The preparation of layered magnets needs the knowledge of growth techniques which are focused on the growth of Fe/V(001) superlattices. Such films have been structurally investigated by X-rays reflectivity and diffraction. The magnetic investigations have been carried out by magneto-optic Kerr effect (MOKE), Superconducting Quantum Interference Device (SQUID) magnetometry and polarized neutron reflectivity (PNR). This latter technique has been used in cooperation with the Institute Laue Langvin (Grenoble, France) and Ruhr Universität (Bochum, Germany). The cross-over in universality class is shown in a series of layered magnets where a δ-doping layer of Fe has been embedded between two layers of Pd showing that the magnetization depends on the effective magnetic thickness of the polarized Pd. A model for the cross-over has been developed in terms of magnetic excitations. The interlayer exchange coupling (IEC) mediated by a non-magnetic spacer has been reviewed focusing the attention on the recent theoretical and experimental works based on Fe/V(001) superlattices. The IEC can be tailored at will by reversibly alloying of the spacer with H: this has been proved in Fe/V(001) double layers showing that in the two dimensional limit, the universality class is not affected by the coupling. The magnetic order-disorder transitions in Fe/V(001) superlattices do not seem to belong to any universality class. A phenomenological model which accounts for the effective coupling at the boundaries has been developed. The influence of the inherent ordering temperatures of single magnetic layers has been investigated in Fe/V(001) superlattices proving that the weakest ferromagnetic layer affects the overall magnetic ordering. A new kind of layered magnet has been developed to increase the effect of the boundaries. PNR measurements show that the universality class depends on which length-scale is investigated.

No description available.

A dependência com a temperatura dos tempos de relaxação nuclear e da forma da linha de ressonância dos núcleos F0, nos antiferromagnetos diluidos Fex Zn1-x F2 e Mnx Zn1-x F2, foram estudados na faixa TN&#8804;T&#8804;300K. Os tempos de relaxação spin, rede (1/T1) dos núcleos F0, os quais não estão acoplados aos spins Fe (ou Mn) através da interação hiperfina transferida, foram medidos e calculados teoricamente em função da concentração x. Os resultados teóricos apresentam uma boa concordância com os dados experimentais para concentrações na faixa 0.1&#8804;x&#8804;0.8. A dependência com a temperatura de 1/T1, para TN<T&#8804;300K foi entendido qualitativamente em ambos sistemas. Os resultados obtidos para 1/T1, em temperaturas próximas de TN foram utilizados para estudar os \"Efeitos de Campo Aleatório\" no comportamento crítico do Mn.65Zn.35 F2 com o campo aplicado paralelamente e perperdicularmente ao eixo fácil (C). Encontrou-se neste caso, que a temperatura de transição TN decresce substancialmente com o aumento da intensidade do campo somente quando H0 || C. Os resultados experimentais obtidos estão de acordo com as teorias do Efeito de Campos Aleatórios em antiferromagnetos anisotrópicos diluídos. A divergência crítica do segundo momento da linha de ressonância não homogeneamente alargada do F0 foi estudada acima de TN. Os resultados experimentais concordam com os cálculos de Heller para o alargamento não homogêneo causado por Efeitos de Campos Aleatórios. Observou-se que a forma da linha se altera na região crítica. Longe de TN ela é Gaussiana e para t&#8804;10-2 ela mostra uma tendência a se tornar Lorentziana. Abaixo de TN a sua meia largura aumenta, seguindo qualitativamente o aumento da magnetização das sub-redes. / The temperature dependence of the nuclear relaxation rates and line shapes of the F0 resonance in the diluted antiferromagnet Fex Zn1-x F2 and Mnx Zn1-x F2 are studied over a large temperature range TN&#8804;T&#8804;300K. The high (room) temperature spinlattice relaxation rates (1/ T1) of the F0 nuclei, which are not transfer hiperfine coupled to the Fe (or Mn) spins, have been measured and calculated as a function of the concentration x. Good agreement with experiment is found for the theoretical results, which have been obtained in the range 0.1&#8804;x&#8804;0.8. The temperature dependence of TN<T&#8804;300K was qualitatively understood \"Random Field Effects\" on the critical behavior of Mn.65Zn.35 F2, for fields applied parallel and perpendicular to the easy (C) axis. It was found that the transition temperature TN depressed substantially with field only for H0 || C. The experimental results are in general accord with the theory for Random Field Effects in desordered, anisotropic antiferromagnets. The critical divergence of the in homogeneously broadened F0 NMR was studied in Fe.6Zn.4F2 above TN. The experimental results agree with Heller\'s calculation of the NMR line broadening by Random Field Effects. With H0 || C the line shape changes from Gaussian towards Lorentzian for t&#8804;10-2 and below TN its line width increase qualitatively following increase in the sublattice magnetization.

A dependência com a temperatura dos tempos de relaxação nuclear e da forma da linha de ressonância dos núcleos F0, nos antiferromagnetos diluidos Fex Zn1-x F2 e Mnx Zn1-x F2, foram estudados na faixa TN&#8804;T&#8804;300K. Os tempos de relaxação spin, rede (1/T1) dos núcleos F0, os quais não estão acoplados aos spins Fe (ou Mn) através da interação hiperfina transferida, foram medidos e calculados teoricamente em função da concentração x. Os resultados teóricos apresentam uma boa concordância com os dados experimentais para concentrações na faixa 0.1&#8804;x&#8804;0.8. A dependência com a temperatura de 1/T1, para TN<T&#8804;300K foi entendido qualitativamente em ambos sistemas. Os resultados obtidos para 1/T1, em temperaturas próximas de TN foram utilizados para estudar os \"Efeitos de Campo Aleatório\" no comportamento crítico do Mn.65Zn.35 F2 com o campo aplicado paralelamente e perperdicularmente ao eixo fácil (C). Encontrou-se neste caso, que a temperatura de transição TN decresce substancialmente com o aumento da intensidade do campo somente quando H0 || C. Os resultados experimentais obtidos estão de acordo com as teorias do Efeito de Campos Aleatórios em antiferromagnetos anisotrópicos diluídos. A divergência crítica do segundo momento da linha de ressonância não homogeneamente alargada do F0 foi estudada acima de TN. Os resultados experimentais concordam com os cálculos de Heller para o alargamento não homogêneo causado por Efeitos de Campos Aleatórios. Observou-se que a forma da linha se altera na região crítica. Longe de TN ela é Gaussiana e para t&#8804;10-2 ela mostra uma tendência a se tornar Lorentziana. Abaixo de TN a sua meia largura aumenta, seguindo qualitativamente o aumento da magnetização das sub-redes. / The temperature dependence of the nuclear relaxation rates and line shapes of the F0 resonance in the diluted antiferromagnet Fex Zn1-x F2 and Mnx Zn1-x F2 are studied over a large temperature range TN&#8804;T&#8804;300K. The high (room) temperature spinlattice relaxation rates (1/ T1) of the F0 nuclei, which are not transfer hiperfine coupled to the Fe (or Mn) spins, have been measured and calculated as a function of the concentration x. Good agreement with experiment is found for the theoretical results, which have been obtained in the range 0.1&#8804;x&#8804;0.8. The temperature dependence of TN<T&#8804;300K was qualitatively understood \"Random Field Effects\" on the critical behavior of Mn.65Zn.35 F2, for fields applied parallel and perpendicular to the easy (C) axis. It was found that the transition temperature TN depressed substantially with field only for H0 || C. The experimental results are in general accord with the theory for Random Field Effects in desordered, anisotropic antiferromagnets. The critical divergence of the in homogeneously broadened F0 NMR was studied in Fe.6Zn.4F2 above TN. The experimental results agree with Heller\'s calculation of the NMR line broadening by Random Field Effects. With H0 || C the line shape changes from Gaussian towards Lorentzian for t&#8804;10-2 and below TN its line width increase qualitatively following increase in the sublattice magnetization.

Within the last decades low-dimensional frustrated quantum spin systems have attracted great interest in the field of modern research. In these systems a competition of various magnetic interactions takes place, leading to an energetically degenerated magnetic ground state, and thus to the occurrence of exotic, unconventional physical properties at low temperatures. This thesis focuses on the quasi one-dimensional frustrated spin chain system linarite, PbCuSO4(OH)2. In this compound the basic building blocks are CuO4 plaquettes which are connected to each other along one crystallographic direction, analogue to a chain. The frustration in linarite is established due to the competition between the magnetic interactions. The nearest-neighbor magnetic spins are coupled ferromagnetically along the chain via a coupling constant J1, while the next-nearest neighbors are coupled antiferromagnetically via a coupling constant J2. For this configuration it is not possible to satisfy all magnetic couplings simultaneously, hence the system is magnetically frustrated. In this work, comprehensive thermodynamic and nuclear magnetic resonance (NMR) studies demonstrate that linarite is one of the richest and most fascinating compounds in the class of low-dimensional frustrated magnets. By means of susceptibility, magnetization, specific heat, magnetocaloric effect, magnetostriction, and thermal-expansion measurements a rich magnetic phase diagram could be mapped out below a temperature of 2.8 K. The phase diagram contains five different magnetic regions/phases for an external magnetic field pointing along the chain direction. Based on the thermodynamic studies it was possible to calculate the exchange integrals within the frustrated J1-J2 model and extensions of it by using various theoretical approaches. The magnetic microscopic nature of the different long-range magnetic phases present in linarite were investigated by NMR measurements and by collaborative neutron scattering experiments. The ground state (phase I) is identified as an incommensurate elliptical helical structure. Via a theoretical modelling the 1H-NMR spectrum of the ground state could be explained, revealing a rearrangement of the zero-field structure in an external magnetic field of 2.0 T used for the NMR studies. By further increasing the external field the system undergoes a complex spin flop transition in two steps (phase I - phase III - phase IV). In phase III a phase separation takes place where one part of the spins form a circular spiral structure while the remaining fraction form a simple antiferromagnetic structure. In phase IV the remaining circular spiral structure vanishes, so that all spins collectively form the antiferromagnetic collinear phase. The most peculiar physical properties studied in this thesis take place in region V at high fields, showing only tiny features in the thermodynamic properties. The magnetic spins in region V form a sine-wave modulated spin-density structure as identified via NMR and neutron investigations. It is discussed whether region V is related to a multipolar phase or if the spin-density wave structure could possibly coexist with such a phase.

Within the last decades low-dimensional frustrated quantum spin systems have attracted great interest in the field of modern research. In these systems a competition of various magnetic interactions takes place, leading to an energetically degenerated magnetic ground state, and thus to the occurrence of exotic, unconventional physical properties at low temperatures. This thesis focuses on the quasi one-dimensional frustrated spin chain system linarite, PbCuSO4(OH)2. In this compound the basic building blocks are CuO4 plaquettes which are connected to each other along one crystallographic direction, analogue to a chain. The frustration in linarite is established due to the competition between the magnetic interactions. The nearest-neighbor magnetic spins are coupled ferromagnetically along the chain via a coupling constant J1, while the next-nearest neighbors are coupled antiferromagnetically via a coupling constant J2. For this configuration it is not possible to satisfy all magnetic couplings simultaneously, hence the system is magnetically frustrated. In this work, comprehensive thermodynamic and nuclear magnetic resonance (NMR) studies demonstrate that linarite is one of the richest and most fascinating compounds in the class of low-dimensional frustrated magnets. By means of susceptibility, magnetization, specific heat, magnetocaloric effect, magnetostriction, and thermal-expansion measurements a rich magnetic phase diagram could be mapped out below a temperature of 2.8 K. The phase diagram contains five different magnetic regions/phases for an external magnetic field pointing along the chain direction. Based on the thermodynamic studies it was possible to calculate the exchange integrals within the frustrated J1-J2 model and extensions of it by using various theoretical approaches. The magnetic microscopic nature of the different long-range magnetic phases present in linarite were investigated by NMR measurements and by collaborative neutron scattering experiments. The ground state (phase I) is identified as an incommensurate elliptical helical structure. Via a theoretical modelling the 1H-NMR spectrum of the ground state could be explained, revealing a rearrangement of the zero-field structure in an external magnetic field of 2.0 T used for the NMR studies. By further increasing the external field the system undergoes a complex spin flop transition in two steps (phase I - phase III - phase IV). In phase III a phase separation takes place where one part of the spins form a circular spiral structure while the remaining fraction form a simple antiferromagnetic structure. In phase IV the remaining circular spiral structure vanishes, so that all spins collectively form the antiferromagnetic collinear phase. The most peculiar physical properties studied in this thesis take place in region V at high fields, showing only tiny features in the thermodynamic properties. The magnetic spins in region V form a sine-wave modulated spin-density structure as identified via NMR and neutron investigations. It is discussed whether region V is related to a multipolar phase or if the spin-density wave structure could possibly coexist with such a phase.

Rare-earth nickel borocarbids RNi2B2C are particularly suitable for investigations on one of the most interesting problems in modern solid-state physics: these compounds display competition and coexistence of superconductivity and magnetism. Depending on the R3+ ion, the transition temperatures are in an experimentally easy accessible range of 1 K to 25 K. This thesis presents experimental studies on the interplay of both ordering phenomena. Neutron diffraction is used to determine the magnetic order and the resulting changes of the crystal structure. Experiments are performed on polycrystalline and single crystal samples in dependence on temperature and external magnetic fields. The Ni-B stoichiometry of the tetragonal RNi2B2C compounds is systematically varied and the magnetic R3+ ions are partially substituted by other magnetic or nonmagnetic R?3+ ions. The experimental results are compared with macroscopic magnetic and electrical properties. For HoNi2B2C three different magnetic structures are found in a narrow temperature range. While for two magnetic structures the Ho3+ moments are modulated along the c axis, a third magnetic structure with a modulation in a direction is observed. Both, partial substitution of Ho3+ ions and variation of the Ni-B stoichiometry, strongly modify the formation of these different types of magnetic order. The comparison with the concomitant changes of the superconducting properties yields the following scenario for HoNi2B2C-based compounds: superconductivity coexists with both magnetic structures with modulations in c direction. However, the onset of magnetic order weakens the superconductivity. For the magnetic structure with modulation along the a axis, components of the magnetic moments arise in c direction. The resulting local magnetic fields on Ni sites yield a strong suppression of the superconductivity. The observed competition between superconductivity and the magnetic structure with modulation along the a axis strongly suggests that the modification of the electronic structure due to the superconducting state influences the magnetic ordering. As a further impact of the magnetism in RNi2B2C compounds with R = Ho, Dy, Tb and Er changes of the crystal structure are investigated. Using high-resolution neutron diffraction, tetragonal-to-orthorhombic lattice distortions are found. They are induced by those magnetic structures with either parallel or anti-parallel alignement of R3+ magnetic moments. The direction of the lattice distortions, the dependence of their size on the square of the effective ordered magnetic moment and on the type of the R3+ ions indicate that the magneto-elastic interactions are determined by crystal-field effects. This fact also facilitates the elucidation of the magnetic phase diagrams by neutron diffraction experiments in external magnetic fields. For a given phase, absence or presence of magneto-elastic lattice distortions restrict the set of possible magnetic structures. For HoNi2B2C the magnetic phases reported in literature are confirmed. The experimental results for DyNi2B2C are interpreted using a simple model to determine the magnetic structures. Based on mean field calculations, the differences in the magnetic structures for increasing and decreasing magnetic fields can be understood as very strong hysteresis effects in connection with first-order phase transitions. / Seltenerd-Nickel-Borkarbid-Verbindungen RNi2B2C sind bestens zur Untersuchung eines der interessantesten Probleme der modernen Festkörperphysik geeignet: Diese Substanzen weisen Konkurrenz und Koexistenz von Supraleitung und Magnetismus auf, wobei die vom R3+-Ion abhängigen Übergangstemperaturen in einem experimentell gut zugänglichen Bereich von 1 K bis 25 K liegen. Die vorliegende Dissertation stellt experimentelle Arbeiten zum Wechselspiel der beiden Ordnungsphänomene vor. Für poly- und einkristalline Proben werden die magnetischen Ordnungen und resultierende Veränderungen der Kristallstruktur mittels Neutronendiffraktion in Abhängigkeit von der Temperatur und vom äußeren Magnetfeld bestimmt und mit den makroskopischen magnetischen und elektrischen Eigenschaften verglichen. Hierbei werden die tetragonalen RNi2B2C-Verbindungen gezielt in ihrer Ni-B-Stöchiometrie variiert sowie die magnetischen R3+-Ionen partiell durch andere magnetische als auch unmagnetische R?3+-Ionen substituiert. Für HoNi2B2C werden in einem engen Temperaturbereich drei verschiedene magnetische Strukturen nachgewiesen. Während in zwei magnetischen Ordnungen die Ho3+-Momente entlang der c-Achse moduliert sind, wird für die dritte magnetische Ordnung eine Modulation in a-Richtung beobachtet. Sowohl durch die partielle Substitution der Ho3+-Ionen als auch durch die Ni-B-Stöchiometrievariation wird die Ausprägung der magnetischen Strukturen stark modifiziert. Der Vergleich mit den ebenfalls veränderten supraleitenden Eigenschaften ergibt das folgende Bild für die HoNi2B2C-Verbindungen: Die Supraleitung koexistiert mit den beiden c-Achsen-modulierten magnetischen Strukturen, das Einsetzen der magnetischen Ordnung führt jedoch zu einer Schwächung der Supraleitung. Die a-Achsen-modulierte magnetische Struktur weist Momentkomponenten in c-Richtung auf, die auf Grund der resultierenden lokalen Magnetfelder an den Ni-Plätzen eine starke Unterdrückung der Supraleitung bewirken. Die beobachtete Konkurrenz zwischen der Supraleitung und der a-Achsen-modulierten magnetischen Struktur gibt andererseits einen starken Hinweis darauf, daß die Modifizierung der elektronischen Struktur im supraleitenden Zustand auf das magnetische System rückwirkt. Als weitere Auswirkung des Magnetismus kommt es in RNi2B2C-Verbindungen mit R = Ho, Dy, Tb und Er zu Veränderungen der Kristallstruktur. Mittels hochauflösender Neutronendiffraktion werden magnetisch induzierte, tetragonal-zu-orthorhombische Gitterverzerrungen für diejenigen magnetischen Ordnungen nachgewiesen, bei denen die magnetischen Momente der R3+-Ionen parallel bzw. antiparallel ausgerichtet sind. Die Richtung der Gitterverzerrung, die Abhängigkeit ihrer Größe vom Quadrat des geordneten magnetischen Momentes als auch von der Art der R3+-Ionen deuten darauf hin, daß die magneto-elastischen Wechselwirkungen durch Kristallfeldeffekte bestimmt werden. Diese Einsicht unterstützt auch die Aufklärung der magnetischen Phasendiagramme mittels magnetfeldabhängiger Neutronenbeugungsexperimente. Für eine magnetische Phase schränkt das Auftreten bzw. Fehlen der magneto-elastischen Effekte die Vielfalt der möglichen magnetischen Strukturen ein. Die aus der Literatur bekannten magnetischen Phasen von HoNi2B2C werden bestätigt. Für DyNi2B2C werden die experimentellen Ergebnisse unter Nutzung eines einfachen Modelles interpretiert und die magnetischen Strukturen bestimmt. Anhand von Molekularfeldrechnungen können die Unterschiede in den magnetischen Strukturen für ansteigendes und für abnehmendes Magnetfeld als sehr starke Hystereseeffekte in Zusammenhang mit Phasenübergängen erster Ordnung gedeutet werden.

Die intermetallischen Verbindungen der RCu2-Serie (R = Seltenerd-Elemente) zeigen eine ungewöhnliche Vielfalt von magnetischen Strukturen in Abhängigkeit von Temperatur und äußerem Magnetfeld. Diese Vielfalt ist verursacht durch das Wechselspiel von indirekter Austauschwechselwirkung und anisotropem kristallelektrischen Feld. Die RCu2-Verbindungen kristallisieren in der CeCu2-Struktur, welche als orthorhombische Verzerrung der hexagonalen AlB2-Struktur verstanden werden kann. Ziel der vorliegenden Arbeit ist es, RCu2-Verbindungen mit magnetischer Röntgenbeugung bzw. resonanter magnetischer Röntgenstreuung zu untersuchen, teilweise ergänzt durch Neutronenbeugungsexperimente. Dem zur Neutronenbeugung komplementären Charakter der Röntgenmethoden entspricht es, dass dabei spezielle Details der magnetischen Strukturen untersucht werden. Zusätzlich zur Untersuchung der magnetischen Eigenschaften und Strukturen und zur Suche nach den Ursachen für die auftretenden Phasenübergänge werden magnetoelastische Kopplungen in der pseudohexagonalen Substanzgruppe betrachtet (R = Nd, Gd, Tb, Dy). Der Zusammenhang von kristalliner und magnetischer Struktur wird auch unter Variation und Mischung der Elemente auf dem Seltenerd-Platz erforscht (Tb(1-x)DyxCu2, Tb(1-x)PrxCu2, Dy(1-x)YxCu2). Für die Untersuchung des elementspezifischen Magnetismus bei Vorhandensein mehrerer magnetischer Atomsorten in einem Kristall ist dabei die resonante magnetische Röntgenstreuung die einzig verfügbare Methode. Im Tb0.5Dy0.5Cu2 wird dabei ein unerwartetes Verhalten im Temperaturverlauf der magnetischen Strukturen beobachtet. Ergebnisse aus ergänzenden Neutronenbeugungsexperimenten werden ebenfalls vorgestellt und mittels Rietveld-Verfeinerung der kristallinen und magnetischen Strukturen ausgewertet. Im Ergebnis der Arbeit sind die untersuchten magnetischen Strukturen näher bekannt. Magnetoelastische Wechselwirkungen werden unter Verwendung von Beugungsmethoden neuartig gemessen. / The intermetallic compounds of the RCu2 series (R = rare earths) show a large variety of magnetic structures depending on temperature and external field, mainly caused by the interplay of an indirect exchange interaction and the anisotropy of the crystalline electric field. The RCu2 compounds crystallize in the CeCu2 structure, which can be described as an orthorhombic distortion of the hexagonal AlB2 structure. The aim of the present work is the investigation of RCu2 compounds by using resonant and nonresonant magnetic x-ray scattering, supplemented by neutron scattering. Because of the complementarity of magnetic x-ray and neutron scattering this investigation reveals new details of the magnetic structures of the studied compounds. Magnetic properties and magnetic structures are investigated as well as magnetic phase transitions and magneto-elastic coupling in the pseudohexagonal compounds. The correlation between the crystallographic structure and the magnetic structures is studied for different rare earths (R = Nd, Gd, Tb, Dy) but also for partial substitution of magnetic rare earths by magnetic or nonmagnetic ions on the rare earth site (Tb(1-x)DyxCu2, Tb(1-x)PrxCu2, Dy(1-x)YxCu2). Resonant magnetic x-ray scattering is the only method available to investigate the element specific magnetism in crystals with different magnetic ions. By the study of the Tb resonance and the Dy resonance on Tb0.5Dy0.5Cu2 an unexpected developement of the magnetic structures with temperature is observed. The results of supplementary neutron scattering experiments are presented and analyzed by Rietveld refinement of the crystallographic and magnetic structures, mainly focussed on TbCu2. In summary, new insights into the different magnetic structures of RCu2 compounds are gained. The strong magneto-elastic coupling is studied by different scattering methods applied to this problem for the first time.

Rare-earth nickel borocarbids RNi2B2C are particularly suitable for investigations on one of the most interesting problems in modern solid-state physics: these compounds display competition and coexistence of superconductivity and magnetism. Depending on the R3+ ion, the transition temperatures are in an experimentally easy accessible range of 1 K to 25 K. This thesis presents experimental studies on the interplay of both ordering phenomena. Neutron diffraction is used to determine the magnetic order and the resulting changes of the crystal structure. Experiments are performed on polycrystalline and single crystal samples in dependence on temperature and external magnetic fields. The Ni-B stoichiometry of the tetragonal RNi2B2C compounds is systematically varied and the magnetic R3+ ions are partially substituted by other magnetic or nonmagnetic R?3+ ions. The experimental results are compared with macroscopic magnetic and electrical properties. For HoNi2B2C three different magnetic structures are found in a narrow temperature range. While for two magnetic structures the Ho3+ moments are modulated along the c axis, a third magnetic structure with a modulation in a direction is observed. Both, partial substitution of Ho3+ ions and variation of the Ni-B stoichiometry, strongly modify the formation of these different types of magnetic order. The comparison with the concomitant changes of the superconducting properties yields the following scenario for HoNi2B2C-based compounds: superconductivity coexists with both magnetic structures with modulations in c direction. However, the onset of magnetic order weakens the superconductivity. For the magnetic structure with modulation along the a axis, components of the magnetic moments arise in c direction. The resulting local magnetic fields on Ni sites yield a strong suppression of the superconductivity. The observed competition between superconductivity and the magnetic structure with modulation along the a axis strongly suggests that the modification of the electronic structure due to the superconducting state influences the magnetic ordering. As a further impact of the magnetism in RNi2B2C compounds with R = Ho, Dy, Tb and Er changes of the crystal structure are investigated. Using high-resolution neutron diffraction, tetragonal-to-orthorhombic lattice distortions are found. They are induced by those magnetic structures with either parallel or anti-parallel alignement of R3+ magnetic moments. The direction of the lattice distortions, the dependence of their size on the square of the effective ordered magnetic moment and on the type of the R3+ ions indicate that the magneto-elastic interactions are determined by crystal-field effects. This fact also facilitates the elucidation of the magnetic phase diagrams by neutron diffraction experiments in external magnetic fields. For a given phase, absence or presence of magneto-elastic lattice distortions restrict the set of possible magnetic structures. For HoNi2B2C the magnetic phases reported in literature are confirmed. The experimental results for DyNi2B2C are interpreted using a simple model to determine the magnetic structures. Based on mean field calculations, the differences in the magnetic structures for increasing and decreasing magnetic fields can be understood as very strong hysteresis effects in connection with first-order phase transitions. / Seltenerd-Nickel-Borkarbid-Verbindungen RNi2B2C sind bestens zur Untersuchung eines der interessantesten Probleme der modernen Festkörperphysik geeignet: Diese Substanzen weisen Konkurrenz und Koexistenz von Supraleitung und Magnetismus auf, wobei die vom R3+-Ion abhängigen Übergangstemperaturen in einem experimentell gut zugänglichen Bereich von 1 K bis 25 K liegen. Die vorliegende Dissertation stellt experimentelle Arbeiten zum Wechselspiel der beiden Ordnungsphänomene vor. Für poly- und einkristalline Proben werden die magnetischen Ordnungen und resultierende Veränderungen der Kristallstruktur mittels Neutronendiffraktion in Abhängigkeit von der Temperatur und vom äußeren Magnetfeld bestimmt und mit den makroskopischen magnetischen und elektrischen Eigenschaften verglichen. Hierbei werden die tetragonalen RNi2B2C-Verbindungen gezielt in ihrer Ni-B-Stöchiometrie variiert sowie die magnetischen R3+-Ionen partiell durch andere magnetische als auch unmagnetische R?3+-Ionen substituiert. Für HoNi2B2C werden in einem engen Temperaturbereich drei verschiedene magnetische Strukturen nachgewiesen. Während in zwei magnetischen Ordnungen die Ho3+-Momente entlang der c-Achse moduliert sind, wird für die dritte magnetische Ordnung eine Modulation in a-Richtung beobachtet. Sowohl durch die partielle Substitution der Ho3+-Ionen als auch durch die Ni-B-Stöchiometrievariation wird die Ausprägung der magnetischen Strukturen stark modifiziert. Der Vergleich mit den ebenfalls veränderten supraleitenden Eigenschaften ergibt das folgende Bild für die HoNi2B2C-Verbindungen: Die Supraleitung koexistiert mit den beiden c-Achsen-modulierten magnetischen Strukturen, das Einsetzen der magnetischen Ordnung führt jedoch zu einer Schwächung der Supraleitung. Die a-Achsen-modulierte magnetische Struktur weist Momentkomponenten in c-Richtung auf, die auf Grund der resultierenden lokalen Magnetfelder an den Ni-Plätzen eine starke Unterdrückung der Supraleitung bewirken. Die beobachtete Konkurrenz zwischen der Supraleitung und der a-Achsen-modulierten magnetischen Struktur gibt andererseits einen starken Hinweis darauf, daß die Modifizierung der elektronischen Struktur im supraleitenden Zustand auf das magnetische System rückwirkt. Als weitere Auswirkung des Magnetismus kommt es in RNi2B2C-Verbindungen mit R = Ho, Dy, Tb und Er zu Veränderungen der Kristallstruktur. Mittels hochauflösender Neutronendiffraktion werden magnetisch induzierte, tetragonal-zu-orthorhombische Gitterverzerrungen für diejenigen magnetischen Ordnungen nachgewiesen, bei denen die magnetischen Momente der R3+-Ionen parallel bzw. antiparallel ausgerichtet sind. Die Richtung der Gitterverzerrung, die Abhängigkeit ihrer Größe vom Quadrat des geordneten magnetischen Momentes als auch von der Art der R3+-Ionen deuten darauf hin, daß die magneto-elastischen Wechselwirkungen durch Kristallfeldeffekte bestimmt werden. Diese Einsicht unterstützt auch die Aufklärung der magnetischen Phasendiagramme mittels magnetfeldabhängiger Neutronenbeugungsexperimente. Für eine magnetische Phase schränkt das Auftreten bzw. Fehlen der magneto-elastischen Effekte die Vielfalt der möglichen magnetischen Strukturen ein. Die aus der Literatur bekannten magnetischen Phasen von HoNi2B2C werden bestätigt. Für DyNi2B2C werden die experimentellen Ergebnisse unter Nutzung eines einfachen Modelles interpretiert und die magnetischen Strukturen bestimmt. Anhand von Molekularfeldrechnungen können die Unterschiede in den magnetischen Strukturen für ansteigendes und für abnehmendes Magnetfeld als sehr starke Hystereseeffekte in Zusammenhang mit Phasenübergängen erster Ordnung gedeutet werden.

Die intermetallischen Verbindungen der RCu2-Serie (R = Seltenerd-Elemente) zeigen eine ungewöhnliche Vielfalt von magnetischen Strukturen in Abhängigkeit von Temperatur und äußerem Magnetfeld. Diese Vielfalt ist verursacht durch das Wechselspiel von indirekter Austauschwechselwirkung und anisotropem kristallelektrischen Feld. Die RCu2-Verbindungen kristallisieren in der CeCu2-Struktur, welche als orthorhombische Verzerrung der hexagonalen AlB2-Struktur verstanden werden kann. Ziel der vorliegenden Arbeit ist es, RCu2-Verbindungen mit magnetischer Röntgenbeugung bzw. resonanter magnetischer Röntgenstreuung zu untersuchen, teilweise ergänzt durch Neutronenbeugungsexperimente. Dem zur Neutronenbeugung komplementären Charakter der Röntgenmethoden entspricht es, dass dabei spezielle Details der magnetischen Strukturen untersucht werden. Zusätzlich zur Untersuchung der magnetischen Eigenschaften und Strukturen und zur Suche nach den Ursachen für die auftretenden Phasenübergänge werden magnetoelastische Kopplungen in der pseudohexagonalen Substanzgruppe betrachtet (R = Nd, Gd, Tb, Dy). Der Zusammenhang von kristalliner und magnetischer Struktur wird auch unter Variation und Mischung der Elemente auf dem Seltenerd-Platz erforscht (Tb(1-x)DyxCu2, Tb(1-x)PrxCu2, Dy(1-x)YxCu2). Für die Untersuchung des elementspezifischen Magnetismus bei Vorhandensein mehrerer magnetischer Atomsorten in einem Kristall ist dabei die resonante magnetische Röntgenstreuung die einzig verfügbare Methode. Im Tb0.5Dy0.5Cu2 wird dabei ein unerwartetes Verhalten im Temperaturverlauf der magnetischen Strukturen beobachtet. Ergebnisse aus ergänzenden Neutronenbeugungsexperimenten werden ebenfalls vorgestellt und mittels Rietveld-Verfeinerung der kristallinen und magnetischen Strukturen ausgewertet. Im Ergebnis der Arbeit sind die untersuchten magnetischen Strukturen näher bekannt. Magnetoelastische Wechselwirkungen werden unter Verwendung von Beugungsmethoden neuartig gemessen. / The intermetallic compounds of the RCu2 series (R = rare earths) show a large variety of magnetic structures depending on temperature and external field, mainly caused by the interplay of an indirect exchange interaction and the anisotropy of the crystalline electric field. The RCu2 compounds crystallize in the CeCu2 structure, which can be described as an orthorhombic distortion of the hexagonal AlB2 structure. The aim of the present work is the investigation of RCu2 compounds by using resonant and nonresonant magnetic x-ray scattering, supplemented by neutron scattering. Because of the complementarity of magnetic x-ray and neutron scattering this investigation reveals new details of the magnetic structures of the studied compounds. Magnetic properties and magnetic structures are investigated as well as magnetic phase transitions and magneto-elastic coupling in the pseudohexagonal compounds. The correlation between the crystallographic structure and the magnetic structures is studied for different rare earths (R = Nd, Gd, Tb, Dy) but also for partial substitution of magnetic rare earths by magnetic or nonmagnetic ions on the rare earth site (Tb(1-x)DyxCu2, Tb(1-x)PrxCu2, Dy(1-x)YxCu2). Resonant magnetic x-ray scattering is the only method available to investigate the element specific magnetism in crystals with different magnetic ions. By the study of the Tb resonance and the Dy resonance on Tb0.5Dy0.5Cu2 an unexpected developement of the magnetic structures with temperature is observed. The results of supplementary neutron scattering experiments are presented and analyzed by Rietveld refinement of the crystallographic and magnetic structures, mainly focussed on TbCu2. In summary, new insights into the different magnetic structures of RCu2 compounds are gained. The strong magneto-elastic coupling is studied by different scattering methods applied to this problem for the first time.