Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session P10: Focus Session: Multiferroics II |
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Sponsoring Units: FIAP Chair: Serge Nakhmanson, Argonne National Laboratory Room: 304 |
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P10.00001: Exchange interactions between soft ferromagnetic thin films and multiferroic BiFeO$_{3}$. Invited Speaker: Controlling the magnetization of a thin ferromagnetic (FM) film using an electric field is a Holy Grail of nowadays spintronics as it would revolutionize the addressing of magnetic memory elements. One strategy is to combine the magnetoelectric coupling of multiferroic materials like BiFeO$_{3}$ (BFO) [1] with the exchange coupling (EC) observed in FM / antiferromagnetic (AFM) systems such as in BFO/CoFeB bilayers [2]. BFO is a material of choice as it is one of the very few room-temperature AFM multiferroics. The two types of studied structures consist in FM layers of CoFeB deposited on BFO/STO films as well as thick permalloy layers sputtered onto BFO single crystals. They have been investigated by MagnetOptic Kerr Effect (MOKE) measurements. A macroscopic shift H$_{E}$ of the FM loops is a signature of exchange-bias (EB) in bilayers where the FM spins are coupled to the uncompensated AFM ones. We will show that the complex angular dependences of H$_{E}$ and H$_{C}$ result from the competition between the anisotropies of the FM and AFM layers and the strength of the EC. We will also compare the magnetic properties of the FM layers in relation with the ferroelectric structure of the underlying BFO. In their virgin state, the crystals are in a single ferroelectric and AFM domain with a cycloidal magnetic structure whereas thin films, in which the cyloid is suppressed, are in a highly multidomain state. This comparative study allows us to determine the nature and location of the spins involved in the mechanism of EC. Finally, we present the electric field effect on H$_{E}$ and H$_{C}$ of these systems. Our previous work on BFO crystals demonstrated that during electrical poling, any change of the polarisation direction induces a spin flop of the AFM moments. We will show here that a 90\r{ } rotation of the anisotropy axes can be obtained in domains where the polarisation is electrically flipped. \\[4pt] [1] PRL, \textbf{100}, 227602 (2008). \\[0pt] [2] PRL, \textbf{100}, 017204 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P10.00002: First principles study of magnetostructural instabilities in magnetic perovskite oxides Jun Hee Lee, Lucia Palova, Karin M. Rabe First principles phonon dispersion relations are reported for a range of magnetic perovskite oxides in cubic high-symmetry reference structures. Materials considered include EuTiO$_3$ and BiFeO$_3$. For each system, the dominant lattice instabilities are identified. These are frozen-in, singly and in combination, and the structures are optimized in the resulting space groups. From this, we identify distinct low-energy alternatives to the ground-state structure. We focus particularly on the dependence of the lattice instabilities and structural energetics of low-lying phases on the magnetic order, and extract key magnetostructural coupling parameters. The analysis is applied to predict possible structural and magnetic phase transitions as a function of epitaxial strain and/or of composition in low-concentration solid solutions ($A_{1-x}A'_xB$O$_3$, $AB_{1-x}B'_x$O$_3$, $A_{1-x}A'_xB_{1-y}B'_y$O$_3$ for small $x$,$y$). [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P10.00003: Absence of Critical Thickness in an Ultrathin Improper Ferroelectric Film Na Sai, Craig J. Fennie, Alexander A. Demkov We study the ferroelectric stability and surface structural properties of an oxygen-terminated hexagonal YMnO$_3$ ultra-thin film using density functional theory. Under an open circuit boundary condition, the ferroelectric state with the spontaneous polarization normal to the (0001) surface, is found to be metastable in a single domain state despite the presence of a depolarizing field. We establish a connection between the result and the role of improper ferroelectric transition. Our results imply that improper ferroelectric ultrathin films can have rather unique properties that are distinctive from those of very thin films of ordinary ferroelectrics. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P10.00004: Influence of step heights and terrace lengths of bottom electrodes on structural and ferroelectric properties of BiFeO$_{3}$ thin films Jae-Wan Park, Seung-Hyub Baek, Chang-Beom Eom The ferroelastic domain structures of epitaxial ferroelectric thin films are critical to control the ferroelectric properties. We have already demonstrated the selection of ferroelastic domain structure variants in epitaxial BiFeO$_{3}$ films and consequently significant improvement in ferroelectric switching behavior and leakage current by employing miscut in cubic (001) SrTiO$_{3 }$substrates. We have also observed significant step bunching in the SrRuO$_{3}$ bottom electrode and BiFeO$_{3}$ thin films on high miscut substrates resulting in significant surface roughness. In particular, the formation of step bunching causes different step heights and terrace lengths which can affect the growth behaviors of subsequent epitaxial films. We have studied the influence of step bunching of SrRuO$_{3}$ bottom electrodes on structural and ferroelectric properties of BiFeO$_{3}$ thin films grown on SrRuO$_{3}$ bottom electrodes by sputtering. We have varied the step heights and terrace lengths of SrRuO$_{3}$ bottom electrodes on 0.2$^{o}$ miscut (001) SrTiO$_{3}$ substrates by controlling of the growth conditions such as laser energy and pulse frequency in pulsed laser deposition. We will discuss the relationship between underlying step structures and the ferroelectric properties of epitaxial BiFeO$_{3}$ films. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P10.00005: Functional Properties at Domain Walls in BiFeO$_{3}$: Electrical, Magnetic, and Structural investigations Qing He, C.-H. Yang, P. Yu, M. Gajek, J. Seidel, R. Ramesh, F. Wang, Y.-H. Chu, L. W. Martin, N. Spaldin, A. Rother BiFeO$_{3}$ (BFO) is a widely studied robust ferroelectric, antiferromagnetic multiferroic. Conducting-atomic force microscopy studies reveal the presence of enhanced conductivity at certain types of domain walls in BFO. We have completed detailed TEM studies of the physical structure at these domain walls as well as in-depth DFT calculations of the evolution of electronic structure at these domain walls. These studies reveal two major contributions to the observed conduction: the formation of an electrostatic potential at the domain walls as well as a structurally-driven change in the electronic structure (i.e., a lower band gap locally) at the domain walls. We will discuss the use of optical characterization techniques as a way of probing this change in electronic structure at domain walls as well as detailed IV characterization both in atmospheric and UHV environments. Finally, the evolution of magnetism at these domain walls has been studied through the use of photoemission measurements. Initial findings point to a significant change in the magnetic order at these domain walls in BFO. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P10.00006: Electronic conduction in doped multiferroic BiFeO$_{3}$ Chan-Ho Yang, Jan Seidel, Sang-Yong Kim, M. Gajek, P. Yu, M.B. Holcomb, L.W. Martin, R. Ramesh, Y.H. Chu Competition between multiple ground states, that are energetically similar, plays a key role in many interesting material properties and physical phenomena as for example in high-$T_{c}$ superconductors (electron kinetic energy vs. electron-electron repulsion), colossal magnetoresistance (metallic state vs. charge ordered insulating state), and magnetically frustrated systems (spin-spin interactions). We are exploring the idea of similar competing phenomena in doped multiferroics by control of band-filling. In this paper we present systematic investigations of divalent Ca doping of ferroelectric BiFeO$_{3}$ in terms of structural and electronic conduction properties as well as diffusion properties of oxygen vacancies. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P10.00007: Ferroelectric switching behavior of (001) mono-domain BiFeO$_{3}$ thin films Seung-Hyub Baek, Ho-Won Jang, Chad Folkman, Chang-Beom Eom, Yulan Li, Benjamin Winchester, Long-qing Chen, Christopher Nelson, Xiao-qing Pan, Ramamoorthy Ramesh BiFeO$_{3}$ has drawn a great deal of attention as a single phase multiferroic material for the magnetoelectric device and non-volatile memory applications. BiFeO$_{3}$ has a magnetoelectric coupling effect between [111] polarization and (111) anti-ferromagnetic order, which allows manipulation of magnetic property by electric field. However, the anti-ferromagnetic plane can be switched only by non-180$^{o}$ polarization switching due to this coupling geometry. Thus, for magnetoelectric device applications, it is crucial to selectively control 71$^{o}$ (or 109$^{o})$ switching. Here, we report the selective control of ferroelectric switching by the size of switched area; local field by AFM tip and uniform field by large area top electrodes. The origin of this behavior will be discussed using phase-field simulations. This result implies that the geometry of magnetoelectric devices should be determined by considering the size of switching area. Moreover, this result can be expanded to other rhombohedral systems such as PMN-PT and PZT. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P10.00008: Systematic investigation of morphotropic phase boundaries in rare-earth doped BiFeO3 Daisuke Kan, Shigehiro Fujino, Vartharajan Anbusathaiah, Valanoor Nagarajan, Makoto Murakami, Sung–Hwan Lim, Arun Luykx, Dwight Hunter, Manfred Wuttig, Ichiro Takeuchi We have investigated structural and ferroelectric properties of BiFeO$_{3}$ doped with rare-earth (RE) elements La, Sm, Gd, Dy, Lu using thin film composition spreads. Previously, we had reported on discovery of a morphotropic phase boundary in (Bi,Sm)FeO$_{3}$.[1] Thin film composition spreads of (Bi,RE)FeO$_{3}$ were fabricated by combinatorial pulsed laser deposition. From scanning xray diffraction, rhombohedral to pseudo-orthorhombic structural transitions are observed. The composition at which the structural transition takes place changes with radii of the RE element: the smaller the ion, the smaller the substitution concentration. This shows that the chemical pressure effect is the cause of the transition. The ferroelectric - antiferroelectric transition is observed for RE doped BiFeO$_{3}$ with smaller radii than the Bi$^{3+}$ ion at the compositions which coincide with the structural transition for each RE dopant. The detailed correlation between the structural properties and ferroelectric and piezoelectric properties will be discussed. This work is supported by DMR, NSF DMR, ARO and the W. M. Keck Foundation.\\[3pt][1] S. Fujino et al., APL 92, 202904 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P10.00009: Ideal Nanocheckerboard $BiFeO_3-BiMnO_3$ from First Principles Lucia Palova, Karin Rabe, Premala Chandra Motivated by recent nanocheckerboard patternings of oxide materials, we use first principles calculations to characterize a prototypical atomic-scale checkerboard of $BiFeO_3-BiMnO_3$ and to compare its properties to those of its bulk constituents. We find this checkerboard has a multiferroic ground state with nonzero ferroelectric polarization and a nonzero magnetic moment, thereby combining desirable features of bulk $BiFeO_3$ and $BiMnO_3$. Unlike either of its parent compounds, structural distortion of the checkerboard stabilizes different magnetic states; this magnetostructural effect can be used to switch between states with zero and nonzero magnetization. The role of oxygen-octahedron rotations and strain in the magnetic ordering of the nanocheckerboard will be examined in detail. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P10.00010: Mechanism of Spontaneous Electric Polarization Flop in TbMnO$_{3}$ Hajime Sagayama, Nobuyuki Abe, Taka-hisa Arima, Kazuaki Iwasa Orthorhombic perovskite TbMnO$_{3}$ is one of the typical multiferroic systems. Spontaneous electric polarization (P) along the $c$-axis which originates from the spiral configuration of Mn$^{3+}$ spins rotating in the \textit{bc}-plane appears below $T_{C}$ ($\sim $27K). P//$c$ is turned to the direction along the a-axis by applying a magnetic field along $a$- or $b$-axis. Magnetic structure analysis and a spin-polarized neutron diffraction study of $^{160}$Gd$_{0.7}$Tb$_{0.3}$MnO$_{3}$ strongly suggest that P//$a$ in TbMnO$_{3}$ in high magnetic fields also originates from spin spiral rotating in the \textit{ab}-plane as in the case of P//$c$. It has been pointed out that anisotropic Tb $f$-electron magnetic moments play an important role for the complicated electric polarization flop. In this study, we have confirmed the change of spin basal plane of TbMnO$_{3}$ from the \textit{bc-} to \textit{ab-} plane by applying a magnetic field along the $b$-axis using spin-polarized neutron diffraction technique. We observed that a magnetic fields induce a C-type antiferromagnetic structure caused by the local anisotropy of Tb magnetic moments. We have succeeded in explaining the electric polarization flop of TbMnO$_{3}$ in terms of a coupling between Mn$^{3+}$ spins and anisotropic Tb magnetic moments. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P10.00011: Non-$d^0$ Mn-driven ferroelectricity in antiferromagnetic BaMnO$_3$ James Rondinelli, Nicola Spaldin Using first-principles density functional theory calculations we predict a ferroelectric ground state structure -- driven by off-centering of the magnetic Mn ion -- for perovskite-structure BaMnO$_3$. Our finding is surprising, since the competition between energy-lowering covalent bond formation, and energy-raising Coulombic repulsions (the 2nd order Jahn-Teller effect) usually only favors off-centering for non-magnetic $d^0$ ions. It is consistent, however, with the recent observation [S. Bhattacharjee, E. Bousquet and P. Ghosez, Arxiv e-prints {\bf 811}, 0811.2344 (2008)], that large lattice constants can stabilize polar off-centering of magnetic ions, by lowering the short-range Coulomb repulsions that favor the centrosymmetric phase. We calculate the Born effective charges for the compound, and find anomalously large values for Mn and O, consistent with the large calculated ferroelectric polarization of 30 $\mu$C/cm$^2$. We also describe the changes that occur in the electronic structure when the system transitions from a centrosymmetric to polar state. Finally, we suggest possible routes by which the cubic perovskite phase can be stabilized over the usual hexagonal phase. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P10.00012: Antiferromagnetic Resonance in Multiferroic YMnO$_3$ and LuMnO$_3$ Sergei Zvyagin Multiferroic rare-earth manganites have attracted much attention because of the coexistence of ferroelectric and magnetic orders. Combining conventional far-infrared Fourier-transform and THz-range free electron laser electron spin resonance (ESR) spectroscopy techniques, magnetic excitations in hexagonal multiferroic YMnO$_3$ and LuMnO$_3$ in the antiferromagnetically (AFM) ordered phase have been studied. The gap in the excitation spectrum ($\sim 42$ and $\sim 48$ cm$^{-1}$ for YMnO$_3$ and LuMnO$_3$, respectively) was observed directly. Similar slope of the frequency-field dependences of AFM resonance modes, $\sim$ 0.5 cm$^{-1}$/T, was revealed for both compounds. A fine structure of AFM resonance absorption has been revealed by means of high-resolution ESR techniques, which can be explained taking into account a finite interaction between the neighboring Mn$^{3+}$ layers. The work was done in collaboration with M. Ozerov, D. Kamensky, E. \v{C}i\v{z}m\'{a}r, J. Wosnitza, A.K. Kolezhuk, D. Smirnov, H.D. Zhou, and C.R. Wiebe. [Preview Abstract] |
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