[pt] Propomos um hamiltoniano de spin para descrever as propriedades térmicas e magnéticas de uma série de borocarbetos. O efeito do campo elétrico cristalino é aproximado por um campo magnético efetivo.Aplicamos a teoria de onda de spin e encontramos a relação de dispersão e a densidade apenas se desloca rigidamente em energia. A partir destes resultados calculamos a contribuição dos magnons ao calor específico molar com e sem campo magnético externo. O efeito do campo externo, porém é pequeno.A seguir calculamos a magnetização de subrede a T=0K, a qual sofre uma pequena redução devido às flutuações quânticas, em relação ao valor iônico obtido pela regra de Hund. Encontrmos também a magnetização em função da temperatura, com e sem aplicação de campo magnético. Com esse modelo conseguimos descrever bastante bem os dados experimentais de calor específicoda série. / [en] We propose a spin hamiltonian to describe the magnetic and thermal properties of a series of borocarbides. The effect of the crystal electric field is simulated by an effective magnetic field. By applying spin wave theory we have found the dispersion relation and the density of state for the magnos as well. In presence of an external magnetic field this density os state shifts rigidly in energy. From these results we have calculated the magnon contribuition to the molar specific heat without applied magnetic field. the effect of the field, however, is very small. We also have found the sublattice magnetization at T = 0K which is slightly reduced with respect to the ionic value given by the Hund's rule. Then we have calculated the sublattice magnetization as a function of T, both in presence and in absence of the magnetic field. With this model we succed in fitting fairly well the experimental data for the specific heat of the borocarbides series.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / Experimental data show that the dissolved impurities of the Mn presents a magnetic moment when substitute the Ni atom in pseudoquaternary borocarbide Y (Ni(1−x)Mx)2B2C, this is not observed if the impurities are Fe and Co. In this work we do a theoretical study of the superconducting order parameter variation and a possible formation of impurities magnetic moment to T < Tc. We have used the LAPW method, based in Density Functional Theory, to calculate the density of state of pure compound. To describe the superconductor system it was used the attractive Hubbard model. To describe the effects of the disorder induced by the dissolved impurities was used the coherent potential approximation. Using the Nambu s formalism to represent the Hamiltonian and the Green s functions of the effective medium (g) and of the perturbed system (G), we calculate the magnetic moment of the impurities from the diagonal terms and the variation of the order parameter from the off-diagonal one. The calculated date to magnetic moment show a qualitative agreement with the experimental results to Mn impurities. To the Fe and Co cases no magnetic moment was observed, in agreement with the experimental results too. The calculated order parameter shows a decrease with concentration to the Mn and Fe atoms. We investigate, through of the a calculation first principles (LAPW), the possible formation of the magnetic moment in this compound to concentration of 25 and 12.5%. The obtained results show that to concentration of 25%, both the impurities, Mn and Fe, presents a magnetic moment. The magnetization on the Co site was small when compared with the above cases. In this way we conclude that the Co atom not present a magnetic moment in this system. / Medidas experimentais mostram que impurezas de Mn formam momento magnético quando substituem o Ni no borocarbeto pseudoquaternário Y (Ni(1−x)Mx)2B2C, enquanto que as impurezas de Fe e de Co não exibem tal comportamento. Neste trabalho foi feito um estudo teórico da variação do parâmetro de ordem supercondutor e da possível formação de momento magnético nos sítios dessas impurezas para T < Tc. Para isso, utilizamos o m´etodo LAPW, baseado na Teoria do Funcional da Densidade, para calcular a densidade de estados do composto puro. Para descrever o sistema supercondutor foi utilizado o modelo de Hubbard atrativo. Os efeitos de desordem induzidos pela adição de impurezas de metais de transição M no material, foram tratados dentro da aproximação do potencial coerente. Usando o formalismo de Nambu para representar o hamiltoniano e as funções de Green associadas aos propagadores do sistema efetivo (g) e do sistema perturbado (G), calculamos o momento magn´etico das impurezas através dos termos diagonais e as variações do parâmetro de ordem efetivo a partir do termo fora da diagonal. Os resultados obtidos comprovam o que é observado experimentalmente, ou seja, o Mn forma momento magn´etico que aumenta com a concentração, enquanto que nas impurezas de Fe e Co nenhuma magnetização foi observada. Além disso, o parâmetro de ordem diminui com a concentração de Mn e Fe. Além desse estudo, também investigamos, através de um cálculo de primeiros princípios (LAPW), a possível formação de momento magnético neste composto para concentrações de 25 e 12.5%. Os resultados obtidos mostraram que para x = 25%, tanto as impurezas de Mn quanto as de Fe, apresentaram um momento magnético. A magnetização no síıtio do Co foi bastante pequena comparada com as duas anteriores e dessa forma pode-se concluir que esta impureza não forma momento quando diluída neste sistema.

Die Dissertation beschreibt Untersuchungen der intermetallischen Verbindungen TbxY1-xNi2B2C bzw. TbxEr1-xNi2B2C, Tb2PdSi3 sowie Dy2PdSi3, die sich auszeichnen durch physikalische Erscheinungen, die durch die Wechselwirkung der magnetischen Momente der Seltenerd (SE)-Ionen mit den Leitungselektronen hervorgerufen werden. Sie umfasst Beiträge zur Konstitution der Legierungssysteme, methodische Untersuchungen zur Einkristallzüchtung, zur Charakterisierung der Homogenität und Perfektion der Kristalle und zu physikalischen Eigenschaften (Supraleitung, Magnetismus). Die für das Erstarrungs- und Schmelzverhalten der intermetallischen Phasen relevanten quasi-binären Schnitte der Tb-(Y, Er)-Ni-B-C bzw. Tb(Dy)-Pd-Si-Phasendiagramme, die bisher noch nicht bekannt waren, konnten bestimmt werden. Damit wurden die Prozessparameter der Einkristallzüchtung durch tiegelfreies Zonenschmelzen optimiert. Die unterschiedlichen Erstarrungstypen, peritektische Erstarrung (SENi2B2C) bzw. kongruentes Erstarren (SE2PdSi3), wurden durch unterschiedliche Ziehgeschwindigkeiten bei der Einkristallzüchtung berücksichtigt. An den massiven Einkristallen wurden Homogenitätsuntersuchungen hinsichtlich Elementkonzentration und physikalischer Eigenschaften (Tc, Tn, RRR) als Funktion der Längskoordinate durchgeführt, die geringe Eigenschaftsunterschiede der einkristallinen Proben nachweisen. Aus der Zusammensetzungsänderung über die Kristalllänge konnte in Verbindung mit in-situ Messungen der Zonentemperatur ein einfaches Prozessmodell des Zonenschmelzens der untersuchten Verbindungen entwickelt werden. Die Korrelation von magnetischen und supraleitenden Eigenschaften der Borokarbidmischreihen werden primär vom Verhältnis der Seltenen Erden bestimmt aber auch durch die Konzentration der weiteren Elemente. An Einkristallen konnte eine Anisotropie des oberen kritischen Feldes der Supraleitung von TbxY1-xNi2B2C gezeigt werden, die durch die magnetischen Tb-Ionen bestimmt wird. Die Untersuchungen der Tb2PdSi3- und Dy2PdSi3-Einkristalle ergaben eine Anisotropie der magnetoelektrischen Eigenschaften. / In this thesis investigations of the intermetallic compounds TbxY1-xNi2B2C, TbxEr1-xNi2B2C, Tb2PdSi3 and Dy2PdSi3 are presented. These compounds exhibit interesting physical phenomena caused by the interaction of the rare earth (RE) magnetic moments on the conduction electrons. Moreover, contributions on the constitution of the alloy systems, basic investigations of crystal growth process, homogeneity, microstructure and physical properties (superconductivity, magnetism) of the crystals are given. The quasi-binary sections of the Tb-(Y, Er)-Ni-B-C and Tb(Dy)-Pd-Si-phase diagrams which are relevant for the crystallisation of the different intermetallic phases have been determined for the first time. They were utilised for optimisation of the process parameters of single crystal growth by floating-zone melting. Because of the different solidification modes of RENi2B2C (peritectic solidification) and RE2PdSi3 (congruent solidification) different growth velocities have been employed in crystal growth. The composition and the physical properties (Tc, Tn, RRR) have been investigated as function of the crystal axis co-ordinate. As these properties show only a slight shift over the crystal length samples are representative for the whole crystal. From the slight composition shift over the crystal length a process model of the floating zone growth has been developed utilising the in-situ measurements of the zone-temperature. The correlation of magnetic and superconducting properties of the borocarbide solid solution compounds are mainly governed by the RE-fraction but they are also influenced by the concentration of the other elements. For TbxY1-xNi2B2C single crystals an anisotropic upper critical field of superconductivity has been detected which is induced by the magnetic Tb-ions. The investigations of Tb2PdSi3 and Dy2PdSi3 revealed an anisotropy of the magnetoresistive properties.

The goal of this thesis is to study the microstrucutre and texture of Yttrium-Nickel- Borocarbide and Samarium-Cobalt thin film heterostructures prepared by Pulsed Laser Deposition and to establish structure-property relations for these materials in order to improve their properties and design new structures. Coincidence site lattice epitaxy was explored in most of these heterostructures (substrate, buffer and film) and studied in detail for each case. Epitaxial thin films of the superconducting borocarbide compound YNi2B2C were grown on single crystal MgO (100) substrates without and with Y2O3 buffer layer using pulsed laser deposition (PLD). In both cases YNi2B2C grows with [001] normal to the substrate. However, the in-plane texture depends on the starting condition. For samples without buffer layer, oxygen from the substrate diffuses into the film and forms an Y2O3 reaction layer at the interface. As a consequence, a deficiency of Y is generated giving rise to the formation of secondary phases. On the other hand, using an artificial Y2O3 buffer layer secondary phases are suppressed. The texture of the Y2O3 layers determines the texture of the YNi2B2C film. The superconducting properties of the borocarbide films are discussed with respect to texture and phase purity. To prevent the formation of an impurity phase at the interface, it was the aim of this preliminary investigation to study YNi2B2C films deposited onto single crystal MgO (100) substrates with an Ir buffer layer. The Ir buffer layer shows a strong cube-on-cube texture onto MgO(100) and also prevents the formation of an Y2O3 interlayer. However, during deposition of YNi2B2C the buffer layer disappears by Ir diffusion into the borocarbide film. The YNi2B2C film exhibits a c-axis texture consisting of four components. As a consequence of these effects, the superconducting transition Tc90 equals up to 13K, but with a transition width of 4K. In the second part of this work, hard magnetic Sm-Co/Cr films were epitaxially grown on MgO(100) and (110) substrates. They were characterized by X-ray pole figure measurements and transmission electron microscopy. For films deposited on MgO(100) at 700ºC, orientations are found with the c-axis aligned in-plane and out-of-plane. By lowering the deposition temperature to 370ºC, the out-of-plane orientations disappeared. Further lowering to 350ºC leads to the formation of amorphous regions in the SmCo5 film. For films grown on MgO(110) the Cr buffer deposition temperature plays an important role. When deposited at 700°C Cr(211) and (100) growth is observed leading to two different types of SmCo5 in-plane orientations. By lowering the Cr-buffer deposition temperature to 300ºC only one buffer and one SmCo5 orientation exists: Cr(-211)[0-11] and SmCo5(10-10)[0001]. The exact orientation relationships between substrate, buffer and films are explained and their correlation with magnetic properties are discussed.

Die Dissertation beschreibt Untersuchungen der intermetallischen Verbindungen TbxY1-xNi2B2C bzw. TbxEr1-xNi2B2C, Tb2PdSi3 sowie Dy2PdSi3, die sich auszeichnen durch physikalische Erscheinungen, die durch die Wechselwirkung der magnetischen Momente der Seltenerd (SE)-Ionen mit den Leitungselektronen hervorgerufen werden. Sie umfasst Beiträge zur Konstitution der Legierungssysteme, methodische Untersuchungen zur Einkristallzüchtung, zur Charakterisierung der Homogenität und Perfektion der Kristalle und zu physikalischen Eigenschaften (Supraleitung, Magnetismus). Die für das Erstarrungs- und Schmelzverhalten der intermetallischen Phasen relevanten quasi-binären Schnitte der Tb-(Y, Er)-Ni-B-C bzw. Tb(Dy)-Pd-Si-Phasendiagramme, die bisher noch nicht bekannt waren, konnten bestimmt werden. Damit wurden die Prozessparameter der Einkristallzüchtung durch tiegelfreies Zonenschmelzen optimiert. Die unterschiedlichen Erstarrungstypen, peritektische Erstarrung (SENi2B2C) bzw. kongruentes Erstarren (SE2PdSi3), wurden durch unterschiedliche Ziehgeschwindigkeiten bei der Einkristallzüchtung berücksichtigt. An den massiven Einkristallen wurden Homogenitätsuntersuchungen hinsichtlich Elementkonzentration und physikalischer Eigenschaften (Tc, Tn, RRR) als Funktion der Längskoordinate durchgeführt, die geringe Eigenschaftsunterschiede der einkristallinen Proben nachweisen. Aus der Zusammensetzungsänderung über die Kristalllänge konnte in Verbindung mit in-situ Messungen der Zonentemperatur ein einfaches Prozessmodell des Zonenschmelzens der untersuchten Verbindungen entwickelt werden. Die Korrelation von magnetischen und supraleitenden Eigenschaften der Borokarbidmischreihen werden primär vom Verhältnis der Seltenen Erden bestimmt aber auch durch die Konzentration der weiteren Elemente. An Einkristallen konnte eine Anisotropie des oberen kritischen Feldes der Supraleitung von TbxY1-xNi2B2C gezeigt werden, die durch die magnetischen Tb-Ionen bestimmt wird. Die Untersuchungen der Tb2PdSi3- und Dy2PdSi3-Einkristalle ergaben eine Anisotropie der magnetoelektrischen Eigenschaften. / In this thesis investigations of the intermetallic compounds TbxY1-xNi2B2C, TbxEr1-xNi2B2C, Tb2PdSi3 and Dy2PdSi3 are presented. These compounds exhibit interesting physical phenomena caused by the interaction of the rare earth (RE) magnetic moments on the conduction electrons. Moreover, contributions on the constitution of the alloy systems, basic investigations of crystal growth process, homogeneity, microstructure and physical properties (superconductivity, magnetism) of the crystals are given. The quasi-binary sections of the Tb-(Y, Er)-Ni-B-C and Tb(Dy)-Pd-Si-phase diagrams which are relevant for the crystallisation of the different intermetallic phases have been determined for the first time. They were utilised for optimisation of the process parameters of single crystal growth by floating-zone melting. Because of the different solidification modes of RENi2B2C (peritectic solidification) and RE2PdSi3 (congruent solidification) different growth velocities have been employed in crystal growth. The composition and the physical properties (Tc, Tn, RRR) have been investigated as function of the crystal axis co-ordinate. As these properties show only a slight shift over the crystal length samples are representative for the whole crystal. From the slight composition shift over the crystal length a process model of the floating zone growth has been developed utilising the in-situ measurements of the zone-temperature. The correlation of magnetic and superconducting properties of the borocarbide solid solution compounds are mainly governed by the RE-fraction but they are also influenced by the concentration of the other elements. For TbxY1-xNi2B2C single crystals an anisotropic upper critical field of superconductivity has been detected which is induced by the magnetic Tb-ions. The investigations of Tb2PdSi3 and Dy2PdSi3 revealed an anisotropy of the magnetoresistive properties.

The goal of this thesis is to study the microstrucutre and texture of Yttrium-Nickel- Borocarbide and Samarium-Cobalt thin film heterostructures prepared by Pulsed Laser Deposition and to establish structure-property relations for these materials in order to improve their properties and design new structures. Coincidence site lattice epitaxy was explored in most of these heterostructures (substrate, buffer and film) and studied in detail for each case. Epitaxial thin films of the superconducting borocarbide compound YNi2B2C were grown on single crystal MgO (100) substrates without and with Y2O3 buffer layer using pulsed laser deposition (PLD). In both cases YNi2B2C grows with [001] normal to the substrate. However, the in-plane texture depends on the starting condition. For samples without buffer layer, oxygen from the substrate diffuses into the film and forms an Y2O3 reaction layer at the interface. As a consequence, a deficiency of Y is generated giving rise to the formation of secondary phases. On the other hand, using an artificial Y2O3 buffer layer secondary phases are suppressed. The texture of the Y2O3 layers determines the texture of the YNi2B2C film. The superconducting properties of the borocarbide films are discussed with respect to texture and phase purity. To prevent the formation of an impurity phase at the interface, it was the aim of this preliminary investigation to study YNi2B2C films deposited onto single crystal MgO (100) substrates with an Ir buffer layer. The Ir buffer layer shows a strong cube-on-cube texture onto MgO(100) and also prevents the formation of an Y2O3 interlayer. However, during deposition of YNi2B2C the buffer layer disappears by Ir diffusion into the borocarbide film. The YNi2B2C film exhibits a c-axis texture consisting of four components. As a consequence of these effects, the superconducting transition Tc90 equals up to 13K, but with a transition width of 4K. In the second part of this work, hard magnetic Sm-Co/Cr films were epitaxially grown on MgO(100) and (110) substrates. They were characterized by X-ray pole figure measurements and transmission electron microscopy. For films deposited on MgO(100) at 700ºC, orientations are found with the c-axis aligned in-plane and out-of-plane. By lowering the deposition temperature to 370ºC, the out-of-plane orientations disappeared. Further lowering to 350ºC leads to the formation of amorphous regions in the SmCo5 film. For films grown on MgO(110) the Cr buffer deposition temperature plays an important role. When deposited at 700°C Cr(211) and (100) growth is observed leading to two different types of SmCo5 in-plane orientations. By lowering the Cr-buffer deposition temperature to 300ºC only one buffer and one SmCo5 orientation exists: Cr(-211)[0-11] and SmCo5(10-10)[0001]. The exact orientation relationships between substrate, buffer and films are explained and their correlation with magnetic properties are discussed.

Im Rahmen dieser Doktorarbeit werden de Haas-van Alphen-Untersuchungen an den nichtmagnetischen Borkarbidsupraleitern LuNi2B2C und YNi2B2C präsentiert. Aus den Quantenoszillationen in der normalleitenden Phase in Kombination mit Bandstrukturrechnungen konnten Informationen über die verzweigte Fermiflächenarchitektur und über die Elektron-Phonon-Kopplung der Borkarbide gewonnen werden. Die Kopplung ist stark anisotrop und fermiflächenabhängig. Dies spricht für einen Mehrbandmechanismus der Supraleitung in der Materialklasse. Zusätzlich konnten de Haas-van-Alphen-Oszillationen mehrerer Fermiflächen unterhalb von Bc2 tief in der Shubnikov-Phase beobachtet werden. Das Verhalten dieser Oszillationen lässt sich nicht mit bisher bekannten Theorien beschreiben. Allerdings weist das Bestehen der Oszillationen weit unterhalb von Bc2 auf ein Bestehen von elektronischen Zuständen in der Shubnikov-Phase hin.

Im Rahmen dieser Doktorarbeit werden de Haas-van Alphen-Untersuchungen an den nichtmagnetischen Borkarbidsupraleitern LuNi2B2C und YNi2B2C präsentiert. Aus den Quantenoszillationen in der normalleitenden Phase in Kombination mit Bandstrukturrechnungen konnten Informationen über die verzweigte Fermiflächenarchitektur und über die Elektron-Phonon-Kopplung der Borkarbide gewonnen werden. Die Kopplung ist stark anisotrop und fermiflächenabhängig. Dies spricht für einen Mehrbandmechanismus der Supraleitung in der Materialklasse. Zusätzlich konnten de Haas-van-Alphen-Oszillationen mehrerer Fermiflächen unterhalb von Bc2 tief in der Shubnikov-Phase beobachtet werden. Das Verhalten dieser Oszillationen lässt sich nicht mit bisher bekannten Theorien beschreiben. Allerdings weist das Bestehen der Oszillationen weit unterhalb von Bc2 auf ein Bestehen von elektronischen Zuständen in der Shubnikov-Phase hin.

In dieser Arbeit werden niederfrequente optische Messungen an vier neuartigen Supraleitern vorgestellt. Im Bereich von 100 GHz bis 3 THz zeigen die vier untersuchten Systeme – LuNi2B2C, Ba(Fe0,9Co0,1)2As2, T’-Pr2CuO4 und Si:Ga – ein sehr unterschiedliches Verhalten. Die beiden erst genannten Supraleiter sind Mehrbandsupraleiter, bei denen die Cooper-Paarkopplung unterschiedlich für verschiedene Fermiflächen ist. T’-Pr2CuO4 ist ein undotierter Kupratsupraleiter, der nach bisheriger Lehrmeinung nicht existieren dürfte. Mit THz-Spektroskopie konnte erstmals die Bildung einer Meißner-Phase in T’-Pr2CuO4 mit optischen Methoden beobachtet werden. Eine gewisse Sonderstellung nimmt Si:Ga als amorpher Supraleiter ein. Si:Ga wird durch Ionenimplantation von Gallium in einen Siliziumwafer hergestellt. Es besteht die Hoffnung, mit Si:Ga halb- und supraleitende Logikblöcke in großem Maßstab auf einem Chip vereinen zu können, da die Ionenimplantation mit den Produktionsprozessen der Halbleiterindustrie kompatibel ist.

The present work reports on thermal low-temperature properties of rare earth transition metal borocarbides such as specific heat, thermal conductivity and thermopower. The influence of structural disorder, caused by stoichiometric variations and substitutions of the rare earth element or the transition metal, on the thermal and superconducting low-temperature properties is investigated. The structural disorder results in the reduction of the superconducting transition temperature Tc, of the Sommerfeld value gamma, of the upper critical magnetic field Hc2(0), of the negative curvature of the H-dependence of the T-linear specific heat contribution gamma(H), and in the reduction of the positive curvature of Hc2(T) near Tc. But isoelectronic rare earth substitutions do not result in the transition from clean to dirty limit. Due to Pt-substitutions similar reductions of thermal and superconducting properties are observed. The behaviour of Hc2(0) and the concentration dependence of the positive curvature of Hc2(T) near Tc point to the transition from clean to quasi-dirty limit in the case of Pt-substitutions. / In der vorliegenden Arbeit werden Untersuchungen zu thermischen Tieftemperatureigenschaften, wie der spezifischen Wärmekapazität, der Wärmeleitfähigkeit und der Thermokraft, an supraleitenden Seltenerd-Übergangsmetall-Borkarbiden vorgestellt. Es wurde der Einfluß von gezielt hervorgerufener Unordnung im kristallographischen Aufbau, die durch isoelektronische Substitutionen des Seltenerd-Elements und des Übergangsmetalls sowie durch Söchiometrievariationen erzeugt wurde, auf die thermischen und supraleitenden Tieftemperatureigenschaften untersucht. Folge der strukturellen Unordnung ist eine Reduzierung der charakteristischen Eigenschaften, wie der Sprungtemperatur der Supraleitung Tc, der Sommerfeldkonstanten gamma, des oberen kritischen Magnetfelds Hc2(0), der negativen Krümmung in der Feldabhängigkeit des T-linearen Beitrags zur spezifischen Wärme gamma(H) sowie eine Reduzierung der positiven Krümmung in der Temperaturabhängigkeit von Hc2(T). Isoelektronische Substitutionen auf dem Seltenerd-Platz führen aber nicht zum Erreichen des dirty limit. Eine Reduzierung der relevanten supraleitenden und thermischen Eigenschaften durch Pt-Beimengungen wird ähnlich wie im Falle der Lu-Substitution festgestellt. Die Konzentrationsabhängigkeit von Hc2(0) sowie die Krümmung von Hc2(T) weisen hier auf einen Übergang vom clean limit zum quasi-dirty limit durch die Pt-Substitution hin.