Session N17: Focus Session: Manganite Dynamics and Structure

Magnetic structure and dynamics of Rh-doped Sr$_2$IrO$_4$ probed by resonant x-ray scattering

The physics of 5d iridates has recently attracted considerable attention due to the potential for novel electronic and magnetic ground states driven by strong spin-orbit coupling (SOC). One material which has attracted particular interest is the layered perovskite Sr$_2$IrO$_4$, which has been proposed as the first experimental realization of a spin-orbital Mott insulator with a j$_{eff}$ = 1/2 ground state [1,2]. It has been shown that by substituting Ir$^{4+}$ (5d$^5$) ions for Rh$^{4+}$ (4d$^5$), the strength of the SOC in this system can be tuned through a series of electronic phase transitions [3]. We have performed resonant magnetic x-ray scattering (RMXS) and resonant inelastic x-ray scattering (RIXS) measurements to determine the effect of Rh-doping on the magnetic structure and excitation spectrum of Sr$_2$Ir$_{1-x}$Rh$_x$O$_4$. We find that increasing Rh concentration results in (i) suppression of the magnetic transition temperature, (ii) a doping-induced change in magnetic structure, (iii) alteration of the magnon dispersion relation, and (iv) significant reduction of magnon lifetimes. [1] B.J. Kim et al, PRL 101, 076402 (2008). [2] B.J. Kim et al, Science 323, 1329 (2009). [3] T.F. Qi et al, PRB 86, 125105 (2012).   

Strain control of spin and orbital transitions in La$_2$NiO$_4$

We have studied the electronic and magnetic structure of the layered nickelates, La$_2$NiO$_4$ within density functional theory. We show that biaxial strain induces a high spin to low spin transition, which coincides with a significant change in the $x^2-y$ and $3z^2-r^2$ orbital occupancy. We discuss the role of the on-site Coulomb interaction, the crystal field, and prospects for the strain control of the spin and orbital state.   

Magnetic and Orbital Orders Coupled to Negative Thermal Expansion in Mott Insulators, {Ca$_{2}$Ru$_{1-x}$M$_{x}$O$_{4}$} (M = 3d transition metal ion)

{Ca$_{2}$RuO$_{4}$} is a structurally-driven Mott insulator with a metal-insulator transition at $T _{MI}$ = 357K, followed by a well-separated antiferromagnetic order at $T _{N}$ = 110 K. Slightly substituting Ru with a 3d transition metal ion M effectively shifts $T _{MI}$ and induces exotic magnetic behavior below $T _{N}$. Moreover, M doping for Ru produces negative thermal expansion in {Ca$_{2}$Ru$_{1-x}$M$_{x}$O$_{4}$} (M = Cr, Mn, Fe or Cu); the lattice volume expands on cooling with a total volume expansion ratio, $\Delta$V/V, reaching as high as 1\%. The onset of the negative thermal expansion closely tracks $T _{MI}$ and $T _{N}$, sharply contrasting classic negative thermal expansion that shows no relevance to electronic properties. In addition, the observed negative thermal expansion occurs near room temperature and extends over a wide temperature interval [1, 2]. These findings underscores new physics driven by a complex interplay between orbital, spin and lattice degrees of freedom.\\[4pt] [1] T.F. Qi, O.B. Korneta, S. Parkin, L.E. DeLong, P. Schlottmann and G. Cao, Phys. Rev. Lett. 105 177203 (2010)\\[0pt] [2] T. F. Qi, O. B. Korneta, S. Parkin, Jianping Hu and G. Cao, Phys. Rev. B 85 165143 (2012)   

THz magneto-electric excitations in the chiral compound Ba3NbFe3Si2O14

The langasite Ba$_3$NbFe$_3$Si$_2$O$_{14}$ displays a chiral structure and orders magnetically with a Neel temperature $T_N$=27K. We have determined its terahertz (THz) spectrum by means of synchrotron-radiation measurements. Three different types of excitation are present. The first one, at 13cm$^{-1}$, disappears at T$_N$ and is assigned to magnons. The others, at 23cm$^{-1}$ and 29cm$^{-1}$, persist up to four times $T_N$. According to their selection rules, they are interpreted as rotational modes of the lattice whose magneto-electric activity reveals a structural transition into a polar helical state.   

Orbital physics in RIXS

In contrast to magnetism, phenomena associated with the orbital degrees of freedom in transition metal oxides had always been considered to be very difficult to observe. However, recently resonant inelastic x-ray scattering (RIXS) has established itself as a perfect probe of the orbital excitations [1] and orbital order [2] in transition metal oxides. Here we give a brief overview of these recent theoretical and experimental advances which have inter alia led to the observation of the separation of the spin and orbital degree of freedom of an electron~[1, 3].\\[4pt] [1] J. Schlappa, K. Wohlfeld, K. J. Zhou, M. Mourigal, M. W. Haverkort, V. N. Strocov, L. Hozoi, C. Monney, S. Nishimoto, S. Singh, A. Revcolevschi, J.-S. Caux, L. Patthey, H. M. R{\o}nnow, J. van den Brink, T. Schmitt, Nature 485, 82 (2012).\\[0pt] [2] P. Marra, K. Wohlfeld, J. van den Brink, Phys. Rev. Lett. 109, 117401 (2012).\\[0pt] [3] K. Wohlfeld, M. Daghofer, S. Nishimoto, G. Khaliullin, J. van den Brink, Phys. Rev. Lett. 107, 147201 (2011).   

Magnetism and Magnetic Order in La$_{2}$CuRuO$_{6}$

Long-range magnetic order has been observed in the insulating double perovskite compound La$_{2}$CuRuO$_{6}$. This monoclinic compound shows a rock salt like ordering of the B sites in the double perovskite A$_{2}$BB'O$_{6}$ lattice. We show that elevated processing temperatures improve the magnetic properties of the material, possibly by reducing the number of antisite defects between the Cu and Ru ions. In polycrystalline samples, microwave resonance and dc SQUID susceptibility measurements indicate a ferrimagnetic or antiferromagnetic ground state at low temperatures (T \textless\ 19 K). Specific heat measurements also show a transition consistent with the magnetization data.   

Hourglass dispersion in overdoped single-layered manganites

Two non-metallic single-layered manganites with a doping level well above half doping, Nd$_{\mathrm{0.33}}$Sr$_{\mathrm{1.67}}$MnO$_{\mathrm{4}}$ and Pr$_{\mathrm{0.33}}$Ca$_{\mathrm{1.67}}$MnO$_{\mathrm{4}}$, exhibit an incommensurate ordering of magnetic, charge and orbital degrees of freedom. Inelastic neutron scattering experiments reveal an hourglass-like excitation spectrum very similar to that seen in various cuprates superconductors, but only for sufficiently short correlation lengths. We find the characteristic features of the hourglass dispersion as the enhanced intensity at the merging of the incommensurate branches, the rotation of intensity maxima at higher energy, and suppression of the outwards-dispersing branches at low energies. The correlation length of the magnetic ordering and the large ratio of intra- to inter-stripe couplings can be identified as the decisive parameters causing the hourglass shape of the spectrum.   

Magnetic excitations in a 5d-based double perovskite Ba$_2$FeReO$_6$

There is great interest in double perovskite materials, from a fundamental viewpoint of studying correlated electron magnetism as well as spintronics applications. We report theoretical calculations and experimental powder inelastic neutron scattering data on magnetic excitations in the 5d-based double perovskite Ba$_2$FeReO$_6$. We find evidence of multiple spin wave branches consistent with local moment magnetism on Fe sublattice coexisting with highly correlated and spin-orbit coupled local moments on Re.   

Effects of Doping and Coulomb Correlations on Tc and Competing Phases in Half-metallic Double Perovskites

Double perovskites such as $Sr_{2}FeMoO_{6}$ (SFMO) are rare examples of materials with half-metallic ground states and a ferrimagnetic $T_{c}$ much greater than room temperature. We have shown that the electronic and magnetic properties of SFMO are well described by a generalized double exchange model [1] for itinerant electrons from Mo coupled to localized Fe spins. However, the simplest model proves inadequate when SFMO is electron-doped by La-substitution on the Sr sites. Ignoring Coulomb correlations for the itinerant electrons, the ferromagnetism of Fe spins becomes progressively weaker with electron doping, and eventually the model is unstable to a metallic antiferromagnetic ground state. This is in contradiction with experiments [2], which find a ferromagnetic $T_{c}$ increasing with carrier concentration and no evidence for an antiferromagnetic state up to $SrLaFeMoO_{6}$. In this talk we will show that the Hubbard U on the Mo-site is responsible for the observed doping trends. We will show that correlations stabilize the ferromagnetism, with the observed $T_{c}(n)$ behavior, and that the antiferromagnetic metal is not a competitive state for reasonable values of n.\newline[1] O. Erten et al, PRL 107, 257201 (2011)\newline[2] J. Navarro et al, PRB 64, 092411 (2001).   

Predictions for Spin Resolved Spectral Function and Optical Conductivity in Half-metallic Double Perovskites

We present the effects of thermal fluctuations and anti-site (AS) disorder on the spin resolved spectral function $A(k,\omega)$ and optical conductivity $\sigma(\omega)$ for half-metallic double perovskite Sr$_2$FeMoO$_6$, which holds great promise in spintronics applications. While both $T\neq 0$ and AS destroy the half metallic state, they produce distinct effects. Increasing $T$ produces smooth broadening in the energy distribution curves of $A(k,\omega)$ while AS produces localized states at specific energies with broad momentum distribution curves for spin up. Our results can be tested directly in spin resolved ARPES experiments. We also calculate $\sigma(\omega)$ by evaluating the Kubo formula in the exact eigenstate basis. We show for $T\neq 0$ the height of the secondary peak in $\sigma(\omega)$, also seen in experiments, tracks the polarization $P$ of conduction electrons, whereas for disordered samples at $T=0$, the weight of the secondary peak indicates the amount of AS. From the spin resolved conductivity, we show that small ($<10\%$) amounts of AS prevalent in real samples has little impact on the spin polarization of the DC current. The features of the optical spectrum provide a relatively simple experimental probe of the polarization and amount of disorder.   

Mapping chemical/structural order in double perovskite Sr2-xGdxMnTiO6 by atomic resolution electron microscopy

We report on visualizing the chemical and structural order of double perovskite Sr2-xGdxMnTiO6. The antisite disorder of Mn and Ti is detected even at atomic scale at all x, resulting in Mn-rich and Ti-rich regions. For x ?0.75, the majority of manganese ions are in Mn3+ state and are centered in Jahn-Teller distorted MnO6octahedra. The Fourier transformation of atomic resolution images along the [110] zone axis reveals a superstructure that corresponds to the tilting of oxygen octahedra and that doubles the unit cell along [001]c. This superstructure is spatially inhomogeneous and coincides with the regions where B-site ion (Mn/Ti) is displaced along the [110] direction. We discuss these findings in the frame of possible local ferroelectricity and in the light of strong electroresistance observed in Sr1.25Gd0.75MnTiO6.   

Structural transition in an unusual $5d$-electron ferromagnetic insulator

Double-perovskite barium sodium osmate (Ba$_2$NaOsO$_6$) is a ferromagnetic (FM) insulator ($T_c\sim6.8$ K, ordered moment $\sim$0.2 $\mu_B$ per formula unit) with $<110>$ easy axis. We present precision x-ray diffraction studies on single crystals to understand structural symmetry in this rare FM compound. At room temperature there is a subtle splitting of crystal Bragg peaks indicating the global symmetry to be weakly tetragonal. At or slightly above $T_c$, the material becomes orthorhombic. These changes are crucial in lifting the degeneracy of $d$-state manifold and are likely associated with orbital ordering.   

Magnetic properties and electronic structure of Cr$_2$(Te$_{\mathrm{1-x}}$W$_{x})$O$_6$

We report magnetic properties of Cr$_{2}$(Te$_{\mathrm{1-x}}$W$_{x})$O$_{6}$ system combining neutron powder diffraction measurements and first principles electronic structure calculations. Both the end members possess an ordered inverse-trirutile structure, in which there are bilayers of Cr-O separated by a W(Te)-O layer, yet Cr$_{2}$TeO$_{6}$ and Cr$_{2}$WO$_{6}$ display distinct magnetic structures and antiferromagnetic transition temperatures: $T_{\mathrm{N}}$ $\sim$ 92 K for Cr$_{2}$TeO$_{6}$ with antiferromagnetic spin alignment within bilayers, while $T_{\mathrm{N}}$ $\sim$ 45 K for Cr$_{2}$WO$_{6}$ with spins aligned ferromagnetically within the bilayer. Spins belonging to neighboring bilayers are antiferromagnetically coupled for both the compounds. For the mixed system Cr$_{2}$(Te$_{\mathrm{1-x}}$W$_{x})$O$_{6}$, both $T_{\mathrm{N}}$ and sublattice magnetization ($M_{\mathrm{s}}$) reach a minimum ($T_{\mathrm{N}}$ $\sim$ 0 K) for $x$ $\sim$ 0.6, suggesting the existence of a quantum critical point. Electronic structure calculations using \textit{ab initio} density functional theory correctly give the ground state spin configurations for the end compounds (x$=$0,1). We suggest that unoccupied W 5d states play a key role in intra-bilayer ferromagnetic ordering seen in the x$=$1 system.