Session D24: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides -- Bulk Multiferroics

Low Temperature Phonon Properties of Orthorhombic REMnO3

We present the temperature dependent phonon spectra of orthorhombic-LuMnO3 and DyMnO3. The temperature dependent phonon spectra results are compared with the XAFS measurement results to probe for structural changes in the low temperature region which may coincide with ferroelectric behavior.   

High pressure structure of Orthorhombic Multiferroic Systems

It is know that pressure modifies the ferroelectric properties of orthorhombic perovskite systems. However, only limited structural studies have been performed. Here we report structural measurements of the impact of pressure on Orthorhombic REMnO$_{3}$ multiferroic systems using x-ray diffraction and x-ray absorption to probe the structural changes. Comments on the influence of pressure on the electrical polarization will be made. This work is supported by DOE Grant DE-FG02-07ER46402.   

Simultaneous magnetic ordering and electric polarization in single-crystal BaMnO$_{3}$ and its derivatives

We report results of a structural, magnetic, dielectric and thermal study of single crystal BaMnO$_{3}$ and its derivatives as a function of temperature and magnetic field. The central findings of this study are (1) simultaneous occurrence of magnetic ordering and electric polarization near room temperature and (2) strong dependence of dielectric properties on slight impurity doping. The results will be presented and discussed along with comparison draw with other related systems.   

Sinusoidal and cycloidal F-type spin structures in multiferroic orthomanganites

We performed resonant x-ray scattering on F-type (0 $q$ 0) reflections of TbMnO$_3$ and Eu$_{0.75}$Y$_{0.25}$MnO$_3$ at Mn $L$-edge, where modulation $q$ are near 0.28 and 0.25, respectively. By using circularly and linearly polarized light, we could confirm that F-type spin structure in TbMnO$_3$ is $c$- axis sinusoid above ferroelectric $T_C$ and $bc$-plane cycloid below $T_C$, while F-type spin structure in Eu$_{0.75}$Y$_{0.25} $MnO$_3$ is not cycloid but $c$-axis sinusoid down to base temperature. F-type spin structures in multiferroic orthomanganites can be explained as canted spin moment developed by Dzyaloshinskii-Moriya interaction and are also consistent in symmetric analysis.   

Local Structure of Hexagonal Multiferroic HoMnO3 at High Temperature

The local structure of HoMnO3 has been measured for temperatures beyond the ferroelectric transition. The detailed structural changes about Ho and Mn are ascertained. Comparisons of the low and high temperature phases are made. This work is supported by DOE Grant DE-FG02-07ER46402.   

Structural instabilities in Aurivillius compound Bi$_4$Mn$_3$O$_{12}$ from First Principles

Layered perovskite oxide materials are good candidates for the potential synthesis of natural multiferroic materials. One approach is to choose a ferroelectric host and to incorporate a magnetically active species. The Aurivillius layered perovskites are chosen because most are ferroelectric. Described by the formula [Bi$_2$O$_2$][A$_{n-1}$B$_n$O$_{3n+1}$], they are formed by stacking Bi$_2$O$_2$ slabs with $n$ perovkitelike blocks. A ferroelectric prototype compound is Bi$_4$Ti$_3$O$_{12}$ (BIT), where bismuth also occupies the $A$ sites. Using first-principles calculations, we investigate here the three-layer Aurivillius Bi$_4$Mn$_3$O$_{12}$ (BIM) that results of substituting all Ti$^{4+}$ B-site cations in the BIT lattice by Mn$^{4+}$ cations. We report the structural instabilities in the high-symmetry tetragonal structure (space group symmetry $I4/mmm$). We find an unstable $E_u$ phonon mode, which mainly involves movements of the Bi ions in the perovskite $A$ sites with respect to the TiO$_6$ octahedra. This instability, also observed in non-magnetic BIT and associated to the in-plane electronic polarization, suggests the presence of ferroelectricity in BIM. We also explore different collinear spin orderings of the magnetic Mn atoms and its effect on the structural instabilities.   

Raman and X-Ray Diffraction Studies on Doped LiMn$_{2}$O$_{4}$

We have carried out Raman and X-Ray diffraction measurements on polycrystalline Cr doped LiMn$_{2}$O$_{4}$. Structural analysis using the XRD data suggests that for less than 50{\%} substitution of Cr at Mn sites the samples have the cubic spinel structure with Fd3m space group. The diffraction peaks shift towards higher angles with increasing Cr content suggesting contraction of the lattice with increasing Cr. We observe strong Raman modes near 630 cm$^{-1}$ and 580 cm$^{-1}$ corresponding to A$_{1g}$ and F$_{2g}$ symmetry respectively. Specifically, A$_{1g}$ corresponds to the stretching of the Mn$^{3+}$O$_{6}$ octahedra and F$_{2g}$ involves Mn$^{4+}$O$_{6}$ stretching. Thus their relative intensity as a function of doping can be considered as representative of the relative amount of Mn$^{3+}$ and Mn$^{4+}$. We find that the intensity ratio I (F$_{2g})$/ I(A$_{1g})$ increases with increasing Cr doping clearly indicating the decrease in the Mn$^{3+}$ content in LiCr$_{x}$Mn$_{2-x}$O$_{4}$ for 0 $\le $ x $\le $ 1. As Mn$^{3+}$ is mainly responsible for the Jahn -- Teller distortion, chromium substitution in LiMn$_{2}$O$_{4}$ on Mn sites suppresses the Jahn -- Teller distortion. These results are also consistent with recent theoretical results of suppression of Jahn-Teller distortion on Cr doping done using density functional theory.   

Progress towards an electron electric dipole moment search in Europium-Barium Titanates

We report on recent progress towards an electron electric dipole moment (EDM) search using mixed Europium Barium Titanates. In particular, we have synthesized and characterized Eu$_{0.5}$Ba$_{0.5}$TiO$_3$ ceramics and identified them as a good candidate for a solid state electron EDM search. These samples are ferroelectric below $200\mbox{ K}$ and paramagnetic above $1.8\mbox{ K}$. Because of the spontaneous electric polarization in these ceramics, the seven unpaired $4f$ electrons of the Eu$^{2+}$ ions in the lattice feel a large effective electric field, on the order of $10\mbox{ MV/cm}$. The estimated sensitivity due to this large electric field and large densities ($n\approx 10^{22}$~cm$^{-3}$) is approximately $d_e\sim10^{-28}e\mbox{cm}$ after ten days of averaging.   

Second Harmonic Generation in MnTiO$_{3}$

Optical Second Harmonic Generation (SHG) is a non-linear process observed in non-centrosymmetric materials. We have performed SHG in MnTiO$_{3}$ in the wavelength range of 900~nm -1500~nm. MnTiO$_{3}$ is anti-ferromagnetically ordered below 63 K ($T_{N}$ =63.5K). We study SHG as a function of temperature and magnetic field both above and below the N\'{e}el temperature to observe the effect of the spin order. When a magnetic field above the critical field ($B_{C }$= 6~T) is applied along the c-axis, MnTiO$_{3}$ exhibits spin flopping, and is thought to show ferrotoroidic symmetry.\footnote{H. Toyosaki et.al, \textit{APL} \textbf{93}, 072507 (2008)} We measured the magnetic field dependence of the second harmonic spectra of MnTiO$_{3}$ from 0~T to 10~T at temperatures ranging from 1.5~K to 150~K.   

Robust charge and magnetic order in the multiferroic LuFe$_2$O$_4$ under an external electric field

We will discuss elastic neutron scattering measurements on the charge- and magnetic-ordered multiferroic material LuFe$_2$O$_4$. An external electric field with strength up to 20~kV/cm applied at low temperature ($\sim$150~K), does not affect either the charge or magnetic structure. At higher temperatures ($\sim 360$~K), the resistivity of the sample is low, and an electric current has been applied instead. A reduction of the charge and magnetic peak intensities has been observed when the sample is cooled with the current being applied. However, using resistance- and intensity-temperature curves, we found that the observation is entirely due to internal sample heating by the current, where the actual sample temperature is higher than that read by the thermal sensor. Our results suggest that the charge and magnetic order in LuFe$_2$O$_4$ are robust, and neither the current applied at high temperature nor the field at low temperature was able to break these orders.   

Infrared characterization of rare earth orthoferrite materials

The rare earth orthoferrite (REFeO$_3$) family of compounds displays a remarkable range of magnetic and dielectric properties due to complex interplay between charge and spin degrees of freedom. This coupling leads to unusual phase diagrams and novel physics. We use infrared optical measurements, along with magnetic susceptibility and specific heat, to characterize single crystals of REFeO$_3$ (RE = Dy, Sm, Gd, Er, Tm). Temperature evolution of the phonon structure is evaluated in conjunction with the magnetic and thermal response to identify trends in the coupling of ordered states.   

High Temperature Far Infrared Emissivity, Reflectivity, and X-ray Absorption of BiFeO$_{3}$

We outline the temperature dependent lattice dynamics of BiFeO$_{3}$ from 4 K to melting by combining far infrared emissivity and reflectivity. The TO low temperature unstable phonon ceases softening at $\sim $400 K. We verified that higher than $\sim $700 K not only thermal fluctuations but also off-stoichiometry chemical disorder are taking place partially reducing long-range order. Strong overlapping, merging, and internal mode band smearing, suggest tripling in an idealized high temperature paraelectric cubic phase. X-ray absorption near edge structure (XANES) shows Fe$^{3+}$ turning into Fe$^{2+}$ while the Bi edge downshift suggests reduction under heating in a very complex configuration. Overall, our structural measurements reveal defect induced irreversible lattice changes in an environment lacking of long-range coherence. By contrast, far infrared wavelengths above Tc $\sim $1090 K do not show this local symmetry breaking only yielding three very broad bands as it would be, in a macroscopic view, if the cubic phase Pmcm (Z=1) is adopted. We do not detect an insulator to metal transition before melting   

Two Distinct Ferroelectric Phases in Multiferroic Y-type Hexaferrite Ba$_{2}$Mg$_{2}$Fe$_{12}$O$_{22}$

The magnetic phase diagram of the $Y$-type hexaferrite Ba$_{2}$Mg$_{2}$Fe$_{12}$O$_{22}$ has been studied using single-crystal neutron diffraction. The result indicates successive phase transitions where the magnetic modulation wave number changed discontinuously, when a magnetic field is applied and the temperature is varied. For the low-temperature spin-driven ferroelectric state, we have found a 6-fold structure with q=(0 0 1/2) in weak magnetic fields and a 2-fold structure with q=(0 0 3/2) in strong magnetic fields, between which a first-order transition intervenes accompanied by a hysteresis.   

Complex Band Structure of MgO, CaO, SrO, BaO insulators: DFT-LDA vs. QSGW theory

The recent discovery of a very large magneto-resistance in highly crystalline tunnel junctions with MgO barrier triggered extensive experimental research in the field. The complex band structure of the barrier material is an important property of corresponding MTJs [1]. This property have been studied theoretically on the basis of the density functional theory within the local density functional approximation (DFT-LDA) [1,2]. In this work we compare DFT-LDA and QSGW theory results for lowest evanescent states decay constants for a number of oxides MgO, CaO, SrO, BaO. We employ recently developed QSGW theory [3] to calculate electronic band structure. Then the attenuation constant for evanescent states in the band gap have been calculated by interpolating electron dispersion relation using polynomial fit, procedure analogous to one suggested and tested in [1]. We find that attenuation constant for evanescent states does not simply scales with the value of band gap. [1]. W. H. Butler, X.-G. Zhang, and T. C. Schulthess, and J. M. MacLarenm$, $Phys. Rev. B \textbf{63}, 054416 (2001) [2]. K. D. Belashchenko, J. Velev, and E. Y. Tsymbal, Phys. Rev. B \textbf{72}, 140404(R) (2005) [3] M. van Schilfgaarde, T. Kotani, and S. V. Faleev, Phys. Rev. Lett. \textbf{96}, 226402 (2006);   

DFT investigation of weak ferromagnetic and antiferroelectric properties of tetragonal BiCrO$_{3}$

The magnetic and ferroelectric properties of a very thick tetragonal BiCrO$_{3}$ film are studied using density functional theory. The experimental lattice constants were used but the atomic positions within the unit cell had to be calculated. This resulted in a centrosymmetric Pnma ground state, in agreement with the experimental antiferroelectric result. The magnetization is G type antiferromagnetic with a spin-orbit induced canting that is larger than the experimental value. A simple model of the local electric polarization of the Bi and their twelve surrounding oxygens is consistent with the experimental polarization induced by a large electric field. A comparison with experimental and calculated bulk BiCrO$_{3}$ in the C2/c structure will be given.