Session T32: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides - Manganites

Colossal magnetodielectric effect in DyMn2O5: Electromagnons or rare earth?

We report on the results of spectroscopic studies of the excitations responsible for the colossal magnetodielectric effect in DyMn2O5 [1]. On one hand, many RMn2O5 compounds have electromagnons capable of inducing large steps in the dielectric constant. On the other hand, rare earth ions can posses electric dipole moments and also can produce dielectric anomalies. Both types of excitations are expected in the experimentally difficult low energy range 0.1-1 meV. We use high frequency dielectric, Fourier transform and back-wave oscillator spectroscopies in combination with low temperature and magnetic field up to 9 T to clarify the origin of the dielectric anomaly in DyMn2O5. [1] N. Hur, S. Park, P. A. Sharma, S. Guha, and S-W. Cheong, Colossal Magnetodielectric Effects in DyMn2O5, PRL 93, 107207 (2004).   

Magnetism-Driven Ferroelectricity in GdMn$_{2}$O$_{5}$

REMn$_{2}$O$_{5}$(RE=rare-earth) is one of the well-studied multiferroics which exhibits the reversible switching of ferroelectric polarization under the application of external magnetic fields. It is known that the ferroelectricity in REMn2O5 originates from the symmetric exchange interaction between Mn ions. However, the role of the rare-earth elements has never been elucidated. In order to reveal the full magnetic structure and the contribution of rare-earth magnetism to the ferroelectricity, we have studied the detailed magnetic and structural properties of high-quality single-crystalline GdMn$_{2}$O$_{5}$ (space group, Pbam). We have performed the resonant x-ray scMn$_{2}$O$_{5}$attering experiment, and comprehensive measurements of physical properties of the system, including magnetic susceptibility, dielectric constant and ferroelectric polarization with the variation of temperature and magnetic fields.   

Raman and Infrared studies of the multiferroics TbMn2O5 and YMn2O5

Orthorhombic manganites of the $RMnO_{3}$ and $RMn_{2}O_{5}$ families develop an electric polarization induced, flipped and flopped by application of a magnetic field. The $Tb(Y)Mn_{2}O_{5}$ Anti-ferromagnetic order is incommensurate between $T_{N}=42K$ ($44K$) and $T_{C}=38K$ and becomes commensurate between $T_{C}=38K$ and $T=24K$ ($18K$); at this temperature ferroelectricicity appears and remains incommensurate below $24K$ ($18K$)[1,2]. We have studied the $TbMn_{2}O_{5}$ and $YMn_{2}O_{5}$ Raman active phonons as a function of temperature and compared their behavior to similar multiferroic compounds $Bi(Dy)Mn_{2}O_{5}$ phonons where the $A_{g}$ high frequencies modes soften between $T^{*}\sim 70K$ and $T_{N}$ and harden below $T_{N}\sim 42K$ [3]. The over-hardening of the phonon frequencies above anharmonicity at low temperatures confirm that the spin-lattice interaction plays an important role in the magnetoelectric properties. The $TbMn_{2}O_{5}$ infrared active phonon frequencies are also studied as a function of temperature and under applied magnetic field (up to 10 Tesla). As predicted theoretically the $TbMn_{2}O_{5}$ infrared-active phonon frequencies soften below $T_{N}$. Evolutions of the phonon frequencies under the applied magnetic field are reported and analyzed.   

Absence of ferroelectricity in hexagonal InMnO$_3$

So far, it was believed that hexagonal (h-) InMnO$_3$ exhibit the same type of multiferroic order as the other compounds from the h-RMnO$_3$ family (R = Sc, Y, Dy - Lu), including, in particular, a unit-cell-tripling improper ferroelectric order. Here we present experimental evidence for the \textit{absence} of ferroelectricity in hexagonal InMnO$_3$ based on three different techniques: x-ray diffraction (XRD), piezoresponse force microscopy (PFM) and optical second harmonic generation (SHG). XRD data are ambiguous because they can be described likewise by the non-ferroelectric $P\overline{3}c$ structure and by the ferroelectric $P6_3cm$ structure present in the other h-RMnO$_3$ compounds. However, PFM at room temperature and SHG measurements at low temperature uniquely reveal the absence of ferroelectric order in InMnO$_3$. We therefore propose that InMnO$_3$ exhibits antiferrodistortive, but non-ferroelectric order according to the $P\overline{3}c$ symmetry. Density functional calculations show that the relative energy between the $P\overline{3}c$ and $P6_3cm$ structures is determined by a competition between electrostatic and covalency effects, with an \textit{absence} of covalency favoring the ferroelectric structure.   

Long range order beyond vortices in h-REMnO$_{3}$

Fascinating vortices were discovered recently in ferroelectric domain patterns in hexagonal (h)-YMnO$_{3}$. One of the important ingredients for these vortex domain patterns is mutual interlocking of ferroelectric and structural antiphase domain walls. In contrast to expected vortex domain patterns, we have found intriguing stripe domain patterns in other h-REMnO$_{3}$ (RE=Ho, Er, Tm, Yb, Lu). These stripe domain patterns appear to indicate the presence of long-range-ordered state as the true ground state in h-REMnO$_{3}$. On the other hand, vortex domain patterns suggest the presence of a Kosterlitz-Thouless-like transition. We argue that this significant difference stems from very slow kinetics associated the ordering of six possible degrees of freedom.   

Electronic and Structural Properties Near the Ferroelectric Transition in Multiferroic Hexagonal RMnO$_{3}$

Combined local and long range structural measurements were conducted on RMnO$_{3}$ for temperatures extending significantly above the ferroelectric transition temperature, (T$_{FE})$. We find in hexagonal RMnO3 no large atomic (bond distance or thermal factors) or electronic structure changes on crossing T$_{FE}$. The born effective charge tensor is found to be highly anisotropic at the O sites indicating very strong hybridization of the charge. The tensor does not change significantly above T$_{FE}$ revealing no charge redistribution and suggests an unusual transition. Molecular dynamic simulations on large supercells are used to provide a general picture of the transition. This work is supported by DOE Grants DE-FG02-07ER46402 (NJIT) and DE-FG02-07ER46382 (Rutgers University).   

Role of the apical oxygen in RMnO3 (R = Ho and Lu) low temperature magneto-electric effect

Multiferroic materials are promising candidates for new innovative devices, particularly in the field of memory storage. The strong coupling between magnetic ordering and ferroelectricity characterizing these compounds allows the modulation of the electric polarization (magnetic moment) with an external magnetic (electric) field. Hexagonal RMnO$_{3}$ (Ho to Lu) compounds are type-I multiferroics in which ferroelectricity and magnetism have different sources giving a relative weak magneto-electric coupling with a large polarization. In this case ferroelectricity is induced at a relative high temperature (T$_{C }\sim $ 800K) following a structural transition, while magnetic ordering of Mn$^{3+}$ and R$^{3+}$ occurs at lower temperatures (T $<$ 100K). In order to determine which atoms play a major role in the giant low temperature magneto-electric effect, we study the evolutions of infrared active phonon frequencies in HoMnO$_{3}$ and LuMnO$_{3}$ under applied magnetic field below T$_{Ho }$= 5K. By comparing the renormalized force constants and the Born-effective charges, apical oxygen role in Ho$^{3+}$-Mn$^{3+}$ superexchange interaction is particularly underlined.   

Piezoelectric-response of charged 180$^{\circ}$ ferroelectric domain walls

We report ambient piezoresponse force microscopy (PFM) studies of the multiferroic hexagonal manganite HoMnO$_3$ performed on the cleaved (110) surface of a single crystal specimen. By changing the sample orientation with respect to the cantilever, we observed an unexpected out-of-plane PFM signal at domain walls which depends on domain wall orientation, in addition to the expected in-plane PFM signal in domains. Further studies confirmed that the domain wall PFM signal results from an out-of-plane displacement, which can be explained by a simple model of local elastic response with conservation of unit cell volume at head-on domain walls.   

A First Principle exploration of A site ordered Ho$_{0.5}$A$_{0.5}$MnO$_{3}$(A=Ge, Sn, Pb, As, Sb, Bi, Se, Te)

In this work a first principle attempt has been made to study the structure and properties of doping lone pair cations to ortho-HoMnO$_{3}$. Electronic structure calculations were carried out to study Ho$_{0.5}$A$_{0.5}$MnO$_{3}$ (A=Ge, Sn, Pb, As, Sb, Bi, Se, Te) under the Generalized Gradient Approximation of Density Functional Theory in an attempt to analyze the effect of lone pair cations towards electric polarization and to predict new multiferroics. Under the first principle calculations, Ho$_{0.5}$A$_{0.5}$MnO$_{3}$ (A=Ge, Sn, As, Sb, Bi, Se, Te) is found to be multiferroic. Doping 50{\%} of Se and Sn to HoMnO$_{3}$ is found to highly enhance the electric polarization compared to parent ortho-HoMnO$_{3}$. O2p- A valence p orbital hybridization is expected to be the cause of this polarization. Thus Ho$_{0.5}$Se$_{0.5}$MnO$_{3}$ and Ho$_{0.5}$Sn$_{0.5}$MnO$_{3}$ are expected to be good candidate multiferroics. A first principle attempt has thus been made to perform an extensive search for new multiferroics in which p-p hybridization is found to have a strong role in causing electric polarization predicting new multiferroics providing a pathway for experimentalists to synthesis new promising multiferroic compounds.   

Synthesis of Perovskite ScMnO$_{3}$ under High Temperature and Pressure

Perovskite type ScMnO$_{3}$ was synthesized under high temperature and pressure starting with hexagonal ScMnO$_{3}$. The detail of the structure is examined by synchrotron x-ray diffraction and IR spectroscopy at room temperature. A highly distorted perovskite phase which is similar to the structure of LaMnO$_{3}$ is identified by XRD Rietveld Refinement. Due to the small Sc ion radius, each Mn site has a distorted MnO$_{6}$ polyhedron. This work is supported by DOE Grant DE-FG02-07ER46402.   

Low Temperature Structural of ScMnO$_{3}$

We present the temperature dependent structural changes of hexagonal ScMnO$_{3}$ probed on multiple length scales. These measurements are compared by IR results. These results are used to assess the structural changes across the N\'{e}el temperature which may coincide and couple with the ferroelectric behavior. This work is supported by DOE Grants DE-FG02-07ER46402 (NJIT), by COMPRES, the Consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreement EAR 10-43050 (X17B3) and EAR 01-35554 (U2A), and by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886 (for use of the National Synchrotron Light Source at Brook Haven National Laboratory).   

Time-resolved imaging of magnetoelectric switching in MnWO$_4$

The interaction of magnetic and ferroelectric order is intrinsically strong in spin-spiral multiferroics. Here the complex magnetic long range order breaks inversion symmetry and induces a spontaneous electric polarization. The interaction allows to switch the magnetic order by an electric field and is thus of great interest for applications. Although such magnetoelectric switching is a major goal in multiferroics, hardly any work was devoted to the dynamic aspects of the actual switching process. Here we report time-resolved optical second harmonic generation measurements of the electric-field-induced reversal of the spin-spiral domains in multiferroic MnWO4. Ferroelectric and magnetic orders appear to remain rigidly coupled even during the non-equilibrium state of the transition. The switching is governed by domain wall motion on the millisecond time scale. Even though, locally domains can disappear within nanoseconds. The slow global response can be explained by an energy estimate: As the dipole energy in the electric field is much weaker than the magnetic anisotropy energy, the electric field is only a weak lever for manipulating the magnetic system. Therefore magnetoelectric switching in this compound is inherently slow.   

Displacement-type ferroelectric transition with magnetic Mn ions in perovskite Sr$_{1-x}$Ba$_{x}$MnO$_{3}$

Almost all the proper ferroelectrics with a perovskite structure discovered so far have no $d$-electrons in the off-center transition metal site, as exemplified by BaTiO$_{3}$ and Pb(Zr,Ti)O$_{3}$. This empirical $d^{0}$ rule is incompatible with the emergence of magnetism and has significantly restricted the variety of multiferroic materials. In this work, we have discovered a displacement-type ferroelectric transition originating from off-center Mn$^{4+}$ ions in antiferromagnetic Mott insulators Sr$_{1-x}$Ba$_{x}$MnO$_{3}$. As Ba concentration increases, the perovskite lattice shows the typical soft mode dynamics, and the ferroelectricity shows up for $x\!\ge\!0.45$. In addition to the large polarization and high transition temperature comparable to BaTiO$_{3}$, we demonstrate that the magnetic order suppresses the ferroelectric lattice dilation by $\sim$70\% and increases the soft-phonon energy by $\sim$50\%, indicating gigantic magnetoelectric effects [1]. This work was supported by the FIRST program on ``Quantum Science on Strong Correlation''. \\[4pt] [1] H. Sakai {\it et al}., Phys. Rev. Lett. {\bf 107}, 137601 (2011).   

Antiferromagnetic pinning of a phase-like mode below T$_{N}$ in NdMnO$_{3}$

We report on reflectivity and emission far infrared spectra of NdMnO$_{3 }$between$_{ }$4K and its dissociation temperature. Phonon bands at 300K are in agreement with orthorhombic Pbnm space group assignments. In addition, a broad strong band. reminiscent to a phase-mode in quasi-one dimension metals, is found at very low frequencies that it is understood originating in charge fluctuations in d-orbitals. There is no distinctive behaviors between 1073 K and 1173 K, where orthorhombic O and O' coexist. Beyond $\sim $700 K a mid-infrared polaron band turns into a Drude tail suggesting hopping conductivity double exchange due to air heating oxidation Mn$^{3+}\to $ Mn$^{4+ }$+1e$^{-}$. Below 300 K phonons are better defined and the low frequency giant dipole acquires strength. Few degrees above the antiferromagnetic transition it broadens as electrons loosing coherence. At T$_{N}\sim $76 K we find strong phonon magnetostriction while the band turns asymmetric locking-in to the underlying magnetic order. Preliminary measurements of hexagonal TmMnO$_{3}$, show that asymmetry split due to lower symmetry and the triangular magnetic lattice two exchange integrals J$_{1}$ and J$_{2}$ in the a-b plane. Similar to a soft mode those two bands and a lower frequency resonance undergo strong hardening down to 4 K.   

Low temperature Electrical and Magnetic studies of Nsutite

Nsutite is a naturally occurring Manganese Oxide of the composition Mn$^{4+}_{1-x}$Mn$^{2+}_{x}$O$_{2-2x}$(OH)$_{2x}$ where x = 0.06 -- 0.07. D.C. electrical transport measurements were carried out on samples from Nsuta, Ghana between 40 K and 400 K. Non-linear I-V curves were observed below 140 K even at very low currents. The resistivity vs. temperature data suggests electron transport is by the variable range hopping mechanism between 140 K and 400 K. Magnetic moment vs. temperature data were obtained between 5K and 300K at high field (10,000 Oe) and low field (1,000 Oe). Both high and low field data suggest paramagnetic behavior with a possible Neel temperature at about 15K, below which the materials exhibits antiferromagnetic behavior. The electrical and magnetic properties as well as high temperature thermal analysis (DSC) data will be discussed.