Venkatraman Gopalan The Pennsylvania State University
Jon-Paul Maria North Carolina State University
Manfred Fiebig Max-Born-Institut
Ce-Wen Nan Tsinghua University
T1: Multiferroics: Past, Present, and Future Perspectives
Monday AM, November 27, 2006
Room 302 (Hynes)
9:30 AM - **T1.1
Historical Milestones on the Route to Maximal Single Phase Multiferroic Complexity.
Hans Schmid 1 Show Abstract
1 , University of Geneva, Geneva Switzerland
Some milestones since the nineteen fifties, having lead to the present day level of understanding of complex single phase multiferroics, will be highlighted, partly based on experience gained with the crystal family of boracites M3B7O13X, where M stands for a bivalent 3d-transition metal ion and X for an ion of the halogens F, Cl, Br or I1.Multiferroics have originally been defined as materials with two or more so-called primary ferroic properties - ferro(i)magnetism, ferroelectricity, ferroelasticity - occurring in a single phase1. In recent times ferrotoroidicity, characterized by a spontaneous toroidal moment, was recognized to complete this family of analogues2.The presence of two or more ferroic properties in a single phase is ruled by stringent symmetry and structural requirements. The occurrence of a spontaneous polarization, magnetization and toroidization (toroidal moment)2 in a common phase is restricted to 9 Heesch-Shubnikov point groups, out of a total of 122. Additional ferroelasticity with centro- or noncentrosymmetry of the prototype phase plays a key role for partial or full coupling between named primary ferroic spontaneous quantities and the associated coupled or non-coupled domain switching, respectively3. However, ferroelasticity can sometimes act as a troublemaker.When considering induced toroidal moments, several novel secondary ferroic phenomena can be postulated, such as electrotoroidic, magnetotoroidic, piezotoroidic effects, toroidal optical SHG and toroidal optical rectification2. No experimental evidence for these effects has been reported so far.Measurements of the linear magnetoelectric effect with strong asymmetry of the off-diagonal tensor components have brought evidence in several substances for the presence of a spontaneous or spin-flop-induced toroidal moment4. The recent first monitoring of ferrotoroidic domains5 has brought further, compelling support for this novel type of primary ferroic. Multiferroic complexity increases with the highest allowed number of ferroic domains of an Aizu-species (prototype/ferroic phase point group pair). This number increases with increasing number of types of ferroic order in a single phase and with decreasing symmetry of that phase. Thus a triclinic triple-ferroic-phase perovskite would reach a maximum of 96 domain states in principle. In case of the additional presence of ferrotoroidic domains5,6 that number would be doubled in principle. As a consequence, applications of coupled switching appear feasible only, if Aizu species with a small number of domain states are used. 1. H. Schmid, Ferroelectrics, 162, 317-338(1994); 2. H. Schmid, Ferroelectrics, 252, 41(2001); 3. H. Schmid, Ferroelectrics, 221, 9-17(1999);; 4. Ref. in: H. Schmid in: M. Fiebig et al(eds.), Magnetoelectric Interaction Phenomena in Crystals, 1-34(2004), Kluwer Academic Pub. 5. Bas B. van Aken, M. Fiebig, J.-P. Rivera and H. Schmid, Symposium T; 6. D.G. Sannikov, Ferroelectrics, 291, 157(2003).
10:00 AM - **T1.2
Progress in Thin Film Magnetoelectric Multiferroics.
Nicola Spaldin 1 Show Abstract
1 Materials Department, University of California, Santa Barbara, California, United States
10:30 AM - **T1.3
Ferroelectricity from Electron Ordering.
Naoahi Ikeda 1 , Shigeo Mori 2 , Kenji Yoshii 3 Show Abstract
1 Physics, Okayama University, Okayama, Okayama pref., Japan, 2 1Department of Physical Science, Osaka Prefecture University, Sakai, Osaka, Japan, 3 Synchrotron Radiation Research Center, Japan Atomic Energy Research Institute , Sayo, Hyogo, Japan
We report a mixed valence oxide RFe2O4 become a ferroelectric originated from the polar electron ordering and free from the ionic displacement. We confirmed Fe3+ and Fe2+ order with a resonant X-ray scattering. The proved ion arrangement possesses an electric polarization. The arrangement of Fe3+ and Fe2+ is explained by the competing interaction between Fe3+ and Fe2+ in triangular lattice. The ferroelectricity arise from the electron distribution shows a direct coupling of degrees of freedom among charge, spin and orbital. RFe2O4 (R=Y or Dy to Lu) is a layered triangular lattice oxide with the spacegroup r-3m(166). The structure is expressed as an alternating stacking of triangular layers composed of rare earth, oxygen and iron ions. The iron triangular plane contains an equal amount of Fe2+ and Fe3+. Comparing to the average valence of Fe2.5+, the Fe2+ and Fe3+ act as an excess half electron (negative charge) and a deficient half electron (positive charge), respectively. Thus the coexistence of Fe2+ and Fe3+ in triangular plane brings a charge frustration on the arrangement for both ions. Taking the competing charge interactions between Fe2+ and Fe3+ into the account the ordering model for Fe2+ and Fe3+ was derived. The model holds an electric polarization since the weight center of Fe2+ and Fe3+ do not coincide. In order to give a proof for the charge ordering we measured the intensity of the superlattice diffraction signal (1/3 1/3 5.5) in the function of X-ray energy near the Fe-K absorption edge. The characteristic enhancements, a maximum at 7.11keV and a minimum at 7.12 keV, show that the difference of the atomic scattering factor for Fe3+ and Fe2+ forms the structure factor of this superlattice. This is the proof for the formation of the superstructure by the ordering from Fe2+ and Fe3+. The index of the super reflection spot indicates that the super cell of Fe2+ and Fe3+ is three times larger than the chemical unit cell within a-b plane. This result gives the same conclusion with the charge ordering model. The presence of the electric polarization by the iron charge ordering gives the consistent explanation for the dielectric response of RFe2O4. For example, LuFe2O4 shows giant dielectric constant up to 5000 with weak temperature dependences below the room temperature. The dielectric relaxation frequency coincides with the iron valence fluctuation frequency, which shows that the polarization switching proceeds with the electron hopping on the iron ions. The electron fluctuation frequency coincides with the characteristic frequency of dielectric response. The electric polarization derived from the pyroelectric current measurement appears below 350K where the charge superstructure develops. This fact indicates that the charge ordering is the order parameter of the electric polarization.reference: N. Ikeda, et al., Nature 436, 1136 (2005).
11:30 AM - **T1.4
Multiferroicity and Colossal Magneto-capacitive Coupling in Transition-metal Compounds.
Joachim Hemberger 1 , Florian Schrettle 1 , Peter Lunkenheimer 1 , Andrei Pimenov 1 , Alexander A. Mukhin 1 2 , Anatoli M. Balbashov 3 , Vladimir Tsurkan 1 4 , Alois Loidl 1 Show Abstract
1 Center for Electronic Correlation and Magnetism, University of Augsburg, 86135 Augsburg Germany, 2 , General Physics Institute of the Russian Academy of Sciences, 119991 Moscow Russian Federation, 3 , Moscow Power Engineering Institute, 111250 Moscow Russian Federation, 4 Institute of Applied Physics, Acadeny of Sciences of Moldova, 2028 Chisinau Moldova (the Republic of)
In recent years multiferroic magnetoelectrics attracted an increasing scientific and technological interest. In this rare class of compounds ferroelectricity (or at least a weak ferroelectric component) and (ferro-)magnetism coexist and both order-parameters are strongly coupled. Prominent examples for such type of materials are the heavy rare earth manganites like TbMnO3, where the partial frustration in the spin-sector leads to spiral magnetic structures inducing finite ferroelectric polarization . The system (Eu:Y)MnO3  offers the possibility to continuously control the orthorhombic distortion of the orbitally ordered perovskite structure and thus to tune the corresponding multiferroic phases without the additional influence of a magnetic rare earth moment. A second example shall be the normal cubic spinels ACr2S4 (A = Cd, Hg, Zn) which in contrast to the manganites do not posses a Jahn-Teller active orbital degree of freedom. CdCr2S4 is a ferromagnetic semiconductor (Tc = 84 K) exhibiting pronounced magneto-resistive and magneto-capacitive effects near the magnetic transition . In this system the dynamics of the dielectric relaxation is strongly influenced by the onset of magnetization.
Besides a detailed characterization of magnetic properties, specific heat, and electric polarization, the discussed materials have been studied using magnetic field dependent broadband dielectric and optical spectroscopy.
 T. Kimura et al., Nature 426, 55 (2003)
 K. Noda et al., J. Appl. Phys. 99, 08S905 (2006)
 J. Hemberger et al., Nature 434, 364 (2005)
This work was partly supported by the Bundesministerium für Bildung und Forschung via grant No. VDI/EKM 13N6917-A and by the Deutsche Forschungsgemeinschaft (SFB 484).
12:00 PM - **T1.5
New Magnetic Twists for Ferroelectricity
Sang-Wook Cheong 1 Show Abstract
1 Department of Physics & Astronomy, Rutgers University, Piscataway, New Jersey, United States
Extraordinary cross-coupling effects between magnetism and dielectric properties in multiferroics where magnetic and ferroelectric orders coexist have been observed recently. For example, the highly reproducible electric polarization reversal in TbMn2O5 and TbMnO3 and unprecedented large change of dielectric constant in DyMn2O5 and DyMnO3 can be activated by an applied magnetic field (H). Furthermore, acoustic phonons in hexagonal–HoMnO3 can be significantly influenced by H. It turns out that the ferroelectricity in those multiferroics arises from non-colinear magnetic orders with inversion symmetry broken, originating from magnetic frustration. Examples of the non-colinear magnetic orders include spiral magnetic orders.
12:30 PM - **T1.6
Ferroelectricity Induced by Magnetic Order.
Tsuyoshi Kimura 1 Show Abstract
1 , Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey, United States
T2: Ferroic Thin Films
Monday PM, November 27, 2006
Room 302 (Hynes)
2:30 PM - T2.1
Enhanced Dielectric Properties in Ferroelectric Barium Titanate Thin Films.
Jon Ihlefeld 1 3 , Bill Borland 2 , Jon-Paul Maria 1 Show Abstract
1 Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 3 Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania, United States, 2 , Dupont Electronic Technologies, Research Triangle Park, North Carolina, United States
2:45 PM - T2.2
First-Principles Study of Domain Evolution in Ferroelectric Ultrathin Films.
Bo-Kuai Lai 1 , Inna Ponomareva 1 , Ivan Naumov 1 , Igor Kornev 1 , Huaxiang Fu 1 , Laurent Bellaiche 1 , Greg Salamo 1 Show Abstract
1 Physics, U of Arkansas, Fayetteville, Arkansas, United States
Over the past decade, ferroelectric thin films have attracted considerable research interest because of their applications in computer memories and radio frequency and microwave devices. With the miniaturization and performance-enhancement trend of the devices, our fundamental understanding on nanoscale ferroelectric thin films is extremely critical to push forward the existing technology and explore new possibilities of ferroelectric applications. The aim of this work is to use first-principles method to reveal and provide unprecedented detailed atomistic features (which are rather challenging to extract from measurements) of the evolution of recently discovered periodic 180o stripe domains in ferroelectric ultrathin films [Phys. Rev. Lett. 89, 067601 (2002)] under external electric fields. Here, we investigate Pb(Zr0.5Ti0.5)O3 (PZT) films that are grown along the  direction (which is chosen to be the z-axis) and assumed to be Pb-O-terminated at all surfaces. They are modeled by 40×24×m supercells that are periodic along the x- and y-axes (which are chosen along the  and  pseudo-cubic directions, respectively), where m is the number of finite (001) B-layers along the non-periodic z-axis. We use the total energy provided by an effective Hamiltonian into Monte-Carlo simulations. Mechanical and electrical boundary conditions are chosen to be close to realistic thin films: the misfit (compressive) strain is –2.65% and the screening corresponds to 81% of the maximum depolarizing field. The external electric field (Ez) is applied along the z-axis. Without the external electric field, the PZT ultrathin films having a thickness of 20 Å form periodic stripe domai