Session Q32: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides - Multiferroics and Magnetoelectrics

Skyrmions in Multiferroics

Magnetic skyrmion is a topologically stable particle-like object, which appears as nanometer-scale vortex-like spin texture in a chiral-lattice magnet. In metallic materials, electrons moving through skyrmion spin texture gain a nontrivial quantum Berry phase, which provides topological force to the underlying spin texture and enables the current-induced manipulation of magnetic skyrmion. Such electric controllability, in addition to the particle-like nature, is a promising advantage for potential spintronic device applications. In this talk, we report the experimental discovery of magnetoelectric skyrmion in an insulating chiral-lattice magnet Cu$_2$OSeO$_3$. We find that the skyrmion can magnetically induce electric polarization through the relativistic spin-orbit interaction, which implies possible manipulation of the skyrmion by external electric field without loss of joule heating. The present finding of multiferroic skyrmion may pave a new route toward the engineering of novel magnetoelectric devices with high energy efficiency.   

Dynamical matrix in magnetoelectrics

In ordinary dielectrics the dynamical matrix at the zone center is a nonanalytic function of degree zero in the wavevector {\bf q}. Its expression (for a crystal of arbitrary symmetry) is well known and is routinely implemented in first principle calculations. The nonanalytic behavior occurs in polar crystals and owes to the coupling of the macroscopic electric field {\bf E} to the lattice. In magnetoelectric crystals both electric and magnetic fields, {\bf E} and {\bf H}, are coupled to the lattice, formally on equal footing. I provide the general expression for the zone center dynamical matrix in a magnetoelectric, where the {\bf E} and {\bf H} couplings are accounted for in a symmetric way. As in the ordinary case, the dynamical matrix is a nonanalytic function of degree zero in {\bf q}, and is exact in the harmonic approximation. Besides the above major result, I will also discuss other related issues: (i) The Lyddane-Sachs-Teller relationship for MEs, where the fields {\bf E} and {\bf H} are (once more) dealt with in a symmetric way; (ii) The microscopic origin of the coupling of magnetic fields to the lattice, which may look counterintuitive; (iii) The relationship to first-principle implementations, where in the simplest cases {\bf E} and {\bf B} (not {\bf H}) are zero.   

Multiferroic behavior in Lu$_{2}$MnCoO$_{6}$

Lu$_{2}$MnCoO$_{6}$ is a new member of the multiferroics with coupling between net magnetization and net electric polarization. Similar to Ca$_{3}$MnCoO$_{6}$, an up-up-down-down order of the magnetic spins is found that breaks spatial-inversion symmetry and creates an electric polarization. Unlike Ca$_{3}$MnCoO$_{6}$, the Co and Mn ions are both in a S = 3/2 state, the ordering temperature is 42 K, and the magnetic field needed to suppress electric polarization is 2 T. We present an experimental study of the multiferroic properties and spin structure including neutron diffraction, electric polarization, magnetization, dielectric constant, and specific heat measurements.   

Structural and Magnetic Properties of Ln$_2$CoMnO$_6$ (Ln = Dy and La) Produced by Combustion Sinthesys

The lanthanum manganites have been intensively studied in recent years. These compounds present a wide variety of properties of great technological and scientific interest. The half-doped lanthanum manganite with Co in Mn site have ferroelectric and ferromagnetic properties with critical temperatures close to room temperature. The mechanisms responsible for magnetoelectric coupling in these materials are not yet understood. In this work we study the effect of Dy doping at La site in the structural and magnetic properties of lanthanum manganite half-doped with Co obtained by combustion method. The dark powder obtained was heat-treated in air and cooled slowly in the oven. The samples were characterized structurally by X-ray diffratometry with Rietveld refinement analysis. Magnetization measurements as a function of temperature and the magnetic field were carried out on the SQUID magnetometer in temperature interval of 5 - 300 K and in magnetic fields up to 7T. The results show an decreases of the magnetic transition temperature when we substitute the La by Dy.   

Emergence of a new order reconciling ferroelectric and antiferrodistortive instabilities in EuTiO$_{3}$

Control of magnetic moments with electric field or electric polarization with magnetic field can open new possibilies to develop future applications of low power sensors, data storages and spintronics. However, in general this magnetoelectric coupling is extremely small for device application. Although, a substantial change of the dielectric constant under magnetic field was observed in the EuTiO$_{3}$ system. This finding offers an insight into how the electric polarization couples with the magnetism through phonon modes to the spin of the Eu atoms. We present recent x-ray diffraction data on a single crystal EuTiO$_{3}$ providing the direct proof of antiferrodistortive (AFD) TiO$^{6}$ octahedral rotations correlated with the quantum paraelectric state. Forming an incommensurate AFD order mediates the competition between antiferroelectric and AFD order. We discuss the origin of magnetoelectric coupling based on the interplay of AFD order and antiferromagnetic interactions.   

Spin-phonon coupling in the rare-earth orthoferrite DyFeO$_3$

The rare-earth orthoferrite (RFeO$_3$) canted antiferromagnets are known to exhibit a wide array of magnetic properties, including spin reorinetation transitions and compensation points between the rare-earth and iron sublattices. Furthermore, strong magnetoelastic coupling has been observed to lead to field-induced multiferroism with a large dielectric polarization. Here we present an infrared optical study of DyFeO$_3$, focusing on the evolution of the phonon structure with temperature. Polarized single-crystal relfectance measurements are supplemented with magnetization and dielectric constant measurements to illuminate the role of spin-phonon coupling in the lattice dynamics.   

A first principles investigation of a hexagonal ferrite LuFeO$_3$

The multiferroic hexagonal manganites RMnO$_3$ (R=Dy-Lu,Y), are a fascinating class of materials that display an unusual, complex interplay between structural, polar and magnetic domains. For example, the electric polarization in these compounds are found to be a by-product of a trimerized (zone-boundary) lattice distortion, arising from the ionic size mismatch between R$^{+3}$ and Mn$^{+3}$ ions. As a direct consequence of this improper ferroelectric transition, the ferroelectric and structural trimer domains are locked; rotation of structural distortion at a structural domain not only flips the polarization, but also rotates the spins. The hexagonal ferrites RFeO$_3$ (R=Lu,Er-Tb) crystallize in the same polar structure as the manganite counterparts. However, unlike the \textbf{M}=0, non-collinear antiferromagnetism in manganites, the ferrites have recently been shown to display week ferromagnetic behaviour[1], the underlying microscopic mechanism of which so far is not understood. In the present study, using first principles density functional calculations, we investigate the structural and magnetic properties of LuFeO$_3$, one of the members of this ferrite series. \\[4pt] [1] A. R. Akbashev, A. S. Semisalova, N. S. Perov and A. R. Kaul, Appl. Phys. Lett \textbf{99}, 122502 (2011).   

Atomic Resolution Valence Mapping in LuFe2O4 in an Aberration Corrected STEM

LuFe2O4 is a multiferroic with the simultaneous existence of ferroelectricity and ferrimagnetism at the highest temperature of any known material. The improper ferroelectricity is attributed to charge ordering in the Fe-O layers, however, a direct measure of the Fe valence on individual columns in the crystal remains elusive. Scanning Transmission Electron Microscopy (STEM) in combination with Electron Energy Loss Spectroscopy (EELS) allows for spatially resolved, chemically sensitive investigation of oxide materials. We used a Nion 5th-order aberration corrected 100 keV dedicated STEM to collect spectroscopic images from a thin film of LuFe2O4 on MgAl2O4 to map the two-dimensional concentrations of every atomic species in the film. The Fe valence on individual columns was measured, however, no statistically significant modulation--as would be consistent with charge ordering--was observed. Finally, changes in the fine structure in the EELS O atoms in the Lu-O and Fe-O planes, was mapped in two-dimensions.   

Anisotropy in the magnetic and multiferroic properties of LuFe$_{2}$O$_{4-\delta }$ single crystals with varying oxygen stoichiometry

LuFe$_{2}$O$_{4}$ is a multiferroic, where the origin of the ferroelectricity is attributed to electron correlations and directly linked to the charge ordering of Fe$^{2+}$ and Fe$^{3+}$ in the lattice. The multiferroic properties of this system are known to be sensitive to the oxygen stoichiometry. Large single crystals of LuFe$_{2}$O$_{4-\delta}$ with varying oxygen stoichiometry have been produced by the floating zone technique. Detailed magnetic susceptibility, dielectric constant and polarization measurements have been carried out along specific crystallographic axes of the single crystals over a wide temperature range to study the anisotropic properties. The effect of altering the Fe$^{2+}$/ Fe$^{3+}$ stochiometry on the physical properties of LuFe$_{2}$O$_{4-d}$ is discussed.   

Charge dynamics in a frustrated charge ordered multiferroic system

Electronic ferroelectricity is known as phenomena where the electric polarization is caused by the electronic charge order without inversion symmetry. This is seen in some transition metal oxides, e.g. LuFe$_{2}$O$_{4}$, and organic salts. It is suggested from the theoretical work [1] that large charge fluctuation and frustration are responsible for the electric polarization. This charge fluctuation is expected to govern dynamical properties. Actually, the measurements of the low frequency dielectric dispersion and the optical conductivity indicate that the large charge fluctuation remains in charge ordered phase in LuFe$_{2}$O$_{4}$. Motivated by these experimental results, we study charge dynamics in charge ordered system on the layered triangular lattice. We adopt the V-t model where the inter-site electron transfers and the inter-site Coulomb interactions are taken into account. We analyze this model by utilizing the exact diagonalization method and focus on effects of frustration in the charge dynamics. In the 3-fold charge ordered phase associated with the electric polarization, the optical conductivity shows multiple-peak structure in a wide energy range. In finite temperature, the low frequency oscillator strength of the optical conductivity and the dynamical charge correlation functions in 3-fold charge ordered phase decrease slower than those in the non-polar 2-fold charge ordered phase. These results imply the strong charge fluctuation in the 3-fold charge ordered phase due to the geometrical frustration. [1] M. Naka et al. Phys. Rev. B. \textbf{77} 224441.   

Multiferroic M-type hexaferrites with room-temperature conical spin structure

Magnetic and magnetoelectric properties have been investigated for single crystals of Sc-doped M-type hexaferrites [1]. Magnetization and neutron diffraction studies have indicated that a longitudinal conical state is stabilized up to room temperature by tuning the Sc concentration. Magnetoelectric measurements have shown that electric polarization can be induced by applying a transverse magnetic field at lower temperatures, and that the spin helicity is nonvolatile and endurable up to near the transition temperature from conical to collinear state. In addition, the behavior of the polarization vector upon the reversal of magnetization varies with temperature, thereby allowing us to control the relation between spin helicity and magnetization vectors with magnetic field and temperature. This work was in part supported by FIRST program on \lq\lq Quantum Science on Strong Correlation\rq\rq \ from JSPS. \\[4pt] [1] Y. Tokunaga, Y. Kaneko, D. Okuyama, S. Ishiwata, T. Arima, S. Wakimoto, K. Kakurai, Y. Taguchi, and Y. Tokura, Phys. Rev. Lett. 105, 257201 (2010)   

The magnetoelectirc effect in the RAl$_{3}$(BO$_{3})_{4}$ (R=Tb, Ho, Er, and Tm)

We study the magnetoelectric (ME) effect of the rare earth aluminum borates, RAl$_{3}$(BO$_{3})_{4}$ (R=Tb, Ho, Er, and Tm). The magnetic, magnetoelectric, and magnetostrictive properties were investigated between 2K and 300K with different orientation of fields up to 70kOe. A giant magnetoelectric polarization, 3600 $\mu $C/m$^{2}$, is found in HoAl$_{3}$(BO$_{3})_{4}$ while a 70kOe transverse magnetic geometry field is applied. This value is significantly larger than that previously reported in all other bulk crystalline linear magnetoelectric or multiferroic materials. Furthermore, the ME polarization decreases with increasing magnetic anisotropy of the rare earth moment. The magnetostrictive measurements show that there is a strong coupling between the 4-f moments and the lattice. Our data further imply that the field-induced ionic displacements in a unit cell give rise to a change of structural symmetry from non-polar to polar symmetry.   

Polar Nanodomains and Giant Converse Magnetoelectric Effect in Charge-Ordered Fe$_{2}$OBO$_{3}$

Charge ordering (CO) is considered to be an important issue that leads to metal-insulator transitions in numerous materials and also shows possible correlations to many notable physical phenomena, such as colossal magnetoresistance, superconductivity and multiferroics. In recent investigations, oxyborate Fe$_{2}$OBO$_{3}$ has been found to show certain structural and physical features in connection with a continuous CO transition [1, 2]. By using\textit{ In-situ }TEM technique, we revealed that the charge-ordering transition characterized by an incommensurate modulation could evidently result in remarkable polar nanodomains at low temperatures. This kind of nanodomain could play a critical role in triggering a high dielectric constant and notable dielectric dispersion as observed in Fe$_{2}$OBO$_{3}$. Moreover, measurements of the magnetoelectric coupling under electrical field demonstrate the existence of giant electrically induced changes in magnetization around the magnetic transition [1, 2]. 1.Y. J. Song et al., Phys. Rev. B 81, 020101(R) (2010). 2.H. X. Yang et al., Phys. Rev. Lett. 106, (2011) 016406.   

Ferroelectric Phase Transition in Pb$_{5}$Cr$_{3}$F$_{19}$ and Coupling of Electric Polarization and Magnetization

The ferroelectric fluoride Pb$_{5}$Cr$_{3}$F$_{19}$ with ferroelectric/paraelectric phase transition at 545 K offers a possibility of multiferroic behavior. The paramagnetic Cr$^{3+}$ ion with electronic spin 3/2 has two inequivalent positions in the unit cell and is responsible for magnetic properties. These properties were measured with a SQUID magnetometer from 2 K to 630 K in addition to our earlier EPR measurements. At the ferroelctric/paraelectric phase transition the lattice parameters (c and a unit cell dimensions) experience relatively big changes leading to alteration of magnetic dipole-dipole and exchange interactions. The temperature dependence of magnetic susceptibility times temperature around the phase transitioin was analyzed following the usual free energy expansion. We obtained that a coupling between the electric polarization and the magnetization is quadratic. A magnetic anomaly was observed below 25 K.   

The origin and coupling mechanism of magnetoelectric effect in \textit{TM}Cl$_{2}$-4SC(NH$_{2})_{2}$ (\textit{TM} = Ni and Co)

Most research on multiferroics and magnetoelectric effects to date has focused on inorganic oxides. Metal organic frameworks (MOF) are a new field in which to search for ferroelectricity and explore new coupling mechanisms between electricity and magnetism. We will present the magnetic and electric properties of NiCl$_{2}$-4SC(NH$_{2})_{2}$, DTN, and CoCl$_{2}$-4SC(NH$_{2})_{2}$, DTC, compounds as a function of temperature, magnetic, and electric field. We gain insights into the coupling mechanism by observing that in DTN the electric polarization closely tracks the magnetic ordering whereas in DTC it does not. For DTN, all electrically polar thiourea, SC(NH$_{2})_{2}$, molecules are tilted in the same direction along the c-axis, breaking spatial inversion symmetry, whereas for DTC, two thiourea molecules are pointing up and the other two thiourea molecules are pointing down direction with respect to c-axis, perfectly canceling the net electrical polarization. Thus the magnetoelectric coupling mechanism is likely magnetostrictive adjustments of the thiourea molecule orientation in response to magnetic order.