Sr2_2IrO4_4/Sr3_3Ir2_2O7_7 superlattice for a model 2D quantum Heisenberg antiferromagnet

Spin-orbit entangled pseudospins hold promise for a wide array of exotic magnetism ranging from a Heisenberg antiferromagnet to a Kitaev spin liquid depending on the lattice and bonding geometry, but many of the host materials suffer from lattice distortions and deviate from idealized models in part due to inherent strong pseudospin-lattice coupling. Here, we report on the synthesis of a magnetic superlattice comprising the single ($n$=1) and the double ($n$=2) layer members of the Ruddlesden-Popper series iridates Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ alternating along the $c$-axis, and provide a comprehensive study of its lattice and magnetic structures using scanning transmission electron microscopy, resonant elastic and inelastic x-ray scattering, third harmonic generation measurements and Raman spectroscopy. The superlattice is free of the structural distortions reported for the parent phases and has a higher point group symmetry, while preserving the magnetic orders and pseudospin dynamics inherited from the parent phases, featuring two magnetic transitions with two symmetry-distinct orders. We infer weaker pseudospin-lattice coupling from the analysis of Raman spectra and attribute it to frustrated magnetic-elastic couplings. Thus, the superlattice expresses a near ideal network of effective spin-one-half moments on a square lattice

The role of epitaxy, chemistry and magnetic fields in multiferroic systems: Investigations with neutron scattering

The complex interplay between ferromagnetism and ferroelectricity found in multiferroic systems represents an impressive opportunity for research and technologies based upon the manipulation of both spin and charge degrees of freedom, but also signifies a considerable challenge to reveal the underlying mechanisms. Together, the projects presented in this thesis represent an array of innovative approaches utilising neutron scattering and complementary techniques to investigate the effect of internal and external influences upon the strong electron correlations in multiferroics with exciting potential for future spintronic applications.In thin film multilayers, where the pertinent physics occurs within only a few unit cells from the interface, it is vital to probe the structural, magnetic and chemical properties with element and depth sensitivity. Using state-of-the-art polarised neutron reflectivity and x-ray magnetic resonant reflectivity, a clear link between modified regions of magnetism and stoichiometry is demonstrated to form at the atomically sharp interface of half metallic ferromagnet La0.67Sr0.33MnO3 / room temperature multiferroic BiFeO3.On the other side, even small influences can play a major role in exhibited ground state, enabling new insight to be gained into intricate electron interactions. Neutron diffraction was used as a powerful tool to explore these properties. Through unique epitaxial constraints of a (110)-oriented SrTiO3 substrate and an intermediate layer of SrRuO3, thin film BiFeO3 is demonstrated to grow as a single domain system and retain the incommensurate spin cycloid found in bulk, opening an exciting new avenue of research in thin film heterostructures. Combined neutron diffraction and theoretical modelling show that the introduction of chemical pressure through cation substitution in doped BiFeO3 significantly improves magnetic properties due to modified superexchange interactions. Finally, using in-situ applied magnetic fields with complementary neutron and bulk techniques, first evidence of potential multiferroicity is shown to arise in the spinel FeCr2S4 in the orbitally ordered state

Square Lattice Iridates

Over the past few years, Sr2IrO4, a single-layer member of the Ruddlesden-Popper series iridates, has received much attention as a close analog of cuprate high-temperature superconductors. Although there is not yet firm evidence for superconductivity, a remarkable range of cuprate phenomenology has been reproduced in electron-and hole-doped iridates including pseudogaps, Fermi arcs, and d-wave gaps. Furthermore, many symmetry-breaking orders reminiscent of those decorating the cuprate phase diagram have been reported using various experimental probes. We discuss how the electronic structures of Sr2IrO4 through strong spin-orbit coupling leads to the low-energy physics that had long been unique to cuprates, what the similarities and differences between cuprates and iridates are, and how these advance the field of high-temperature superconductivity by isolating essential ingredients of superconductivity from a rich array of phenomena that surround it. Finally, we comment on the prospect of finding a new high-temperature superconductor based on the iridate series.1

Nonmagnetic J = 0 State and Spin-Orbit Excitations in K2RuCl6K_{2}RuCl_{6}

Spin-orbit Mott insulators composed of $t^4_{2g}$ transition metal ions may host excitonic magnetism due to the condensation of spin-orbital J=1 triplons. Prior experiments suggest that the 4d antiferromagnet Ca$_2$RuO$_4$ embodies this notion, but a J=0 nonmagnetic state as a basis of the excitonic picture remains to be confirmed. We use Ru L$_3$-edge resonant inelastic x-ray scattering to reveal archetypal J multiplets with a J=0 ground state in the cubic compound K$_2$RuCl$_6$, which are well described within the LS-coupling scheme. This result highlights the critical role of unquenched orbital moments in 4d-electron compounds and calls for investigations of quantum criticality and excitonic magnetism on various crystal lattices

Nonmagnetic J = 0 State and Spin-Orbit Excitations in K2RuCl6K_{2}RuCl_{6}

Spin-orbit Mott insulators composed of $t^4_{2g}$ transition metal ions may host excitonic magnetism due to the condensation of spin-orbital J=1 triplons. Prior experiments suggest that the 4d antiferromagnet Ca$_2$RuO$_4$ embodies this notion, but a J=0 nonmagnetic state as a basis of the excitonic picture remains to be confirmed. We use Ru L$_3$-edge resonant inelastic x-ray scattering to reveal archetypal J multiplets with a J=0 ground state in the cubic compound K$_2$RuCl$_6$, which are well described within the LS-coupling scheme. This result highlights the critical role of unquenched orbital moments in 4d-electron compounds and calls for investigations of quantum criticality and excitonic magnetism on various crystal lattices

Growth and properties of fully strained SrCoOx (x approximate to 2.8) thin films on DyScO3

High quality epitaxial SrCoOx (oxygen deficient SrCoO3) thin films were grown on (110) DyScO3 substrates by pulsed laser deposition.The disappearance of half order peaks in X-ray diffraction as well as the XAS at the O K-edge indicates an oxygen content of x approximate to 2.8 in the thin films. Magnetization measurements reveal that the specific substrate strain suppresses the ferromagnetism found in the corresponding bulk material and the emergence of an antiferromagnetic-type spin correlation as predicted by theoretical calculations. Our work demonstrates that the magnetism can be tuned by in-plane strain in SrCoOx thin films

IRIXS Spectrograph: an ultra high-resolution spectrometer for tender RIXS

The IRIXS spectrograph represents a new design of a ultra high-resolution resonant inelastic X-ray scattering (RIXS) spectrometer that operates at the Ru L3-edge(2840 eV). First proposed in the field of hard X-rays by Shvyd'ko (2015), the X-ray spectrograph uses a combination of laterally graded multilayer mirrors and collimating/dispersing Ge(111) crystals optics in a novel spectral imaging approach to overcome the energy resolution limitation of a traditional Rowland-type spectrometer(Gretarsson et al., 2020). In combination with a dispersionless nested four-bounce high resolution monochromator design that utilizes Si(111) and Al$_2$O$_3$(110) crystals,we achieve an overall energy resolution better than 35 meV full width at half maximum (FWHM) at the Ru L$_3$-edge, in excellent agreement with ray tracing simulations

Enhanced Magnetization of Cobalt Defect Clusters Embedded in TiO2-d Films

High magnetizations are desirable for spintronic devices that operate by manipulating electronic states using built-in magnetic fields. However, the magnetic moment in promising dilute magnetic oxide nanocomposites is very low, typically corresponding to only fractions of a Bohr magneton for each dopant atom. In this study, we report a large magnetization formed by ion implantation of Co into amorphous TiO2-δ films, producing an inhomogeneous magnetic moment, with certain regions producing over 2.5 μB per Co, depending on the local dopant concentration. Polarized neutron reflectometry was used to depth-profile the magnetization in the Co:TiO2-δ nanocomposites, thus confirming the pivotal role of the cobalt dopant profile inside the titania layer. X-ray photoemission spectra demonstrate the dominant electronic state of the implanted species is Co0, with a minor fraction of Co2+. The detected magnetizations have seldom been reported before and lie near the upper limit set by Hund's rules for Co0, which is unusual because the transition metal's magnetic moment is usually reduced in a symmetric 3D crystal-field environment. Low-energy positron annihilation lifetime spectroscopy indicates that defect structures within the titania layer are strongly modified by the implanted Co. We propose that a clustering motif is promoted by the affinity of the positively charged implanted species to occupy microvoids native to the amorphous host. This provides a seed for subsequent doping and nucleation of nanoclusters within an unusual local environmentD.L.C. and J.L.B. acknowledge the support of the Australian Institute Nuclear Science and Engineering. We also acknowledge the support of the ARC Centres of Excellence program

Unique Crystal Structure of Ca2RuO4 in the Current Stabilized Semimetallic State

The electric-current stabilized semimetallic state in the quasi-two-dimensional Mott insulator Ca2RuO4 exhibits an exceptionally strong diamagnetism. Through a comprehensive study using neutron and x-ray diffraction, we show that this nonequilibrium phase assumes a crystal structure distinct from those of equilibrium metallic phases realized in the ruthenates by chemical doping, high pressure, and epitaxial strain, which in turn leads to a distinct electronic band structure. Dynamical mean field theory calculations based on the crystallographically refined atomic coordinates and realistic Coulomb repulsion parameters indicate a semimetallic state with partially gapped Fermi surface. Our neutron diffraction data show that the nonequilibrium behavior is homogeneous, with antiferromagnetic long-range order completely suppressed. These results provide a new basis for theoretical work on the origin of the unusual nonequilibrium diamagnetism in Ca2RuO4.11Nscopu

Pseudospin-lattice coupling in the spin-orbit Mott insulator Sr2IrO4\mathrm{Sr_{2}IrO_{4}}

Spin-orbit entangled magnetic dipoles, often referred to as pseudospins, provide a new avenue to explore novel magnetism inconceivable in the weak spin-orbit coupling limit, but the nature of their low-energy interactions remains to be understood. We present a comprehensive study of the static magnetism and low-energy pseudospin dynamics in the archetypal spin-orbit Mott insulator $\mathrm{Sr_{2}IrO_{4}}$. We find that in order to understand even basic magnetization measurements, a formerly overlooked in-plane anisotropy is fundamental. In addition to magnetometry, we use neutron diffraction, inelastic neutron scattering, and resonant elastic and inelastic x-ray scattering to identify and quantify the interactions that determine the global symmetry of the system and govern the linear responses of pseudospins to external magnetic fields and their low-energy dynamics. We find that a pseudospin-only Hamiltonian is insufficient for an accurate description of the magnetism in $\mathrm{Sr_{2}IrO_{4}}$ and that pseudospin-lattice coupling is essential. This finding should be generally applicable to other pseudospin systems with sizable orbital moments sensitive to anisotropic crystalline environments