Session D33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: Multiferroics

Oxygen rotation driven ferroelectricity enables controllable magnetization-polarization coupling in Ca$_3$Mn$_2$O$_7$

We show how to achieve the electric field switching of magnetism in a multiferroic with a large polarization by having the ferroelectric state arise from the same lattice instability that modulates the spin system. Oxygen octahedron rotations, ubiquitous in perovskites and related materials, are natural candidates for this lattice instability. First-principles calculations are presented for the layered perovskite Ca$_3$Mn$_2$O$_7$, in which rotations induce both ferroelectricity and weak ferromagnetism. The key point is that this rotation pattern is a combination of two non-polar structural modes with different symmetries. We introduce the term ``hybrid'' improper ferroelectricity to describe this phenomenon. Our results suggest a new strategy in magnetoelectronics, whereby control over magnetism is achieved through functional antiferrodistortive oxygen octahedron rotations.\\ N. A. Benedek and C. J. Fennie, arXiv:1007.1003 (2010).   

The search for multifunctional polar materials

One strategy in the search for new polar semiconducting (and possibily magnetic) materials is to check systems already synthesized and reported as polar in the literature to determine the intrinsic magnitude and switchability of the polarization, the band gap and magnetic properties. In many examples where a polar space group was found, neither polarization or band gap measurements were made because the sample as grown was too conductive. Using a combination of ICSD searching and symmetry analysis, we first identify potentially interesting polar materials and screen out those that are reported to definitely be metallic. We then use first-principles density functional theory (DFT) calculations to investigate the ground state structures of these experimentally synthesized materials for which limited data is available. These calculations will help us to develop criteria for screening candidate systems for polar, magnetic and semiconductive behavior, and broaden the search for new examples of these important functional materials.   

Higher-order Ginzburg-Landau Model for Multiferroic Hexagonal Manganites

Hexagonal manganites have been studied intensely as some of the few multiferroic materials with relatively high ordering temperatures. The recent experimental discovery of topological defects in the domain structure of YMnO3 has led to renewed interest in these materials [1, 2, 3]. Though a Landau free-energy model has already been parameterized at low order[4], we show the form of the parameterization with higher-order terms, including for the first time an angular dependence to the structural trimerization mode. Analysis of the resulting model explains clearly the origin of the topological defects in the domain structure, provides further theoretical insight into the contentious issue of the nature of the ferroelectric phase transition, and gives theoretical input into understanding the thickness of ferrelectric domain walls. \\[4pt] [1] Choi et al., Nature Mat. 9, 253 (2010)\\[0pt] [2] Mostovoy, Nature Mat. 9, 188 (2010)\\[0pt] [3] Jungk et al., Appl. Phys. Lett. 97, 012904 (2010)\\[0pt] [4] Fennie et al., Phys. Rev. B 72, 100103 (2005)   

Local and Long-Range High Pressure Structure of Orthorhombic REMnO$_{3}$

Orthorhombic perovskite REMnO$_{3}$ multiferroic systems were prepared by high pressure synthesis and solid state reaction. High pressure synchrotron x-ray diffraction and x-ray absorption measurements were performed to explore the structural changes. The influence of the pressure on the electrical polarization is discussed. Theoretical simulations are utilized to predict the stable magnetic phases based on the experimental parameters. This work is supported by DOE Grant DE-FG02-07ER46402.   

On the Nature of the Ferroelectric Transition in Multiferroic Hexagonal REMnO$_{3}$

Combined local and long range structural measurements were conducted on REMnO$_{3}$ for temperatures extending significantly above the ferroelectric transition temperature (T$_{FE})$. We find in hexagonal REMnO3 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. This work is supported by DOE Grants DE-FG02-07ER46402 (NJIT) and DE-FG02-07ER46382 (Rutgers University).   

Anomalous Phonon Behavior in Orthorhombic LuMnO3 at Low Temperature

We present the pressure dependent phonon spectra of orthorhombic-LuMnO3 which are conducted in the low temperature region (below T$_{N}$ and T$_{L})$. A temperature dependent anomalous phonon coincides with the ferroelectric behavior at low pressure condition. At $\sim $10 GPa, this anomalous phonon exhibits an unusual softening trend which will be suppressed at higher pressure. This work is supported by DOE Grant DE-FG02-07ER46402 (NJIT), by DE-FG02-07ER46402 (Rutgers), by COMPRES (U2A beam line at NSLS), the Consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreement EAR01-35554, U.S. Department of Energy (DOE-BES and NNSA/CDAC) and by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886 (use of NSLS at Brookhaven National Laboratory).   

Induction of novel macroscopic properties by local symmetry violations in spin-spiral multiferroics

Incommensurate (IC) structures are omnipresent in strongly correlated electron systems as high-$T_C$ superconductors, CMR manganites, as well as multiferroics. In each case they are origin of a pronounced symmetry reduction reflecting the complexity of the underlying microscopic interactions. Macroscopically, this can lead to new phases and possibilities to gain control of the host material. Here we report how the IC nature of a spin-spiral multiferroic induces new physical properties by renormalizing the relevant length scales of the system. Local symmetry violations directly manifest in the macroscopic response of the material and co-determine the multiferroic order giving rise to additional domain states. These usually hidden degrees of freedom become visible when non-homogenous fields are applied and condition for instance the second harmonic generation. Our study shows that incommensurabilities play a vital role in the discussion of the physical properties of multiferroics -- they represent a key ingredient for further enhancing the functionality of this class of materials.   

Origin of the magnetic-field controlled polarization reversal in multiferroic TbMn$_{2}$O$_{5}$

The interplay of multi-dimensional complex magnetic order parameters leads to interesting effects like magnetically induced ferroelectricity. A particular interesting example is TbMn$_{2}$O$_{5}$ because of the associated magnetic-field controllable electric polarization. By optical second harmonic generation we show that the gigantic magnetoelectric effect originates in three independent ferroelectric contributions. Two of these are manganese-generated. The third contribution is related to the magnetism of the Tb$^{3+}$ sublattice and has not been identified so far. It mediates the remarkable magnetic-field induced polarization reversal. This model is verified by experiments on the isostructural YMn$_{2}$O$_{5}$ where Y$^{3+}$ ions are nonmagnetic and only two polarization contributions are present and no magnetoelectric coupling is observed. These results underline the importance of the $3d-4f$-interaction for the intricate magnetoelectric coupling in the class of isostructural RMn$_{2}$O$_{5}$ compounds.   

Electronic mechanism for ferroelectricity and strong magneto-electric coupling in charge-ordered multiferroics

We study magneto-electric phenomena in multiferroic materials, which exhibit ferroelectricity due to the charge ordering. Using rare-earth iron oxides as an example, we derive an effective model, which takes into account the Coulomb interaction, magnetic superexchange and spin-orbit effects, and is consistent with the recent X-ray absorption spectroscopy measurements in multiferroic ${\rm LuFe_2O_4}$. Then we demonstrate, how the interplay between quantum fluctuations and geometric frustration stabilizes the charge and ferrimagnetic spin orderings. The strong coupling, due to the double-exchange mechanism, between these orders, leads to a large magneto-electric response. Our results provide a complete physical description of the magneto-electric properties of charge-ordered multiferroics.   

Colossal Magnetoelectric Effect with Competing Multiferroic and Weak-Ferromagnetic Phases

From our investigation of magnetoelectric properties of Eu$_{0.75}$Y$_{0.25}$MnO$_{3}$, where a multiferroic phase competes with a weak ferromagnetic phase in magnetic fields, we found intriguing hysteretic behaviors of physical properties with variation of temperature and magnetic field. These hysteretic behaviors arise from the kinetic arrest/de-arrest processes of the first order magnetic transition, resulting in freezing or melting of a magnetoelectric glass state with the coexistence of two competing phases. We note that most of large magnetoelectric coupling effects in multiferroics are associated with the large change of polarization with magnetic fields, but the control of ferromagnetic-type magnetization by applying electric fields is most relevant to technological applications, which is scarcely observed. This important issue of mutual controllability is achieved in Eu$_{0.75}$Y$_{0.25}$MnO$_{3}$ utilizing dynamical modulations of the coexistence of two contraindicative phases, highly susceptible to the external perturbations such as electric and magnetic fields.   

Pairing and Self-Organization of Vortices and Antivortices in h-YMnO$_{3}$

Fascinating vortices and antivortices with ferroelectric domains were discovered in multiferroic hexagonal YMnO$_{3}$ [1]. Interlocking of ferroelectric and structural antiphase domain walls of h-YMnO$_{3}$ is one of the important ingredients for the topologically-nontrivial domain pattern formation. We have recently investigated the large-scale configuration of vortices and antivortices in h-YMnO$_{3}$ using selective chemical etching. Our results indicate the importance of pairing of vortices and antivortices, and provide valuable insights into understanding the self-organization mechanism of a zoo of vortices and antivortices. Furthermore, we have studied the response of the vortices and antivortices configuration to external stimuli such as external electric fields. \\[4pt] [1] T. Choi et al., Nature Mater. \textbf{9}, 253 (2010).   

Ferroelectricity driven by symmetric exchange striction in orthorhombic HoMnO$_{3}$

Orthorhombic HoMnO$_{3}$ crystallizes in a distorted perovskite structure (space group \textit{Pbnm}). It has been predicted that the spin configuration below the N\'eel temperature corresponds to a collinear E-type antiferromagnetic phase, which accompanies a large ferroelectric polarization originated from local oxygen distortions driven by exchange striction. In order to understand the exact nature of the E-type magnetism-driven ferroelectricity as well as the influence of Ho magnetism on ferroelectricity, we have performed comprehensive measurements of physical properties of the system, including magnetic susceptibility, dielectric constant, ferroelectric polarization and heat capacity with the variation of temperature and magnetic fields.   

Landau theory of composite domain walls and vortices in multiferroic hexagonal manganites

Multiferroic materials with their coexisting magnetic and ferroelectric orders may find applications in memory devices. In hexagonal manganites, where electric polarization is induced by a periodic lattice distortion, ferroelectric and magnetic domain walls are firmly locked\footnote{M. Fiebig et al., Nature 419, 818 (2002).} even though electric polarization and spin ordering are decoupled in the bulk. Recent measurements showed that electric polarization changes sign at the boundaries of structural domains and revealed the existence of unusual vortices where six structural domains merge and the electric polarization changes sign six times around the defect.\footnote{T. Choi et al., Nature Materials 9, 253 (2010).}$^,$\footnote{M. Mostovoy, Nature Materials 9,188 (2010).} We present a phenomenological theory of coupled lattice, charge and spin degrees of freedom in hexagonal manganites, which we use to calculate how electric polarization, structural distortions and magnetic ordering vary at the domain walls and vortices, and how the shape of these defects changes in an applied electric field.   

Analysis of the magnetic structure and spin exchange interactions of multiferroic YBaCuFeO$_{5}$ by first principles DFT calculations

In the layered perovskites RBaCuFeO$_{5}$ (R = Y, Lu, Tm), the CuFeO$_{9}$ dumbbells made up of apex-sharing CuO$_{5}$ and FeO$_{5}$ square pyramids share their basal corners to form perovskite layers, and the resulting CuFeO$_{5}$ slabs are stacked along the c-direction. Recently, these compounds were found to exhibit ferroelectric polarization when a modulated magnetic component is superposed on their antiferromagnetic structure. To help understand this finding, we examined the spin exchange interactions between the Fe$^{3+}$ (d$^{5})$ ions, between the Cu$^{2+}$ (d$^{9})$ ions, and between the Fe$^{3+}$ and Cu$^{2+}$ ions on the basis of DFT+U and DFT+U+SOC calculations for YBaCuFeO$_{5}$. The ferroelectric polarization of YBaCuFeO$_{5}$ was also calculated for several modulated magnetic structures that were constructed based on the cone-model.