the magnetic entropy change ( DSM) as a function of temperature, for the removal of a 50 kOe field. Samples with x < 0,2 exhibited DSMxT curves whose shapes are typical of a superferromagnet, consistently with the existence of Gd clusters in the films. For all the other alloys whose MCE was investigated, the presence of clusters is manifested from the behavior of these curves at low temperatures; at higher temperatures, the evolution of DSM with T strongly indicated the presence of more than one magnetic phase in the alloys A magnetic phase diagram based on the Tf and Tc transition temperatures and their dependence on x is presented in this thesis / Doutorado / Física da Matéria Condensada / Doutor em Ciências