Session Z9: Focus Session: Complex Bulk Oxide: General Properties

Local atomic structure of the layered compound SrFeO$_{2}$

SrFeO$_{2}$ exhibits several unexpected structural and physical properties. Its antiferromagnetic transition temperature T$_{N}$=473K is unusually high for a two-dimensional layered structure. First-principle calculations on SrFeO$_{2}$ showed that the Fe 3d down-spin elections occupy the nondegenerate d$_{z2}$ level rather than the degenerate (d$_{xz}$, d$_{yz})$ levels. This is in good agreement with the absence of a Jahn-Teller instability and the existence of the three dimensional antiferromagnetic ordering because the out-of plane direct Fe-Fe exchange is comparable in strength to the in-plane Fe-O-Fe superexchange. Therefore, it is expected that there is no structure instability in SrFeO$_{2}$. Using the pair distribution function (PDF) analysis to characterize the local structure of SrFeO$_{2}$, we observed that the local symmetry is lower than the average P4/mmm crystal symmetry. In particular, the FeO$_{2}$ planes are buckled, with two unique buckling angles along the a-axis. The buckling angle of Fe-O-Fe is reduced from 180$^{o }$with increasing temperature, accompanied by a reduction of the Fe magnetic moment. Thus the local structure instability correlates with the magnetism where the distortions suppress orbital overlap.   

Phonon anomalies and critical fluctuations associated with charge stripes in La$_{1.67}$Sr$_{0.33}$NiO$_4$

We will report inelastic neutron scattering measurements of the spectrum of charge excitations in the stripe-ordered phase of La$_{2-x}$Sr$_{x}$NiO$_{4}$ ($x = \frac{1}{3} $). We identified clear signature of charge stripes at low energies in the nickelates. Our results imply that dynamic stripes are critical fluctuations associated with the stripe-ordering transition. We also observed a phonon anomaly correlated with dynamic stripes above the static-ordering transition, which occurs at 241~K. Our results elucidate the nature of dynamic charge stripes and their signature in the neutron spectra.   

Rare orbital glass state in single crystalline Y$_{2}$Mo$_{2}$O$_{7}$

Perhaps one of the most curious cases of frustrated pyrochlores, Y$_{2}$Mo$_{2}$O$_{7 }$was first classified as a spin glass in 1986. Conventionally, spin glasses must exhibit some sort of chemical disorder although oxygen vacancies and Y-Mo site mixing is virtually absent in all studies to date. NMR and neutron PDF experiments show the presence of local disorder. While other studies have shown a lattice deformation occurring near T$_{g}$=22K, these distortions cannot be detected globally and may not be enough to explain the spin glass behavior. For 25 years, researchers have struggled to resolve spin glass theory with the data; the problem lies in that, until now, scientists have been unable to grow single crystal samples due to the oxidation of Mo$^{4+}$ to Mo$^{6+}$ at low temperatures. Here, we report the synthesis and characterization of the world's first single crystalline sample of Y$_{2}$Mo$_{2}$O$_{7}$. Unlike powder samples, single crystalline Y$_{2}$Mo$_{2}$O$_{7}$ heat capacity measurements show a T$^{2}$ dependence. Neutron scattering experiments show isotropic, broad, liquid-like collective modes and high-Q diffuse scattering characteristic of an orbital liquid to orbital glass transition at T$_{g}$.   

Anisotropic magnetic properties of the KMo$_{4}$O$_{6}$

Electrical resistivity measurements in the tetragonal KMo$_{4}$O$_{6}$ single crystals show a metal-insulator transition (MIT) near 100K. Magnetization measurements as a function of temperature show no evidence of magnetic ordering at this MIT [1]. Single crystals of KMo$_{4}$O$_{6}$ were obtained by electrolysis of a melt with a molar ratio of K$_{2}$MoO$_{4}$:MoO$_{3}$ = 6:1. The process were carried out at 930\r{ }C with a current of 20-25mA for 52h in argon atmosphere. After that, electrodes were removed from the melt alloying the crystals to cool down to room temperature rapidly. Scanning Electron Microscopy (SEM) showed that the black single crystals were grown on the platinum cathode. Typical dimensions of the single crystals are 1x0.2x0.2mm$^{3}$. X-ray diffractometry confirmed that the single crystals have KMo$_{4}$O$_{6}$ tetragonal crystalline structure with space group P$\overline{4}$. Magnetization measurements were performed parallel and perpendicular to the c-axis from 2 to 300K. The results show anisotropic behavior between both directions. Furthermore, the temperature independence of the magnetization at high temperature and the upturn at low temperature are observed in agreement with previous results [1]. MxH curves measured at several temperatures show nonlinear behavior and a small magnetic ordering. The magnetic ordering seems to be related to the MIT near 100K. This material is based upon support by FAPESP (2009/14524-6 and 2009/54001-6) and CNPq/NSF (490182/2009-7). M. Andrade is CAPES fellow and C.A.M. dos Santos is CNPq fellow. \\[4pt] [1] K. V. Ramanujachary et al., J. Sol. State Chem.102 (1993) 69.   

Cu3Nb2O8: A Multiferroic with Chiral Coupling to the Crystal Structure

We will present recent bulk properties, neutron powder diffraction, and non-resonant x-ray diffraction measurements on a new multiferroic material, Cu$_3$Nb$_2$O$_8$. We demonstrate than an electric polarization evolves simultaneously with a generalised helicoidal magnetic ordering. Contrary to conventional models of multiferroicity, the electric polarization was found to lie perpendicular to the common rotation plane of the magnetic moments. We present a new model applicable to a specific class of crystals; those that support a macroscopic``ferroaxial'' vector and adopt a helicoidal magnetic structure. Our new interpretation, which may explain the multiferroic properties of a number of systems reported in recent literature, is based upon the well-understood inverse Dzyaloshinskii-Moriya interaction, but requires an altogether different interpretation.   

THz and infrared excitation spectrum below the Jahn-Teller transition in Sr3Cr2O8

We report on optical excitations observed recently in Sr3Cr2O8 by THz and infrared spectroscopy. Low-energy excitations below 3 THz are detected by THz time domain spectroscopy. These excitations can be divided into two different classes according to the temperature-dependent properties. One is emergent right below the Jahn-Teller transition temperature, which is determined by specific heat measurement to occur at 285 K [1, 2]. The other appears only below 100 K, where the fluctuations are sufficiently suppressed, consistent with the temperature dependence of low-energy Raman modes [3]. Infrared transmission measurements reveal a broad crystal-field excitation, which can be associated with an electronic transition from E to T2 orbital states. \\[4pt] [1] Zhe Wang \textit{et al}., Phys. Rev. B \textbf{83}, 201102 (2011) \\[0pt] [2] D. L. Quintero-Castro \textit{et al}., Phys. Rev. B \textbf{81}, 014415 (2010) \\[0pt] [3] D. Wulferding \textit{et al}., Phys. Rev. B \textbf{84}, 064419 (2011)   

Magnetic and Orbital Excitations in Sr$_3$CuIrO$_6$

We report resonant inelastic x-ray scattering studies of the one-dimensional Sr$_3$CuIrO$_6$ at the Ir L$_3$ edge, with $\Delta$E=40 meV. At high energies, we find peaks at 0.6 eV, 0.9 eV, 3 eV, 4 eV and 6 eV. These peaks are non-dispersive, well-defined excitations. On the basis of quantum chemistry calculations, we are able to identify the first two as excitations within the $t_{2g}$ manifold and the next two as between the $t_{2g}$ and $e_g$ manifolds, together with charge transfer excitations from the O $2p$ to the Ir 5$d$. The 6 eV feature is another $t_{2g}$ - $e_g$ excitation. From these we are able to determine both the spin-orbit and non-cubic crystal field splittings. We find that they are of comparable strength. In addition, magnetic excitations are observed, corresponding to excitations of the $j_{1/2}$ isospin. We find these are highly dispersive along the chain direction with a bandwidth of 20 meV and a gap of 30 meV. These results allow a complete modeling of the spin and orbital degrees of freedom in this model compound and we conclude an atomic, spin-orbit coupled description works well.   

Probing Spin-Orbit Coupling in Iridium 5d Compounds with X-ray Absorption Spectroscopy

Iridium-based transition metal oxides have begun to attract considerable interest following recent proposals of exotic electronic and magnetic ground states (topological insulators, topological semi-metals, spin liquids, and spin-orbital Mott insulators) driven by large spin-orbit coupling effects. We have performed x-ray absorption spectroscopy (XAS) measurements on a series of iridium-based compounds (such as Ir, IrCl3, IrO2, Na2IrO3, Sr2IrO4, and Y2Ir2O7) in order to investigate the magnitude of the spin-orbit interactions in these systems. By determining the branching ratio of the XAS white-line intensities at the Ir L2 and L3 absorption edges, we obtain a direct measure of the expectation value for the spin-orbit operator $\langle$L $\cdot$ S$\rangle$. These measurements reveal remarkably strong spin-orbit interactions in almost all Ir-based compounds, with branching ratios up to several times larger than the statistical value. The branching ratio is found to be largely independent of electronic properties (metal or insulator), ionization state (Ir$^{3+}$ [5d$^{6}$] or Ir$^{4+}$ [5d$^{5}$]), and chemical composition (oxide or halide), although it does appear to be correlated with local Ir site symmetry and Ir-Ir bond distances.   

Investigation of magnetic phase transitions in B-site disordered PbBxB'1-xO3 (B = Fe, Co and B' = Nb, Ta)

Materials such as PbFe$_{0.5}$Nb$_{0.5}$O$_{3}$ (PFN-0.5)/ PbFe$_{0.5}$Ta$_{0.5}$O$_{3}$ (PFT-0.5) and PbCo$_{0.33}$Nb$_{0.67}$O$_{3}$ (PCN-0.33) are relaxor ferroelectrics. PbFe$_{0.5}$Nb$_{0.5}$O$_{3}$ (PFN-0.5) also shows anti-ferromagnetic order below $\sim$ 143 K. Though multiferroicity is an important property of PFN-0.5, its uniqueness stems from coexisting anti-ferromagnetic and spinglass phases below $\sim$ 12 K. Presently, it is the only known such case in a Heisenberg 3D spin system. We report a first systematic study of the H-T phase diagram of PFN-0.5 and discuss the results in the context of existing theories. In addition, we study the magnetic properties of PFT-0.5 and PCN-0.33. While PFN-0.5 and PFT-0.5 demonstrate similar behavior, PCN-0.33 does not show any anomalies that could signify magnetic ordering. We propose that the observed dramatic differences may result from partial B-site ordering.   

Direct neutron scattering-based observation of spin excitons in LaCoO$_{3}$

We report temperature-dependent small angle neutron scattering (SANS) results on undoped LaCoO$_{3}$ single crystals. The data reveal two main scattering components; a high $q$ signal increasing with $T$ above 50 K, and a low $q$ component that turns on sharply below 60 K. The former is shown to be due to inelastic scattering quantitatively consistent with the excitations associated with the spin-state transition. Of greater interest, the low $q$ scattering is shown to be of Guinier form, revealing scattering from a dilute assembly of magnetic objects of size approximately 15-20 nm. The abrupt onset of this scattering at 60 K, i.e. very close to the point at which prior work provided evidence of formation of interacting magnetic excitons, suggests that this represents the first direct scattering-based observation of magnetic excitons forming around O defects in LaCoO$_{3}$.   

High-pressure studies on Ba-doped cobalt perovskites by neutron diffraction

Cobalt perovskite possess rich structural, magnetic and electrical properties depending on the subtle balance of the interactions among the spin, charge, and orbital degrees of freedom. Divalent hole-doped cobalt perovskites LaA$^{2+}$CoO$_{3}$ exhibit structural phase transitions, metal-insulator transitions, and multi-magnetic phase transitions. High-pressure measurement is believed to mimic the size effects of the doped ions. We performed neutron diffraction experiments on selected Ba-doped LaCoO$_{3}$ under pressures up to 6.3 GPa at SNAP at Spallation Neutron Source of ORNL. This work focuses on the high-pressure effects of the selected Ba-doped samples and the change of the phase diagram with pressure.   

Electric field tuning of the metal-magnetic transition in ionic liquid gated Ca$_3$Ru$_2$O$_7$ flakes

The recent application of ionic liquids in electric field-effect devices has provided for unprecedentedly high surface charge accumulations ($\approx$ 10$^{14}$ cm$^{-2}$) due to the formation of an electronic double layer at their interface with various materials. This technique provides a tool to explore metal-insulator transitions and superconductivity in various materials including transition metal oxides. We extend this technique to the study of the physics of strongly correlated electrons in complex oxides such as the Ruddlesden-Popper (R-P) series Ca$_ {n+1}$Ru$_n$O$_{3n+1}$, where $n$ = 1, 2, $\ldots$ $\infty$. Ca$_ 3$Ru$_2$O$_7$, the $n$ = 2 member of the R-P series, features antiferromagnetic ordering below 56 K and a first-order metal-insulator transition along with a structural transition at 48 K. We developed a technique for fabricating Hall bar devices on Ca$_ 3$Ru$_2$O$_7$ flakes prepared by mechanical exfoliation from bulk crystals, and performed low-temperature measurements using the ionic liquid DEME-TFSI in a top-gate configuration. We will discuss electrical transport results and the metal-magnetic transition with gate voltage tuning in these devices.   

p-Electron Magnetism in anion doped BaTiO$_{3-x}$X$_x$ (X=C,N,B)

We present VASP calculations using the HSE functional for carbon, nitrogen, and boron doped BaTiO$_{3-x}$X$_x$ (X=C,N,B). We calculate a 40-atom supercell and replace one oxygen atom by C,N, or B. For all three substituents we find a magnetically ordered groundstate which is insulating for C and N and halfmetallic for B. The changes in the electronic structure between the undoped and the doped case are dominated by the strong crystal field effects together with the large band splitting for the impurity p-bands. Using an MO picture we give an explanation for the pronounced changes in the electronic structure between the insulating non-magnetic state and the as well insulating magnetic state for doped BaTiO$_3$. p-element doped perovskites could provide a new class of materials for various applications ranging from spin-electronics to magneto-optics.   

Spin-dependent Optical Properties of Multiferroic EuO and GdN

We present a first-principles study on the linear and non-linear optical properties of multiferroic EuO and GdN. In particular, we have decomposed the linear and non-linear optical spectra into spin-dependant terms, i.e., we could obtain optical spectra from individual spin channel. With the help of such method, a regular optical spectrum could present information of spin-dependent band structures, which is of great importance to the study of ferromagnetic insulators.