Complex ordering phenomena in transition metal oxides and oxyhalides

Ordering phenomena like charge, orbital and magnetic ordering play an important role for the understanding of the intriguing properties of transition metal oxides. In this work, the focus was on the study of such ordering phenomena in first row transition metal oxides with either one or two electrons or one or two holes in the 3d-shell, i. e. titanates, vanadates, chromates, titanium and vanadium oxychlorides on the one hand and nickelates and cuprates on the other hand. Experimentally, the pillars of this work are elastic and inelastic neutron scattering, X-ray diffraction, synchrotron measurements and crystal growth. In the perovskite titanate system RTiO3 with trivalent R3+-ions, anomalies in the lattice parameter and thermal expansion at the onset of magnetic ordering have been studied as a function of R-ionic radius across the boundary of ferromagnetic, antiferroorbitally ordered and antiferromagnetic, ferroorbitally ordered compounds in the phase diagram. These anomalies change sign and get strongest at the crossover of the orbital and magnetic ordering schemes which resembles on the metamagnetic transition in Ca2-xSrxRuO4 or on a quantum phase transition in general. Further structural studies reveal that the distortions which are indicative for ferroorbital and antiferroorbital ordering and which distinctly change with the R-ion size and the orbital ordering schemes change much less with temperature. This also indicates that the recent scenario of an orbital liquid state in the paramagnetic phase of RTiO3 is questionable which could also be supported by resonant X-ray diffraction measurements of DyTiO3 which exhibit only few changes of orbital ordering across the magnetic transition. In the hole-doped R1-xCaxTiO3-system evidence for charge ordering was found in various diffraction studies including single crystal neutron diffraction and synchrotron radiation single crystal X-ray diffraction measurements as well as in resonant X-ray diffraction measurements at the synchrotron. In the hollandite system K2V8O16 a novel dimerized phase could be observed which resembles on the monoclinic M2-phase of VO2. In LiV2O5 the electron density could be measured by means of synchrotron radiation single crystal X-ray diffraction and the occupation of a dxy orbital could be observed. In ZnV2O4 an electron density measurement indicates either the occupation of a trigonal a2g orbital or a 'dimerized' phase which might stimulate completely new theoretical calculations for the ground state of this compound. Usually, in fully three-dimensional transition metal oxides antiferromagnetism can be expected to be found in insulating materials only. In CaCrO3, a material with an unusual Cr4+-oxidation state, antiferromagnetism and metallic properties can be observed together. Also in SrCrO3 an antiferromagnetic structure could be affirmed. This compound should be even more metallic than CaCrO3. In powder neutron diffraction measurements of TiOCl an unusual strong flattening of the TiO-planes could be observed at high pressures up to 13 GPa which should induce changes of the electronic properties of this system. In VOCl a yet undisclosed phase transition has been observed at low temperatures which lifts the frustration across two previously equivalent diagonal magnetic exchange 'paths' (interactions) and, hence, gives rise to the exact determination of the magnetic structure. Inelastic neutron scattering experiments on La1.8Sr0.2NiO4 indicate electron phonon coupling in this prototypical diagonal charge-stripe system. These observations might be also relevant for the isostructural cuprate system. For a compound in the spin-glass phase of LSCO, La1.95Sr0.05CuO4, the phonon dispersion of the high-frequency Sigma1 and Delta-1 phonon modes has been studied. For this compound in the spin-glass phase of LSCO incommensurate magnetic peaks or satellites can be observed at diagonal positions around the the planar antiferromagnetic wave vector. Therefore, a diagonal charge stripe phase has been proposed for this compound. Here, no electron phonon coupling could be observed for the Sigma1 mode which should couple to any diagonal charge stripe ordering as was observed for the diagonal incommensurate charge stripes in La1.8Sr0.2NiO4 in this work. Also the direct search for structural superstructure reflections which would be indicative for a diagonal charge stripe order did not reveal any evidence for such a stripe instability. Instead, synchrotron experiments with 100 keV hard X-rays and complementary neutron experiments reveal novel superstructure reflections together with a number of higher harmonic which could be observed at positions in reciprocal space which are rotated by 45° with respect to the positions where diagonal charge stripes would have been expected

Single Crystal Growth of Pure Co3+ Oxidation State Material LaSrCoO4

We report on the single crystal growth of the single-layer perovskite cobaltate LaSrCoO4 that was grown by the optical floating zone method using high oxygen pressures. Phase purity and single crystallinity were confirmed by X-ray diffraction techniques. The pure Co3+ oxidation state was confirmed by X-ray absorbtion spectroscopy measurements. A transition to a spin glass state is observed at ∼7 K in magnetic susceptibility and specific heat measurements

Floating Zone Single Crystal Growth of γ‑CoV₂O₆ with Substantially Enhanced Crystal Size and Quality

Here, we report the growth of centimeter-sized γ-CoV₂O₆ single crystals by the optical floating zone method which are suitable for neutron scattering experiments. Our floating zone grown single crystals have higher magnetic ordering temperatures of 7.8 K, much more clearly visible magnetization plateaus, and substantially enhanced saturation magnetizations compared to flux-grown single crystals, which is also important with regard to the presence of orbital moment contributions within these <i>S</i> = 3/2 systems. Our centimeter-sized single crystals grown by the optical floating zone technique open the way toward the measurement of the peculiar magnetic properties of this intriguing system by means of elastic and inelastic neutron scattering and have been characterized by X-ray and neutron diffraction measurements

Single Crystal Growth and Physical Properties of Pyroxene CoGeO3

We report on the synthesis and physical properties of cm-sized CoGeO3 single crystals grown in a high pressure mirror furnace at pressures of 80 bar. Direction dependent magnetic susceptibility measurements on our single crystals reveal highly anisotropic magnetic properties that we attribute to the impact of strong single ion anisotropy appearing in this system with T-N similar to 33.5 K. Furthermore, we observe effective magnetic moments that are exceeding the spin only values of the Co ions, which reveals the presence of sizable orbital moments in CoGeO3

Hour-glass magnetic excitations induced by nanoscopic phase separation in cobalt oxides

The magnetic excitations in the cuprate superconductors might be essential for an understanding of high-temperature superconductivity. In these cuprate superconductors the magnetic excitation spectrum resembles an hour-glass and certain resonant magnetic excitations within are believed to be connected to the pairing mechanism, which is corroborated by the observation of a universal linear scaling of superconducting gap and magnetic resonance energy. So far, charge stripes are widely believed to be involved in the physics of hour-glass spectra. Here we study an isostructural cobaltate that also exhibits an hour-glass magnetic spectrum. Instead of the expected charge stripe order we observe nano phase separation and unravel a microscopically split origin of hour-glass spectra on the nano scale pointing to a connection between the magnetic resonance peak and the spin gap originating in islands of the antiferromagnetic parent insulator. Our findings open new ways to theories of magnetic excitations and superconductivity in cuprate superconductors

Spin-Induced Multiferroic Behavior in Centrosymmetric Mn 3 WO 6

International audienc

Canted Antiferromagnetism on Rectangular Layers of Fe²⁺ in Polymorphic CaFeSeO

From stoichiometric amounts of CaO, Fe, and Se, pure powders and single crystals of quaternary Ca[FeSe2/2O2/2]∞2 can be obtained by solid-state reaction and self-flux growth, respectively. The as-synthesized compound exhibits a polymorphic crystal structure, where the two modifications have different stacking sequences of [FeSe2/2O2/2]2−∞2 layers. The two polymorphs have similar unit cells but different crystal symmetries (<i>Cmc</i>2₁ and <i>Pnma</i>), of which the former is non-centrosymmetric. Fe is divalent (d⁶) and high-spin, as proven by X-ray spectroscopy, Mössbauer spectroscopy, and powder neutron diffraction data. The latter two, in combination with magnetic susceptibility and specific heat data, reveal a long-range antiferromagnetic spin order (<i>T</i>_N = 160 K) with a minor spin canting. CaFeSeO is an electronic insulator, as confirmed by resistivity measurements and density functional theory calculations. The latter also suggest a relatively small energy difference between the two polymorphs, explaining their intimate intergrowth