Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z30: Focus Session: Iron-based Multiferroics |
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Sponsoring Units: DMP GMAG Chair: Sang-Wook Cheong, Rutgers University Room: 334 |
Friday, March 20, 2009 11:15AM - 11:27AM |
Z30.00001: Multiferroic states in perovskite type orthoferrites Yusuke Tokunaga, Satoshi Iguchi, Yasujiro Taguchi, Takahisa Arima, Yoshinori Tokura Versatile and gigantic magnetoelectric (ME) phenomena have been found for a single crystal of perovskite-type orthoferrite DyFeO$_3$[1]. Below the antiferromagnetic ordering temperature of Dy moments, a linear ME tensor component as large as $\alpha_{zz}\sim 2.4 \times 10^{-2}$ in dimensionless CGS unit is observed. In addition, it is revealed that the application of magnetic field along the c axis induced a ferroelectric order whose large polarization ($\geq$ 0.2 $\mu$C$/$cm$^2$ along the $c$-axis) can be directly reversed by either of magnetic field or electric field. It is noteworthy that this magnetically driven ferroelectric state is even weakly ferromagnetic, i.e., truly multiferroic, in nature. We propose here that the exchange striction working between adjacent Fe$^{3+}$ and Dy$^{3+}$ layer with the respective layered antiferromagnetic components can be the origin of the ferroelectricity with such a large polarization value. It is further argued that the reversal process of electric polarization by magnetic (electric) field is inherently related to the change of the relative phase of antiferromagnetic spin (moment) arrangement of Fe (Dy)\\[0pt] [1] Y. Tokunaga et al., Phys. Rev. Lett. 101, 097205 (2008). [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z30.00002: Composite Domain Walls in Multiferroic Orthoferrites RFeO3 Nobuo Furukawa, Hosho Katsura In order to analyze novel magneto-electric effects such as electric-field controlled magnetization-flips in multiferroic orthoferrites $R$FeO$_3$, we study an effective model which includes $d$-spins on Fe sites and $f$-spins on $R$ sites. Order parameters for $d$ and $f$ spins are coupled through Peierls distoritons. Ginzburg-Landau theory is applied to investigate domain wall structures of the model. As a result, we find various types of solitons corresponding to ferromagnetic, ferroelectric and composite ferromagnetic-ferroelectric domail walls. Dynamics of the domail walls under external fields will also be presented. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z30.00003: Multiferroic BiFeO3 Sang-Wook Cheong BiFeO3 (BFO) is a unique multiferroic in the sense that the magnitude of ferroelectric polarization is large (about 90 microC/cm) - similar with that of standard ferroelectrics such as BaTiO3 and PbTiO3. In addition, both magnetic and ferroelectric temperatures are much high than room temperature. BFO has been extensively studied, but mostly in the form of films. In order to explore the intrinsic properties of BFO and also properties that cannot be measured in film forms, we have investigated comprehensive physical properties of bulk BFO single crystals using a number of techniques such as neutron scattering, piezoelectric force microcopy and transport property measurement. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z30.00004: Polarized Neutron Investigations of BiFeO$_{3}$ William Ratcliff, Seongsu Lee, Taekjib Choi, Ross Erwin, Sang Wook Cheong, Valery Kiryukhin BiFeO$_{3}$ is a multiferroic material at room temperature. Until recently, the only studies on this material were performed on either thin films or powders. We report on the results of neutron diffraction studies performed on single crystals of BiFeO$_{3}$. Polarized neutron diffraction results unambigously reveal that the magnetic structure of this material is chiral. Furthermore, neutron diffraction experiments have shown that it is possible to control magnetic domain populations through the application of an external electric field. These results may suggest directions for future research performed in thin films. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z30.00005: Weak ferromagnetism in a high-pressure phase of FeTiO$_{3}$ with polar lattice distortion Tamas Varga, John Mitchell, Craig Fennie, Stephen Streiffer, Seungbum Hong, Moonkyu Park, Venkatraman Gopalan, Amit Kumar, Eftihia Vlahos, Takeshi Sanehira, Yanbin Wang Today's challenge in multiferroics is to identify materials in which polarization and magnetization -- normally considered contraindicated properties - are strongly coupled. Recent density functional theory calculations have predicted that the family of compounds MTiO$_{3}$ (M = Mn, Fe, Ni) are promising candidates where a polar lattice distortion can induce weak ferromagnetism. The crucial insight is that while the equilibrium one-atmosphere structure of these is ilmenite, they must be transformed to a closely related LiNbO$_{3}$-type structure. We have prepared the corresponding FeTiO$_{3}$ phase at 18 GPa and 1200 $^{\circ}$C. It shows a sharp antiferromagnetic (AF) transition at 111.5 K. FeTiO$_{3}$ also displays ferroelectric domains, and weak ferromagnetism coincident with the AF transition. Possible coupling between its polarization and weak ferromagnetism is discussed based on results of piezoelectric force microscopy (PFM), second harmonic generation (SHG), dielectric, and polarization measurements. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z30.00006: EXAFS Studies of Mulfiferroic Pb(Ti,Fe)O$_{3}$ Ying Zou, Somaditya Sen, Shishir Ray, Mark Williamsen, Tomohiro Shibata, Soma Chattopadhyay, Mali Balasubramanian, Prasenjit Guptasarma Recent suggestions [1] of multiferroic behavior in Pb(Ti,Fe)O$_{3}$ has revived an interest in this text-book ABO$_{3}$ type ferroelectric material. Here, we study the effect of Fe-substitution on the local structural and chemical environment of the parent PbTiO$_{3}$ phase. We have carried out extended x-ray absorption fine structure (EXAFS) measurements at the Fe-K, Ti-K and Pb-LIII edge at the Advanced Photon Source, on a series of single phase samples of PbTi$_{1-x}$Fe$_{x}$O$_{3}$ (0$<$ x $<$ 0.5) synthesized using a sol-gel technique. The near edge fine structure (XANES) reveals that Fe cations are trivalent. A fascinating new result is the observation of charge disproportionation of Pb into Pb2+ and Pb4+, likely a result of charge redistribution arising from Fe3+ substitution. ~Radial distribution function (RDF) study of EXAFS spectra from the Ti-K edge and the Fe-K edge confirms that Fe substitutes Ti up to x=0.5. 1. Palkar et al, Appl. Phys. Lett. 90(2007)172901. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z30.00007: Phase formation, Crystal Lattice and Microstructure Studies of sol-gel derived Pb(Ti,Fe)O$_{3}$ Somaditya Sen, David Gelting, Shishir Ray, Ying Zou, Donald Robertson, Marija Gajdardziska-Josifovska, Larry Buroker, Mark Williamsen, Prasenjit Guptasarma It has recently been suggested[1] that Fe-substituted PbTiO$_{3}$ can exhibit magnetoelectric multiferroic behavior. With an intent to examine whether Fe can fully substitute the lattice in Pb(Ti,Fe)O$_{3 }$and to study its effect on crystal structure, we have synthesized highly phase pure nanopowders from citric acid metal ion chelate complexes stabilized by glycerol in a sol gel. Using variety of probes, we demonstrate that Fe can substitute Ti up to at least 0.5 atoms per formula unit of Pb(Ti,Fe)O$_{3}$. Rietveld refinement of XRD data, from both laboratory and synchrotron sources, demonstrates that crystal structure of Fe substituted phases can be derived from the parent orthorhombic PbTiO$_{3}$ phase. Increasing concentration of Fe up to x=0.3 results in drastic change in lattice parameters and decrease in orthorhombic distortion. These results are supported by detailed studies of XRD, TEM and XAFS. \\[3pt] [1] Palkar et al, Appl. Phys. Lett. 90(2007)172901. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z30.00008: Electric polarization and magneto-dielectric effect in charge ordered system with frustrated geometry Tsutomu Watanabe Recently discovered multiferroics materials, where electric polarization and non-collinear spin structure coexist, are recognized as ferroelectric materials driven by spin ordering. There is another class of ferroelectricity where the electric polarization is attributed to the electronic charge ordering. Layered iron oxide LuFe$_{2}$O$_{4}$ belongs to this class of material. Ferroelectric transition occurs at almost the same temperature with the charge ordering one where Fe$^{2+}$ and Fe$^{3+}$ are aligned in the paired triangular lattices. Therefore, it is expected that the frustration plays important roles on breaking of the space inversion symmetry. We study theoretically a possibility of the charge-driven ferroelectric transition, and, in particularly, focus on the electron quantum transfer effects. We analyzed the V-t model, where t and V are the transfer integral and the Coulomb repulsion between nearest-neighbor sites, respectively, by using the variational Monte Carlo method. We found that the quantum fluctuation and frustration tend to enhance the stabilization of a three-fold charge ordered state and electric polarization, although the polarization is small. In addition, we studied the effect of the spin ordering. It is found that, in some spin ordered structures, the electric polarization is stabilized accompanying the three-fold charge ordered state. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z30.00009: Charge order, dynamics, and magneto-structural transition in multiferroic LuFe$_2$O$_4$ Xiaoshan Xu, Manuel Angst, Tatiana Brinzari, Raphael Hermann, Janice Musfeldt, Andy Christianson, David Mandrus, Brian Sales, Steve McGill, Jong-Woo Kim, Zahirul Islam We investigated the series of temperature and field-driven transitions in LuFe$_2$O$_4$ by optical and M\"{o}ssbauer spectroscopies, magnetization, and x-ray scattering in order to understand the interplay between charge, structure, and magnetism in this multiferroic material. We demonstrate that charge fluctuation has an onset well below the charge ordering transition, supporting the ``order by fluctuation'' mechanism for the development of charge order superstructure. Bragg splitting and large magneto optical contrast suggest a low temperature monoclinic distortion that can be driven by both temperature and magnetic field. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z30.00010: Electronic approaches on orbital anisotropy and spin configuration in Multiferroic LuFe$_{2}$O$_{4}$ K.-T. Ko, H.-J. Noh, B.-G. Park, J.-Y. Kim, J.-H. Park, A. Tanaka, Sung-Baek Kim, S.-W. Cheong The orbital anisotropy and spin configuration of the multiferroic LuFe$_{2}$O$_{4}$ are investigated by the x-ray absorption spectroscopy (XAS) at Fe L$_{2,3}$- and O K-edges, and the theoretical cluster model calculations including the configuration interactions and full multiplets. The x-ray magnetic circular dichroism (XMCD) results show that the system has a surprisingly large orbital moment as large as $m_{O} \quad \sim $ 0.8 $\mu _{B}$/f.u., which also agrees with the theoretical model calculation result. This result also well explains the observed total magnetic moment of 2.9 $\mu _{B}$/f.u.. The polarization dependent XAS enables us to identify the orbital level and occupation, which turns out to be rather different from the band structure prediction. We also found that the polar charge ordering plays an essential role for the 0.7 eV charge gap. Finally, we discuss about the local electronic structure, orbital anisotropy, and the spin configuration of LuFe$_{2}$O$_{4}$. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z30.00011: Multiple magnetic transitions and glass dynamics in LuFe2O4 Hari Srikanth, Manh-Huong Phan, Natalie Frey, Manuel Angst, Brian Sales, David Mandrus LuFe$_{2}$O$_{4}$ is a complex oxide of topical interest as ferroelectricity in this material arises from charge ordering and it also exhibits multiferroic behavior. A good understanding of the magnetic phase diagram has remained elusive primarily due to the complexity of the system as well as the sensitivity to sample quality. In this study, we report on the magnetic properties of high quality LuFe$_{2}$O$_{4}$ single crystals grown by the floating zone method. Temperature dependent DC, AC susceptibility and isothermal magnetization reveal a rich and complex phase diagram. Magnetic transitions at 240K and 175K are accompanied by strong frequency dependence of the real ($\chi $') and imaginary ($\chi $'') parts of the AC susceptibility indicative of glassy behavior. Quantitative fits to the glass model confirm cluster glass dynamics and this is consistent with the presence of ferrimagnetic domains within the Fe-O planes. Magnetocaloric effect (MCE) in these materials will also be presented. We will place our results in the context of work by other groups on this system and clarify the nature of the magnetic phase diagram that emerges from our studies. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z30.00012: Stacking the Collinear Magnetic Phases of the Geometrically-Frustrated Antiferromagnet CuFeO$_{2}$ Randy Fishman, Feng Ye, Jaime Fernandez-Baca The correct stacking of hexagonal layers is used to obtain accurate estimates for the exchange and anisotropy parameters of the geometrically-frustrated antiferromagnet CuFeO$_{2}$. Those parameters are highly constrained by the stability of a collinear metamagnetic phase between fields of 13.5 and 20 T. Constrained fits of the spin-wave frequencies of the ``up up down down'' phase below 7 T are used to identify the magnetic unit cell of the metamagnetic ``up up up down down'' phase, which contains two hexagonal layers and 10 Fe$^{3+}$ spins. The resulting exchange parameters are much smaller than those obtained from an unconstrained fit of the zero-field spin-wave data and successfully describe not only the main branch of spin-wave excitations but also the spin-wave excitations of the two twins in the (H,K,0) plane [1]. Research sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy under contract with UT-Battelle, LLC. \\[0pt] [1] R.S. Fishman, F. Ye, J.A. Fernandez-Baca, J.T. Haraldsen, and T. Kimura, \textit{Phys. Rev. B} \textbf{78}, 140407 (2008). [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z30.00013: Site occupancy and magnetic properties of aluminum substituted barium hexaferrite Amitava Moitra, Sungho Kim, Seong-Gon Kim, Yang-Ki Hong, Steven C. Erwin Aluminum substituted barium hexaferrite has been studied using density functional theory (DFT). The substitution has been carried out for BaFe$_{12-x}$Al$_{x}$O$_{19}$ from $x$ = 1 to $x$ = 3 in steps of 0.5. With the aid of accurate DFT study, our result show that the Al$^{3+}$ ions preferentially occupy the 2a and 12k site, unlike the previously reported 4f2, 2a, 4f1, and 12k sites. Our result confirms the experimental fact that with increasing of Al substitution the total magnetic moment monotonically decreases. We also present a possible reason of the site preference of 2a and 12k. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z30.00014: Magnetoelectric Coupling and Relaxation in Yttrium Iron Garnet Yuichi Yamasaki, Yuki Kohara, Yoshinori Tokura We report the results of the magnetic and dielectric relaxation measurement on yttrium iron garnet Y$_{3}$Fe$_{5}$O$_{12}$ which shows the 2nd order magneto-electric (ME) effect under an applied electric field [1] and the 1st order ME effect by an electric field cooling procedure [2]. We found that the temperature dependence of the dielectric relaxation dynamics coincides with that of magnetic one. This suggests a strong magneto-electric coupling between dielectric and magnetic relaxation dynamics. Indeed the dielectric relaxation strengths are enhanced by an applied magnetic field; namely the amplitude of electric dipole moment can be tuned by magnetic field. This effect accounts for the magnetic field induced change of the static dielectric permittivity and thereby the gigantic 2nd order ME effect as observed. [1] T.H. O'Dell, Phil. Mag. 16, 487 (1967), [2] H. Ogawa et al., JPSJ 56, 452 (1987). [Preview Abstract] |
Friday, March 20, 2009 2:03PM - 2:15PM |
Z30.00015: Dielectric and magnetic Properties of the Non-centrosymmetric Fe-Lagasite Nara Lee, Young Jai Choi, Sang-Wook Cheong The non-centrosymetric compound Ba$_{3}$NbFe$_{3}$Si$_{2}$O$_{14}$ known as the Fe-langasite forms in a unique magnetic triangular lattice of Fe$^{3+}$ spins. The interesting magnetic and dielectric properties may arise from the spin frustration of the triangular magnetic lattice as well as the broken inversion symmetry of the crystallographic structure. In order to understand the complex structure and magnetic spin ordering, we have performed comprehensive experiments on single crystals grown by a floating zone method, including x-ray diffraction analysis and measurements of magnetic susceptibility, dielectric constant and heat capacity under variation of temperature and magnetic field. [Preview Abstract] |
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