<p>This thesis is the result of several experiments designed to probe the low temperature physics underlying the 1D-Ising-like behaviour of chains of spins in the structure of Cobalt Niobate, CoNb2O6. A collection of prior work has been done by several groups prior to this, focusing on mapping the phase diagram above 0.5K. Interest in this material was renewed recently based upon theoretical work and experimental confirmation of the unique structure of the spins in the system. The bulk of this work was done at temperatures below the previously investigated range to probe the unique properties of this system.</p> <p>The material was grown at McMaster University using the optical floating zone technique from oxide powders. The crystal was examined and oriented using single crystal and Laue diffraction and was cut for use in further experiments. Squid magnetometry was used to confirm the material properties and phase transition temperatures, and was compared to literature values.</p> <p>Heat capacity measurements were performed locally down to 2K, and by collaborators at Waterloo in the range from 330mK to 1K. The heat capacity measurement confirmed the 2.9K transition and explored the relaxation time of the material. Cobalt niobate was found to have an exceptionally long relaxation time at low temperatures indicating strong spin-spin interactions. A sharp transition with zero applied field was found to become a broad, smooth feature at 2.9K when a small field was applied.</p> <p>We performed muSR measurements in zero, longitudinal and transverse field. The muSR results confirmed the long relaxation time found by the heat capacity measurements, which may reflect the coupling of the spin system to the lattice. Additionally, the material was never seen to statically order in zero or longitudinal field down to 700mK and up to 1T. The material was found to behave dynamically throughout all the field ranges.</p> / Master of Science (MSc)

The aim of the present work is to clarify the interplay between the complex technological chain of crystal preparation, chemical and structural perfection of grown crystals of intermetallic compounds and oxides and their physical properties. This technological chain includes detailed studies of unknown or insufficiently known phase diagrams, their correlation with growth conditions and optimisation of process parameters for obtaining single crystals with high chemical and physical perfection. The measurements of the physical properties of the grown crystals such as superconductivity, thermoelectric or dielectric properties not only show new features and properties for application of the materials obtained, but also allow conclusions of the crystal perfection. The studies are focused on the following systems: RENi2B2C borocarbides (RE=Y, Tb or Ho) displaying superconductivity, magnetic order and a strong interplay between magnetic and superconducting properties for YNi2B2C, TbNi2B2C, HoNi2B2C, respectively; CeSi2-?Ô and Ru2Si3 as examples of systems with magnetic and promising thermoelectric properties, respectively; MgB2 and LiBC to test of theoretical predictions of the new superconducting intermetallic compounds discovered in the last years; SrTiO3 and SrZrO3 oxide compounds with special dielectric and optical properties. For this wide spectrum of substances necessarily different growth techniques were applied. That is mainly the floating zone (FZ) or travelling solvent floating zone (TSFZ) techniques with optical heating. Flux techniques were used if the vapour pressure of composing elements is high such as for Mg and Li. The crucible free FZ technique is very attractive for the crystal growth of these intermetallic and oxide compounds to avoid contamination with the crucible material, if the melts have very high chemical reactivity, high melting temperatures and if a large crystal size (at least 3-5 mm) is desired for corresponding physical measurements. One special aim in the presented work is the optimisation of the preparation and growth process features with respect to crystal perfection, establishing new relationships between process parameters, crystal perfection, crystallographic structure, composition of grown crystals and the related physical properties. Optimisation of crystal growth process requires own constitutional studies of growth relevant parts of corresponding multicomponent phase diagrams. Therefore, parts of the phase diagrams were experimentally revealed by differential thermal analysis (DTA), optical metallography and EPMA and partially combined with CALPHAD calculations.

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.