We investigated the antiferromagnetic phase transition in the frustrated and multiferroic hexagonal manganites h-YMnO3 (YMO) and h-(Y0.98Eu0.02)MnO3 (YEMO). Elastic neutron scattering was used to study, in detail, the phase transition in YMO and YEMO under zero pressure and in YMO under a hydrostatic pressure of 1.5 GPa. Under conditions of zero pressure, we found critical temperatures of TN = 71.3(1) K and 72.11(5) K and the critical exponent 0.22(2) and b = 0.206(3), for YMO and YEMO, respectively. This is in agreement with earlier work by Roessli et al. Under an applied hydrostatic pressure of 1.5 GPa, the ordering temperature increased to TN = 75.2(5) K, in agreement with earlier reports, while b was unchanged. Inelastic neutron scattering was used to determine the size of the anisotropy spin wave gap close to the phase transition. From spin wave theory, the gap is expected to close with a critical exponent, b0, identical to the order parameter b. Our results indicate that the gap in YEMO indeed closes at TN = 72.4(3) K with b0 = 0.24(2), while the in-pressure gap in YMO closes at 75.2(5) K with an exponent of b0 = 0.19(3). In addition, the low temperature anisotropy gap was found to have a slightly higher absolute value under pressure. The consistent values obtained for b in the two systems support the likelihood of a new universality class for triangular, frustrated antiferromagnets.

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<p>This thesis details a collection of experiments performed on two condensed matter systems, Co-doped BaFe₂As₂ and URu₂Si₂. These two materials are related by their structural type (<em>ThCr₂Si₂</em>-type) serving as great examples of the diversity of material properties present in this family. They are also both superconducting materials and belong to the collection of strongly-correlated electron systems. The interest in studying the Ba(Fe,Co)₂As₂ group of materials is due to the high superconducting transition temperature in these (and related) materials, while the compound URu₂Si₂ was studied due to the presence of a poorly-understood 'hidden order' phase.</p> <p>Muon spin relaxation/rotation/resonance (µSR) was used to measure several single crystals of the series Ba(Fe_2-<em>x</em>Co<em>_x</em>)₂As₂ with Cobalt concentrations <em>x </em>= 0.038, 0.047, 0.061, 0.074, 0.107 and 0.114, and a single crystal of Sr(Fe_0.87Co_0.13)₂As₂. The two samples with the lowest doping, <em>x </em>= 0.038 and <em>x </em>= 0.047, showed strong c-axis magnetism occurring below the magnetic transition, T_SDW. The measurements suggest that the local magnetic field is increasingly disordered as the concentration of Co increases. These samples were shown to exhibit both superconductivity and magnetism, but that the entire sample contains non-zero local magnetic fields, meaning that superconductivity exists in or near regions of strong magnetic order.</p> <p>The remaining compounds (with <em>x </em>= 0.061, 0.074, 0.107, 0.114 and Sr(Fe_0.87Co_0.13)₂As₂) were measured with zero-field (ZF)-µSR and no magnetic ordering was found down to T = 1.65 K. An analytic Ginzburg-Landau model was used to fit the data and obtain absolute values for the penetration depth, λ. A model for the temperature dependence of the density of superconducting carriers, n_s ≈ λ², based on two <em>s</em>-wave gaps describes the data well. Below T_SC, a paramagnetic frequency shift was observed indicative of field-induced magnetism along the c crystallographic direction.</p> <p>Measurements of URu₂Si₂ under chemical and hydrostatic pressure have focused on measuring the spin correlations that are present in the hidden order phase. The chemical pressure that is induced by 5% Re doping perturbs, but does not destroy, the commensurate spin excitations. The spin gap that is present in the parent material is also present under this chemical doping. The hidden order phase survives at least halfway to the quantum critical point to ferromagnetism, but is weakened by the Re substitution.</p> <p>Under hydrostatic pressure of 10.1 kbar, URu₂Si₂ becomes antiferromagnetic, but the spin correlations are found to be qualitatively similar to those of the hidden order phase. The width in reciprocal space (Q-width) of the excitations and their gapped nature remains unchanged upon entering the antiferromagnetic phase. Quantitatively, there is an increase in the magnitude of the gap at Q = (1.4 0 0). This may be a result of the increase in the transition temperature preceding the entry to the antiferromagnetic phase.</p> <p>Due to the large difference in their properties, and hence the motivation for studying Ba(Fe_1-<em>x</em>Co<em>_x</em>)₂As₂ and URu₂Si₂, they will be introduced and presented separately. Chapter 1 will provide the necessary background material on Ba(Fe,Co)₂As₂, while Chapter 2 will provide the background for the work on URu₂Si₂. Chapter 3 will describe the experimental techniques that were used to study these systems.</p> <p>Original research results on Ba(Fe,Co)₂As₂ are presented in Chapter 4. This is mainly focused on µSR measurements of dopings that display superconductivity. Samples that did not order magnetically were measured in the mixed state to measure the vortex lattice to extract the various properties, including the superconducting pairing symmetry. Samples that did order magnetically were measured to analyze the amount of magnetic disorder and discover the extent of coexistence or phase separation between magnetism and superconductivity.</p> <p>Chapter 5 details the original research results on URu₂Si₂. This involved crystal growth of these compounds, and two neutron scattering experiments to measure the spin correlations while perturbing the hidden order state. The first experiment was done on a Re-doped crystal, URu_1.9Re_0.1Si₂. Doping with Re suppresses the hidden order, eventually leading to ferromagnetism at higher dopings. This work showed that the spin correlations are also suppressed, but not as quickly as the hidden order. The second experiment was on pure URu₂Si₂ under hydrostatic pressure. Applied pressure increases the hidden order transition, but eventually leads to antiferromagnetism, the phase in which the experiment was performed.</p> / Doctor of Philosophy (PhD)

<p>The following dissertation presents neutron scattering studies on three specific magnetic insulating oxide materials whose lattice is based on triangular structural motifs. Each of the three materials studied, LuCoGaO₄, Co₃V₂O₈ and Tb₂Ti₂O₇, displays an interesting disordered ground state that is reached by different mechanisms: site disorder, geometric frustration, and quantum fluctuations induced by a transverse magnetic field. The main focus of this work is the characterization of the resulting magnetic ground states and magnetic excitations within these systems.</p> <p>Chapters 3, 4 and 5 contain original work in the form of six research articles that have either been published or have been prepared for publication in peer-reviewed journals.</p> <p>Chapter 3 describes studies of the quasi two-dimensional triangular layered antiferromagnet LuCoGaO₄. This material is found to exhibit a spin glass ground state as a result of geometrical frustration and site disorder inherent in this system. Below the freezing temperature, this system exhibits static, two-dimensional correlations consistent with frozen short-range correlated regions in the plane of the bilayers that extend over roughly five unit cells. The dynamic correlations reveal typical spin glass behavior upon cooling. Furthermore, a resonant gapped spin-wave-like excitation is observed, that can be related to the anisotropy in the system. Such an excitation is relatively uncommon in spin glasses and has been studied for the first time in such detail.</p> <p>Chapter 4 is concerned with the study of the kagome staircase system Co₃V₂O₈. While prone to geometrical frustration due to its underlying kagome structural motif, this material is characterized by predominantly ferromagnetic interactions that lead to an unfrustrated, ferromagnetic ground state. In this chapter, departures from this conventional ground state by different disordering mechanisms are investigated. The first part focuses on the effects of site disorder by introducing quenched nonmagnetic impurities into the system. The growth of single crystals of (Co_1-xMg_x)₃V₂O₈ is reported. These crystals reveal that the ferromagnetic ground state is very sensitive to doping, and show that a low doping concentration of 19% leads to a suppression of the ferromagnetic ground state below 1.5 K. This could be understood as percolation problem on the quasi two-dimensional kagome lattice including site and bond percolation. The second part focuses on the influence of a transverse magnetic field on the ground state of Ising spins, introducing quantum fluctuations that lead to quantum phase transitions at ~6.25, 7 and 13 T. The observed quantum phase transitions are characterized by distinct changes in the magnetic structure and their associated spin excitation spectra.</p> <p>Chapter 5 presents studies on the pyrochlore antiferromagnet Tb₂Ti₂O₇, which is a proposed spin liquid candidate but whose actual ground state is still the topic of current debate. The ground state of Tb₂Ti₂O₇ was revisited by neutron scattering measurements, revealing a new phase in the low temperature low field phase diagram that can be described as a frozen antiferromagnetic spin ice that exhibits distinct elastic and inelastic scattering features.</p> / Doctor of Philosophy (PhD)

Magnetic frustration is the underpinning theme to all of the magnetic oxide systems explored in this dissertation. The materials studied in this thesis belong to two topical families of interest in modern condensed matter physics, namely, the rare-earth titanates R2Ti2O7 and the double perovskites A2BB'O6. Chapter 1 provides the theoretical background necessary to understand the crystalline systems studied in this thesis. Chapter 2 explains the necessity of utilizing neutron scattering and x-ray experiments to tease out the key signatures which were essential to formulating the conclusions made in each study. Chapter 3 outlines the neutron scattering techniques which were employed to investigate the crystal systems. The first objective of this thesis is to understand effect of “stuffing” on the ground state anisotropy of the quantum spin liquid candidate Yb2Ti2O7 via an investigation of the crystal-field excitations in intentionally stuffed samples. The pentultimate study was performed on the monoclinic crystal systems, La2LiRuO6 and La2LiOsO6, to discern the effect of lattice distortions on the spin-orbit induced magnetic ground state of 4d3 and 5d3 double perovskites based on Ru and Os magnetic ions. The final investigation involves an inelastic neutron scattering investigation of magnetic ground states in three d2 double perovskites, Ba2CaOsO6, Ba2MgOsO6 and Ba2ZnOsO6. Here, we make the case for novel octupolar order below their respective transition temperatures T* of 50 K, 49 K and 30 K based on information provided by neutron scattering, heat capacity, muon spin relaxation and synchrotron x-ray diffraction studies. / Thesis / Doctor of Philosophy (PhD)

Yes / Small-angle neutron scattering (SANS) and neutron spin-echo (NSE) have been used to investigate the temperature-dependent solution behaviour of highly-branched poly(N-isopropylacrylamide) (HB-PNIPAM). SANS experiments have shown that water is a good solvent for both HB-PNIPAM and a linear PNIPAM control at low temperatures where the small angle scattering is described by a single correlation length model. Increasing the temperature leads to a gradual collapse of HB-PNIPAM until above the lower critical solution temperature (LCST), at which point aggregation occurs, forming disperse spherical particles of up to 60 nm in diameter, independent of the degree of branching. However, SANS from linear PNIPAM above the LCST is described by a model that combines particulate structure and a contribution from solvated chains. NSE was used to study the internal and translational solution dynamics of HB-PNIPAM chains below the LCST. Internal HB-PNIPAM dynamics is described well by the Rouse model for non-entangled chains.