Resonant X-ray scattering studies of collective electronic states in cuprates and nickelates controlled by isovalent chemical substitution and epitaxial integration

In quantum materials, valence electrons experience strong mutual interactions, which can lead to the spontaneous emergence of multifarious collective electronic and magnetic phases. These include Mott insulating phases, superconductivity, antiferromagnetism, as well as charge and orbital ordered phases. Often, various nearly-degenerate electronic ordering tendencies exist within close proximity of one another. In such cases, small variations in the material's chemical and structural landscape can effectively push the system toward one or the other ordered ground state. In this sense, careful control of both chemical and structural degrees of freedom may be harnessed to design collective states in strongly correlated systems. Transition metal oxides represent a promising platform for the controlled manipulation of collective states, due to the strongly correlated transition metal d states and the wide variety of nearly isostructural compounds, which allow for elaborate chemical substitutions and coherent epitaxial relationships between chemically distinct phases. Progress in our understanding of these systems, and in particular the set of tools available to control them, may allow these materials to form the basis of future information and energy technologies ranging from spintronics and neuromorphic computation to superconducting quantum computers and superconducting magnetic levitation. In this thesis a variety of solid state techniques are used to control the collective behaviour of strongly correlated charge and spin degrees of freedom in Cu and Ni based 3d transition metal oxides. We begin with the study of a copper-oxide high-temperature superconductor with the chemical formula (Ca${}_x$La${}_{1-x}$)(Ba${}_{1.75-x}$La${}_{0.25+x}$)Cu${}_3$O${}_y$ (CLBLCO), which hosts a 123-type structure closely related to that of the well studied compound YBa${}_2$Cu${}_3$O${}_{6+x}$ (YBCO). Isovalent chemical substitution of Ba with Ca causes slight changes in the Cu-O bond angles and distances, resulting in a dramatic variation of both electronic and magnetic correlations. In the context of this thesis, we have used resonant x-ray scattering at the Cu L${}_3$ edge to produce the first report of charge order in the CLBLCO system, as well as to characterize its response to the chemical substitution x and the oxygen content y. In recent years, charge order correlations have been established as a ubiquitous component of the cuprate high-T${}_c$ phase diagram, often evidently competing with superconductivity. In CLBLCO, the charge order onset temperature was found to decrease by up to 90 K as a function of x, in contrast to superconductivity and antiferromagnetism which are both strongly enhanced by the structural variations associated with increasing x. Our results reveal a powerful mechanism by which to control the complex interplay between charge density wave order and superconductivity. In the second part of this thesis we study charge order correlations in the related high-temperature superconductor YBCO. While charge order correlations in bulk crystals of YBCO have been studied extensively since their discovery in 2011-2012, this thesis presents the first systematic study of charge order in epitaxially grown YBCO films. While charge order in the bulk system is characterized by 2-dimensional short range correlations, our resonant x-ray scattering study has revealed the unexpected emergence of 3-dimensionally phase coherent charge order in YBCO films grown on SrTiO${}_3$ substrates. Our results demonstrate how the epitaxial relationship to a chemically and structurally distinct substrate is able to couple to both structural and electronic degrees of freedom, thereby manipulating the collective behaviour of charge order correlations in YBCO. This result is paralleled by the recent observation that 3-dimensional charge order can be induced in the bulk system in the presence of strong magnetic fields or a uniaxial strain field. In the final chapter of this thesis we combine the control of structural degrees of freedom facilitated by heteroepitaxy with the designed integration of two magnetically active materials with distinct anisotropies and exchange interactions. As a model system we have chosen to incorporate ultrathin layers of metallic antiferromagnetic LaNiO${}_3$ with the orthorhombic and magnetically anisotropic DyScO${}_3$ in superlattices grown on DyScO${}_3$ substrates. The resulting heterostructure adopts the structural symmetry of the substrate material, while the coupled magnetic sublattices inherit characteristics associated with both of the constituent materials. Our resonant magnetic x-ray scattering study of this system reveals that the spiral antiferromagnetic structure originating from the strong Ni-Ni exchange interactions is induced on the Dy sites, whereas the strong Ising anisotropy of the Dy moments is fed back across the interface to the otherwise nearly isotropic Ni spins. Furthermore, the large Dy moments respond sensitively to applied magnetic fields thereby acting as "anchors" for the magnetic-field-induced manipulation of the Ni spin-spiral orientation. The manipulation of spiral antiferromagnetic states via external fields opens new perspectives for the control of spin-dependent transport phenomena in antiferromagnetic spintronic devices. In this work we have exploited chemical and epitaxial degrees of freedom to manipulate the lattice, electronic and magnetic structures in the 3d transition metal oxides (Ca${}_x$La${}_{1-x}$)(Ba${}_{1.75-x}$La${}_{0.25+x}$)Cu${}_3$O${}_y$, YBa${}_2$Cu${}_3$O${}_{6+x}$ and LaNiO${}_3$. In doing so, we have contributed to an ongoing effort to understand the correlated phases in these materials, and have helped to characterize the set of tools available to control their collective behaviour. Finally, these case studies highlight the power of resonant x-ray scattering to selectively probe lattice, charge, orbital and spin degrees of freedom in an element-specific manner well suited to multinary transition metal oxide compounds.