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The,purpose of the thesis is to report the inve?tigation of superconducting phase-periodic conductance oscillations in ferromagnetic wires with interfaces to conventional superconductors. The ferromagnetic wires were made of Holmium(Ho), a conical ferromagnet, and Aluminium(AI) is used as superconductor. The distance between the interfaces was much larger than the singlet superconducting penetration depth. The observed oscillations is explained as due to the long-range penetration of an unusual helical triplet component of the order parameter that is generated at the superconductor/ferromagnet interfaces and maintained by the intrinsic rotating magnetization of Ho. The samples were fabricated using electron-beam lithography and the shadow evaporation technique. This method enables the fabrication of the whole structure without breaking vacuum thus avoiding the formation of Ho oxide barriers at the Ho/ Al interfaces. The films were thermally deposited with first layer of Ho and then Al as second layer. An Al loop is created with a Ho segment to form a Andreev interferometer for the study of superconducting proximity effect.

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The hole doped transition metal oxides show remarkable properties that are related to the influence of charge and spin correlations. In this thesis charge correlations in hole doped manganite and nickelate systems are smdied using soft (˂2 keV), normal (10 keV) and high energy x-ray scattering (100 keV).The charge and J aim-Teller (JT) order is studied in the bi-layer manganite La(_2)-(_2r) Sr(_1)+(_2x)Mn(_2)O(_7) for x = 0.5, 0.55 and 0.60 using high energy x-ray scattering. In the x = 0.55 sample the charge ordered state existed in the same checkerboard pattern as observed in the x = 0.5 sample with no change in behaviour. However, in the x = 0.6 doped sample the charge ordered state entered a new regime characterised by incommensurate JT and charge order. The magnitude of the incommensurability was inversely proportional to the intensity of the JT and charge peaks. High resolution x-ray measurements of the charge stripe order in the La(_2-x)Sr(_x)NiO(_4) system in the doping range 0.20 ≤ x ≤ 0.50 are presented. As the doping was moved away in either direction from the commensurate x = 0.33 the charge stripes changed from a highly correlated state to an increasingly disordered charge glass state. At the commensurate 0.5 composition there was no increase in the intensity or correlation of the charge order despite a significantly higher transition temperature than in the lower doped compositions. Finally the first measurements of the orbital order in La(_0.5)Sr(_1.5) MnO(_4) using soft x-ray scattering at the manganese L edges are presented. The measurements directly probe the orbital order unlike the previously reported measurements at the manganese K-edge. Energy scans were carried out on the orbital order and compared with theoretical predictions. From this it was determined that both the Jahn-Teller distortions and direct orbital ordering contribute to the observed scattering.

This thesis describes the investigation of frustrated magnetic systems, focusing on the pyro-chlore antiferromagnets (AFM). The rare-earth pyrochlore oxides R2B2Oj {R+ = rare earth, = transition metal), crystallizes in the Fd3m space group, with the magnetic ions (R) and the non-magnetic transition metal (B) ions situated on the 16c and on the 6d sites, respectively. As a result, the magnetic ions form lattices of corner-sharing tetrahedra-a stage in which frustration is manifested. Erbium titanate Er2Ti2C > 7 and stannate ESO are both frustrated pyrochlore antiferromagnets which are expected to have very similar magnetic properties. However, Er2Ti207 magnetically orders at 7n 1.1 K but Er2Sn207 remains disordered down to F 150 mK. The main aim of this thesis is therefore to investigate the origin of this difference. The magnetic properties of the series Er2Ti2.jrSn.xO7 were studied as a function of jc (jc = 0 2). Firstly, from the powder neutron diffraction patterns obtained, it was shown that the detected order diminishes as jc * 2. However, in contrast to the previously reported result, a long-ranged ordering transition was detected for Er2Ti207 (at 80 mK) as well as for Er2Ti2 < 37 (at 1.2 K). The ordered magnetic structure was determined while Er2Ti207 orders into the Champion state (one of the ground state of the pyrochlore (lll)-XY AFM), Er2Sn207 orders into a ground state of a Heisenberg pyrochlore AFM with dipolar interaction (Palmer-Chalker state). Secondly, the existence of the quantum critical point (QCP) in the composition range 1.5 < x < 2 was investigated by means of low temperature magnetization measurements. The critical composition jccrjticai 1-55 was determined. Thirdly, the single ion anisotropy of the series ETiSnxO (x = 0 2) was studied by analyzing the crystal electric field (CEF) measured by the inelastic neutron scattering. As a result, the single-ion moments were shown to retain a constant size ( 3 Ub) throughout the series. In contrast, the nature of the single ion anisotropy changed from being XY-like for jc = 0 into being Ising-like for jc = 2 (with respect to the < 111)- axes). Finally, the temperature-field phase diagram of the experimental realization of the Heisenberg pyrochlore AFM -Gd2Ti207, was investigated by means of powder neutron diffraction.

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This thesis describes theoretical and experimental investigation of the optical and magnetic effects in nematic liquid crystals and in ferronematics, namely suspensions of ferromagnetic nanoparticles in nematic liquid crystals. In the experimental part, the effect of the nanoparticles shape and functionality on the suspension stability and magneto-optic properties were studied. Suspensions with magnetic nanospheres showed a linear response to low magnetic fields (< 100 Oersted) and a decrease in the effective Frederiks threshold. Ferronematics with magnetic nanorods coated by 4-n-Octyloxybiphenyl-4-carboxylic acid were more stable and showed a larger decrease in the Frederiks threshold than the spherical magnetic nanoparticles coated by Oleic acid. No ferronematic effects were detected in the weakly magnetic hematite nanorod suspensions. The aim of the theoretical part was to develop a realistic numerical model that could simulate the experimental results of the magnetic-field-induced Frederiks transition in nematic and ferronematic cells. The modelling was carried out in two steps. The first step involved modelling the Frederiks transition of an undoped liquid crystal cell in the presence of an easy axis pretilt and a bias, in-plane, magnetic field. The nematic model predicted that applying a bias field would lead to a shift of the threshold response, which would be sensitive to the bias field direction. This prediction was confirmed as an excellent agreement between the model and experimental data was achieved. In the second stage, a new approach to modelling of ferronematics was proposed, which involved extending previous ferronematic theories to include both the ferromagnetic effect of the particles and the intrinsic magnetic properties of the nematics. There were two variable parameters in the model, which characterise the effective ferroparticle-field interaction, and the ferroparticle-nematic director interaction. These parameters for experimental suspensions were obtained by comparing the model with experimental data. The fitting parameters were used to estimate an effective coupling energy between a nematic host and doped nanoparticles. Up to one order of magnitude higher coupling energy was obtained in the magnetite nanorod suspension as compared to the spherical magnetic nanoparticles. The research presented in this thesis demonstrates a route to prepare highly sensitive and stable ferronematic suspensions, contributes to better understanding of the magneto-optic effects in these suspensions, and highlights their potential for applications as tailor-made optical materials in magnetically driven devices.

Die Arbeit befasst sich mit der Elektronenspinresonanz (ESR) stark korrelierter Elektronensysteme mit ferromagnetischen Wechselwirkungen. Es wurden dafür Messungen an den Kondogitter-Systemen CeRuPO und CeOsPO, der Dotierungsreihe CeFeAs1-xPxO, den niederdimensionalen frustrierten Quadratgittern AA’VO(PO4)2 sowie in dem schwach ferromagnetischen Metall Nb1-yFe2+y durchgeführt. Alle Verbindungen zeigen entweder eine ferromagnetische Ordnung oder befinden sich in der Nähe einer ferromagnetischen Instabilität, die die Eigenschaften des stark korrelierten Systems beeinflusst.:Abkürzungsverzeichnis xi Einleitung 1 1 Theoretische Modelle 5 1.1 Auswirkungen des Kristallfeldes 1.2 Phasenübergänge, Landau-Ginzburg Theorie und Quantenphasenübergänge 1.3 Physik Niederdimensionaler Spin-Systeme 1.3.1 Das zweidimensionale XY-Modell 1.3.2 Das J1-J2-Modell auf dem Quadratgitter 1.4 4f-Magnetismus, Kondoeffekt und Kondogitter 1.4.1 Vom freien Elektronengas zur Landau-Fermi-Flüssigkeit 1.4.2 Kondo-Effekt, RKKY-Wechselwirkung und Schwere Fermionen Systeme 1.4.3 Cer: 4f-Magnetismus 1.5 Schwach magnetische Metalle 1.5.1 Pauli-Suszeptibilität und spontan aufgespaltene Bänder 1.5.2 Spinfluktuationen in itineranten Magneten und marginale Fermiflüssigkeit 1.6 Dichtefunktionaltheorie 2 Grundlagen der Elektronenspinresonanz 31 2.1 Prinzip der magnetischen Resonanz und ESR Parameter 2.1.1 Linienform 2.1.2 Intensität 2.1.3 g-Faktor 2.1.4 Linienbreite und Spin-Relaxation 2.2 Experimenteller Aufbau 2.3 ESR in niederdimensionalen Systemen 2.4 ESR an Metallen und Kondogitter Systemen 2.4.1 Auswirkungen der Wechselwirkungen zwischen lokalen Momenten und Leitungselektronen auf die ESR 2.4.2 ESR in Kondogittersystemen 2.5 Leitungselektronenspinresonanz 2.5.1 Linienform in der LESR 2.5.2 Elliot-Yafet Theorie und ihre Verallgemeinerung 2.6 ESR an magnetischen Phasenübergängen 3 ESR in CeTPO (T=Ru, Os) und CeFeAs1-xPxO 59 3.1 Die Eigenschaften von CeTPO (T=Ru, Os) & CeFeAs1-xPxO 3.1.1 Das ferromagnetische Kondogitter CeRuPO 3.1.2 Das antiferromagnetische Kondogitter CeOsPO 3.1.3 Die Dotierungsreihe CeFeAs1-xPxO 3.2 CeRuPO und CeOsPO: Die Bedeutung FM Fluktuationen für die Beobachtbarkeit der ESR in Kondo-Gitter Systemen 3.3 ESR an CeRuPO Einkristallen 3.3.1 Anisotropie und Temperaturverhalten des g-Faktors 3.3.2 Anisotropie und Temperaturverhalten der Linienbreite 3.4 Entwicklung der ESR in der Dotierungsreihe CeFeAs1-xPxO 3.4.1 CeFeAs0.7P0.3O - Supraleitung und Ferromagnetismus 3.5 Zusammenfassung ESR in CeTPO und CeFeAs1-xPxO 4 ESR in den frustrierten Quadratgittern AA’VO(PO4)2 97 4.1 Struktur und magnetische Eigenschaften AA’VO(PO4)2 4.2 ESR an polykristallinen SrZnVO(PO4)2 & BaCdVO(PO4)2 4.2.1 SrZnVO(PO4)2 4.2.2 BaCdVO(PO4)2 4.3 ESR an Pb2VO(PO4)2-Einkristallen 4.3.1 Anisotropie der Resonanz 4.3.2 Temperaturabhängigkeit 4.4 Zusammenfassung der ESR in den frustrierten Quadratgittern 7 4.4.1 Analyse der Linienbreitendivergenz am Phasenübergang 5 Leitungselektronenspinresonanz in Nb1-yFe2+y 125 5.1 Das magnetische Phasendigramm von Nb1-yFe2+y 5.2 LESR in polykristallinen Nb1-yFe2+y 5.2.1 Proben mit ferromagnetischer Ordnung 5.2.2 Proben mit SDW-Ordnung 5.2.3 Zusammenfassung polykristalline Proben 5.3 Bandstruktur und verallgemeinerte Elliot-Yafet-Theorie in NbFe2 5.4 LESR in einkristallinen Proben von Nb1-yFe2+y 5.4.1 Fe-reicher Einkristall: y=0.016 5.4.2 Stöchiometrischer Einkristall: NbFe2 5.4.3 Nb-reicher Einkristall: y=-0.008 5.5 Zusammenfassung LESR in Nb1-yFe2+y 6 Zusammenfassung und Ausblick Anhang A.1 Das zweite Moment der Dipol-Dipol-Wechselwirkung A.2 Untersuchte CeFeAs1-xPxO-Proben Literaturverzeichnis