Made available in DSpace on 2016-06-02T20:16:47Z (GMT). No. of bitstreams: 1 3337.pdf: 3467265 bytes, checksum: 529eb6b6e21347d13906d054f5528d34 (MD5) Previous issue date: 2010-08-31 / Financiadora de Estudos e Projetos / In this work the magnetic properties of iridium oxide synthesized via Pechini s method and doped with different concentrations of cobalt were studied. The proposal was to investigate the possibility of applying it as a diluted magnetic oxide (DMO1). The samples were characterized by x-ray diffraction to study the structural features by Rietveld refinement, which indicated that there is no inconsistency between the observed intensities and the calculated profile. The samples were characterized by magnetization measurements as a function of temperature and applied magnetic field. A paramagnetic behavior was observed in samples doped with low Co concentrations. However, a small change in the magnetism appears in samples with concentrations up to 10 mol% of Co. A new metamagnetic phase was observed for samples with 25% and 30% in mol of Co. / Neste trabalho foram estudadas as propriedades magnéticas do óxido de irídio sintetizado via método de Pechini e dopado com diferentes concentrações de Co, com a proposta de investigar a possibilidade de aplicá-lo como um Óxido Diluído Magnético (DMO, sigla em inglês) 1. As amostras foram caracterizadas por medidas de difração de raios-x através de refinamento Rietveld, indicando que não há incompatibilidade entre as intensidades observadas e as do perfil calculado. Além disso, foram realizadas medidas de magnetização em função da temperatura e do campo magnético aplicado. Foi observado comportamento paramagnético em amostras dopadas com baixas concentrações de Co, entretanto, uma pequena mudança no magnetismo das amostras começou a aparecer em amostras com concentrações de Co a partir de 10 mol%, ficando evidente em altas concentrações de dopante, 25% e 30% em mol, uma fase metamagnética.

Dans cette thèse, on étudie la criticalité quantique ferromagnétique dans trois composés ternaires à base d'uranium, par des mesures thermodynamiques et de transport sur des échantillons monocristallins, à basse température et sous haute pression. URhSi et URhAl sont des systèmes ferromagnétiques itinérants, tandis que UCoAl est un système paramagnétique étant proche d'une instabilité ferromagnétique. Tous ont une phase ordonnée de type Ising. Dans le composé orthorhombique URhSi, on montre que la température de Curie diminue lorsqu'un champ magnétique est appliqué perpendiculairement à l'axe facile d'aimantation, et une transition de phase quantique est attendue autour de 40~T. Dans le système hexagonal URhAl, on établit le diagramme de phase pression--température pour la première fois, lequel indique une transition de phase quantique vers 5~GPa. Dans le composé isostructural UCoAl, on étudie la transition métamagnétique par des mesures d'aimantation, d'effet Hall, de résistivité et de dichroïsme circulaire magnétique des rayons X. On observe des phénomènes de relaxation magnétique intrigants, avec des sortes de marches. L'effet Hall et la résistivité ont été mesurés à des températures de réfrigérateur à dilution, sous pression hydrostatique jusqu'à 2,2~GPa, et sous champ magnétique jusqu'à 16~T. La transition métamagnétique se termine sous pression et champ magnétique au niveau d'un point critique quantique terminal. Dans cette région, il se produit une forte augmentation de la masse effective, et une différence intrigante entre champ montant et descendant apparaît dans la résistivité transverse. Ce pourrait être la signature d'une nouvelle phase, éventuellement reliée aux phénomènes de relaxation observés dans les mesures d'aimantation, et résultant de frustrations au sein du réseau quasi-Kagomé que forment les atomes d'uranium dans cette structure cristalline. / In this thesis we explore the ferromagnetic quantum criticality in three uranium-based ternary compounds, by means of thermodynamical and transport measurements on single crystal samples, at low temperature and high pressure. URhSi and URhAl are itinerant ferromagnets, while UCoAl is a paramagnet being close to a ferromagnetic instability. All of them have Ising-type magnetic ordering. In the orthorhombic compound URhSi, we show that the Curie temperature decreases upon applying a magnetic field perpendicular to the easy magnetization axis, and a quantum phase transition is expected around 40~T. In the hexagonal system URhAl, we establish the pressure--temperature phase diagram for the first time, indicating a quantum phase transition around 5~GPa. In the isostructural compound UCoAl, we investigate the metamagnetic transition with measurements of magnetization, Hall effect, resistivity and X-ray magnetic circular dichroism. Some intriguing magnetic relaxation phenomena are observed, with step-like features. Hall effect and resistivity have been measured at dilution temperatures, under hydrostatic pressure up to 2.2 GPa and magnetic field up to 16~T. The metamagnetic transition terminates under pressure and magnetic field at a quantum critical endpoint. In this region, a strong effective mass enhancement occurs, and an intriguing difference between up and down field sweeps appears in transverse resistivity. This may be the signature of a new phase, supposedly linked to the relaxation phenomena observed in magnetic measurements, arising from frustration on the quasi-Kagome lattice of uranium atoms in this crystal structure.

Magnetické nanočástice a nanostrukturované materiály jsou velkým příslibem v mnoha oblastech, včetně biomedicíny, sanace životního prostředí nebo získávání energie. Proto neustále roste zájem o výzkum jejich jedinečných vlastností a vývoj uskutečnitelných výrobních postupů. Tato práce se zabývá metodou samouspořádávání, spočívající v povrchové precipitaci tenkých vrstev, za účelem výroby epitaxních polí nanoostrůvků ze slitiny FeRh na různých monokrystalických substrátech. Při tomto výrobním postupu zůstává zachována metamagnetická fázová přeměna nanoostrůvků. Morfologie a magnetické vlastnosti samouspořádaných nanomagnetů z FeRh jsou charakterizovány kombinací experimentálních technik a modelování, přičemž bylo zjištěno, že jejich rovnovážné tvary a magnetické uspořádání jsou navzájem propojeny. Kromě toho je navržena cesta pro získání volných nanočástic, která by mohla potenciálně umožnit využití metamagnetických nanostruktur v buněčných kulturách a biomedicíně obecně. Za tímto účelem jsou nanoostrůvky FeRh uvolňovány ze substrátu chemickým leptáním. Chování nanočástic a jejich reakce na teplotu a magnetické pole jsou studovány v kapalném prostředí. Metamagnetické vlastnosti separovaných nanočástic jsou charakterizovány pomocí vibrační magnetometrie.

The crystal structures of the LnAgAs2 and LnAuAs2 compounds were reinvestigated by single-crystal diffraction experiments. Contrary to the respective copper compounds, no stuffed variant of the HfCuSi2 type was found. For CeAuAs2, GdAuAs2 and TbAuAs2, a slight under-occupation of the gold position was determined, the other compounds crystallize in a 1:1:2 ratio. Additionally, LaCuAs2 was synthesized for the first time in a 1:1:2 ratio. Due to the fact that imaging plate diffraction systems were used instead of four-circle diffractometers, satellite reflections could be observed for most of the LnCuAs2 compounds (Ln = Ce, Nd, Sm, Gd, Tb, Ho), CeAuAs2, GdAuAs2 and TbAuAs2. Structure models of GdCuAs2, CeAuAs2, GdAuAs2 and TbAuAs2 were developed, rod and layer groups of the respective structural motives were determined and approximants were presented. / Die Kristallstrukturen der LnAgAs2 und LnAuAs2 Verbindungen wurden mittels Einkristallbeugungsexperimenten neu untersucht. Im Gegensatz zu den jeweiligen Kupferverbindungen wurde keine gefüllte Variante des HfCuSi2-Typs gefunden. Im Falle von CeAuAs2, GdAuAs2 und TbAuAs2 wurde eine geringfügige Unterbesetzung der Gold-Position bestimmt, die anderen Verbindungen kristallisieren in einem 1:1:2-Verhältnis. Weiterhin wurde erstmals LaCuAs2 in diesem Verhältnis synthetisiert. Aufgrund der Verwendung von Bildplattensystemen an Stelle von Vierkreisdiffraktometern konnten Satellitenreflexe für den Großteil der LnCuAs2-Verbindungen (Ln = Ce, Nd, Sm, Gd, Tb, Ho), CeAuAs2, GdAuAs2 and TbAuAs2 beobachtet werden. Strukturmodelle von GdCuAs2, CeAuAs2, GdAuAs2 und TbAuAs2 wurden entwickelt, Stab- und Ebenengruppen der einzelnen Strukturmotive wurden bestimmt und Approximanten präsentiert.

Sr₃Ru₂O₇ is a quasi-two-dimensional metal and has a paramagnetic ground state that is heavily renormalised by electron-electron correlations and magnetic exchange interactions. Inextricably linked to this renormalisation is the metamagnetism of Sr₃Ru₂O₇ - a rapid rise in uniform magnetisation over a narrow range of applied magnetic field. Knowledge of the zero-field physics is essential to any description of the metamagnetism. Light may be shed on the enigmatic ground state of Sr₃Ru₂O₇ by doping the crystal lattice with foreign cations: this is the primary purpose of the original research referred to in this thesis, in which studies of some of the electronic properties of crystals of cation-doped Sr₃Ru₂O₇ are reported. Single crystals of Sr₃(Ru[subscript(1-x)]Ti[subscript(x)])₂O₇ and Sr₃(Ru[subscript(1-x)]Cr[subscript(x)])₂O₇ have been synthesised in an image furnace and some of the properties of these crystals have been measured. Evidence that indicates the emergence of a spin density wave as a function of Ti-doping in Sr₃(Ru[subscript(1-x)]Ti[subscript(x)])₂O₇ is presented. Time-dependent magnetic irreversibility has been observed in samples of Sr₃(Ru[subscript(1-x)]Cr[subscript(x)])₂O₇, thus hinting at the involvement of the RKKY mechanism in these materials. Regarding cation doping out of the conducting RuO₂ planes, samples of (Sr[subscript(1-y)]La[subscript(y)])₃Ru₂O₇ have been grown and investigated. Both the Sommerfeld coefficient and the Fermi liquid A coefficient of (Sr[subscript(1-y)]La[subscript(y)])₃Ru₂O₇ are found to decrease as a function of y (0 ≤ y ≤ 0.02); these observations point towards a reduction in the thermodynamic mass of the Landau quasiparticles. Results from magnetoresistance and magnetisation measurements indicate that the metamagnetism of the (Sr[subscript(1-y)]La[subscript(y)])₃Ru₂O₇ series probably cannot be explained by a rigid band-shift model. Also, some aspects of these data imply that the metamagnetism cannot be fully accounted for by a spin fluctuation extension to the Ginzburg-Landau theory of uniform magnetisation.

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