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Abstract: Materials that undergo a coupled phase transition offer a window into the relationship between electrons, nuclei, and magnetic spins in condensed matter. The development of ultrafast techniques where materials can be probed in the sub-ps time regime have provided the means to provide new insights into the exchanges of energy that occur between these systems. This can be applied to magnetocaloric, memory storage, and spintronics devices. This work investigated the dynamics of the FeRh coupled phase transition, where the magnetic ordering change from Anti-Ferromagnetic (AF) to FerroMagnetic (FM) at temperatures moderately above room temperature. The specific focus of this work is on the structural transformations and the effects of lateral confinement on the transition. An x-ray based probe of anti-parallel Fe spin lattice in the AF phase of FeRh is demonstrated experimentally. Non-resonant x-ray magnetic scattering relies upon long-range spin order being established. We demonstrate the temperature dependence of the long-range ordering and confirm that this order only disappears following complete establishment of the FM moment. As a consequence, it allows for a probe of the mixed AF/FM phase of FeRh. This technique allowed for an estimation of the AF domain size suggesting dimensions are limited by the microstructure of the thin film (~40 nm). Time-resolved X-Ray Diffraction (XRD) studies were carried out at the x-ray Free Electron Laser (x-FEL) at SACLA, Japan. We observed structural changes through the phase transition on a timescale not previously reported and show a fluence dependence that indicates the importance of considering non-equilibrated states in the growth and relaxation dynamics of FeRh. A model is presented which demonstrates that such non-equilibria states can be explained using non-trivial electron-phonon coupling. Complementary heated XRD measurements are consistent with the hypothesis that the paramagnetic phase of FeRh is accessed on ps timescales. The effects of lateral confinement were examined in FeRh nanowire arrays to determine if mesoscale magnetic interactions affect magnetisation dynamics. In order to understand the results obtained, heat dissipation was modelled using finite-element software so as to separate magnetic and thermal contributions. Pump-probe Magneto- Optical Kerr Effect (MOKE) investigations alongside static electrical measurements demonstrate that the orientation of external magnetic fields influences the transition behaviour in FeRh wires. FM stabilisation is observed when the external field is applied along the nanowire length. This orientation dependence was not observed in thin films and is ascribed to the shape anisotropy which may influence the FM domain growth mechanism - shifting the phase transition temperature by up to 10 K at applied magnetic fields of 1 T.

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Abstract: The burgeoning field of quantum materials concerns systems that do not adhere to the traditional theories of condensed matter physics. A key feature of these materials is a strong coupling between structural, electronic and magnetic degrees of freedom, which is especially prominent in 4d and 5d transition- metal oxides. The consequences of this coupling are wide, stabilising a range of emergent phases that are sensitive to perturbation. In this thesis, I develop novel techniques based on neutron and x-ray scattering to characterise and control electron-lattice coupling in 4d and 5d quantum materials. I begin with Ca3Ru2O7, a 4d polar metal that hosts a spin-reorientation transition. Using neutron and resonant x-ray scattering, I reveal a new cy- cloidal magnetic phase, arising from spin-orbit coupling, that rapidly evolves with temperature to mediate the transition. I further show that the cycloid- mediated spin-reorientation can be driven by anisotropic strain, demonstrating the control enabled by coupling to the lattice. I then turn to resonant inelastic x-ray scattering (RIXS), which has re- cently received interest as a new probe of electron-phonon coupling (EPC). Using graphite as a model system, I demonstrate the power of RIXS to probe the momentum-dependent EPC for a range of excited electronic states. Our RIXS data reveal some key deficiencies of current theoretical models of phonon excitations in RIXS, and prompt the development of a new Green’s-function– based model by our collaborators to address these issues. Finally, I present a study of the 5d material Sr2IrO4, a famous jeff = 1/2 spin-orbit Mott insulator. I characterise the phonon spectrum with non- resonant inelastic x-ray scattering, before using RIXS to explore the phonon and magnon excitations. I find a strong EPC similar to that seen in the cuprates, and offer a new interpretation of the magnon dispersion involving coupling to spin-orbit excitons.

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Abstract: his thesis presents the work and results of four elastic and inelastic X-ray and neutron scattering studies on single crystals. The topics, the studied compounds and the applied scattering techniques are manifold. However all of those have a common theme: The characterisation of ordered phases, their characteristic excitations and the their effects on material properties in layered 3d- and 4d-transition metal oxides by neutron and X-ray scattering techniques. The presented work particularly focuses on interesting states such as unconventional superconductivity, magnetism and charge density wave order in square-lattice quantum materials, to compare with model Hamiltonians, to determine the natures of the states, or to characterise the interactions between orders. A neutron time-of-flight spectroscopy study performed at the MAPS instrument at the ISIS Neutron and Muon Source in the UK, reveals an anomalous high-energy spin-wave dispersion in square-lattice S = 1 antiferromagnet La2NiO4, which resembles the anomalous dispersion observed in multiple quantum antiferromagnets. A continuation of this study, at the SEQUOIA instrument at the Spallation Neutron Source at the Oak Ridge National Laboratory in the USA, showed no other signatures of quantum antiferromagentism such as a high-energy scattering continuum or missing spectral weights. Instead it showed that the spectral weights are well described by a classical one-magnon + semi-classical two-magnon model, including antiferromagnetic second-nearest-neighbour Heisenberg coupling and strong anisotropy. Sr2RuO4 is an unconventional superconductor, which was long though to be solid state analogue of the superfluid 3He A-phase, exhibiting a chiral spin-triple state with an odd-parity order parameter and an out-of-plane d-vector. Two polarised neutron diffraction experiments, performed on the instrument IN20 at the Institut Laue–Langevin in France, are presented, which not only reject the previously thought state, but also show a large jump in the spin-susceptibility at Hc2, due a first order transition induced by Pauli paramagnetism. But especially, they reveal a large low temperature and low magnetic field spin susceptibility, in the superconducting state, consistent with an odd-parity state with in-plane d-vector or with an even-parity state with residual density of states. Multiple X-ray diffraction studies were performed at beamline i16 at the Diamond Light Source in the UK, at the sector 6-ID-D at the Advanced Photon Source at the Argonne National Laboratory in the USA, and at the beamline P24 at the PETRA III at the DESY facility in Germany to uncover the atomic displacements associated with the charge density wave (CDW) order in the compound La1.675Eu0.2Sr0.125CuO4, a derivative of the high-Tc superconductor La2−xSrxCuO4. A stripe-like in-plane atomic displacement is observed which is directly related to the stripe-like hole-accumulation of the charge density ordering. In addition, large collective out-of-plane modulations, which appear to screen the charge order and relax internal strain induced by the stripe-like displacements, are observed. At last, clear signatures of coupling, of the microscopic structure, to distortions affiliated with the low-temperature tetragonal (LTT) structure are revealed. These appear as periodic modulations of the CuO6-octahedra tiltings and hence, stabilise the S3 irreducible representation of the LTT structure.

Open Access

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Abstract: We demonstrate a probe of long-range antiferromagnetic (AF) order in FeRh thin films using non-resonant magnetic x-ray scattering. In particular, x-rays at energies below the Fe K-edge have been used for the observation of magnetic Bragg peaks. Due to the low efficiency of the magnetic scattering, a grazing incidence geometry was used to optimise the diffracted intensity from the thin film samples. Based on Scherrer analysis, we estimate a coherence length similar to previous reports from x-ray magnetic linear dichroism (XMLD) experiments, indicating that domain sizes are limited to 40 nm which is consistent with the grain size. The temperature dependent behaviour of the AF order shows an inverse correlation with the emergence of the ferromagnetic (FM) moment, as expected from the phase diagram.

Open Access

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Abstract: Helical structures continue to inspire, prompted by examples such as DNA double-helix and alpha-helix in proteins. Most synthetic polymers also crystallize as helices, which relieves steric clashes by twisting, while keeping the molecules straight for their ordered packing. In columnar liquid crystals, which often display useful optoelectronic properties, overall helical chirality can be induced by inclusion of chiral chemical groups or dopants; these bias molecular twist to either left or right, analogous to a magnetic field aligning the spins in a paramagnet. In this work, however, we show that liquid-crystalline columns with long-range helical order can form by spontaneous self-assembly of straight- or bent-rod molecules without inclusion of any chiral moiety. A complex lattice with Fddd symmetry and 8 columns per unit cell (4 right-, 4 left-handed) characterizes this “antiferrochiral” structure. In selected compounds it allows close packing of their fluorescent groups reducing their bandgap and giving them promising light-emitting properties.