Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W37: Focus Session: Complex Oxide Thin Films -- BiFeO3 Multiferroics |
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Sponsoring Units: DMP GMAG Chair: Ichiro Takeuchi, University of Maryland Room: E147-E148 |
Thursday, March 18, 2010 11:15AM - 11:51AM |
W37.00001: Electrical conduction at domain walls in multiferroic BiFeO3 Invited Speaker: We present recent results on electrical conductivity at ferroelectric domain walls in BiFeO3. The origin and nature of the observed conductivity is probed using a combination of conductive atomic force microscopy, high resolution transmission electron microscopy and first-principles density functional computations. We show that a structurally driven change in both the electrostatic potential and local electronic structure (i.e., a decrease in band gap) at the domain wall is linked to the observed electrical conductivity. Additionally, we observe an anomalous photovoltaic effect arising from structurally driven steps of the electrostatic potential that occur at ferroelectric domain walls. We explore ways of tuning the conductivity by chemical doping. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:03PM |
W37.00002: Neutron Diffraction Investigations of BiFeO$_3$ Films W. Ratcliff II, Daisuke Kan, Silvia Capelli, Gary McIntyre, Ichiro Takeuchi The study of the magnetic structure of bulk BiFeO$_3$ started with powders and evolved to single crystals when large ones became available. Single crystal studies definitively showed that the magnetic structure in the bulk was a long wavelength ``cycloid'' [4]. However, there have not been similarly detailed studies of the magnetic structure in thin films. Thus, to investigate how the thin film geometry affects the multiferroicity of BiFeO$_3$, we pursued a study of the magnetic structure of BiFeO$_3$ films as a function of film thickness and substrate orientation. BiFeO$_3$ thin films with thicknesses up to 1 micron were epitaxially grown on (100), (110), and (001) oriented SrTiO$_3$ substrates by PLD. We used D10 at the ILL to measure the 1 micron thick BiFeO$_3$ film for magnetic structure determination. Polarized and unpolarized neutron diffraction experiments were performed on BT9 and BT7 at the NCNR. We will discuss the change in the magnetic structure as a function of film thickness and orientation of the substrate. [Preview Abstract] |
Thursday, March 18, 2010 12:03PM - 12:15PM |
W37.00003: Origin of Modified Interface Dielectric Properties in BFO Thin Films Albina Borisevich, Hye Jung Chang, Mark Huijben, Mark Oxley, Satoshi Okamoto, Manish Niranjan, John Burton, Evgeny Tsymbal, Ying Hao Chu, Pu Yu, Ramamoorthy Ramesh, Sergei Kalinin, Stephen Pennycook BiFeO$_{3}$ - La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (BFO-LSMO) interfaces were investigated by high-resolution scanning transmission electron microscopy (STEM) revealing a local suppression of the octahedral tilts and local increase of the out-of-plane lattice parameter. The combination of direct structural mapping using bright field and annular dark field images with electron energy loss spectroscopy (EELS) was used to correlate the atomic structure, polarization, strain fields and dielectric behavior locally on the atomic level. EELS compositional mapping at the interfaces used principal component analysis combined with neural network interpolation. In the low loss energy region the BFO side of the interface could not be identified with BFO, LSMO or STO (SrTiO$_{3})$, suggesting that the first 2 nm of BFO has modified dielectric properties. First principles calculations show that the observed suppression of octahedral tilts results in a decreasing BFO band gap or possibly even metallic behavior. [Preview Abstract] |
Thursday, March 18, 2010 12:15PM - 12:27PM |
W37.00004: Reversible Electric Control of Exchange Bias in an Oxide Based Multiferroic Field Effect Device Stephen Wu, Shane Cybart, Pu Yu, R. Ramesh, R.C. Dynes We report the fabrication and measurement of multiferroic/ferromagnet, BiFeO$_{3 }$(BFO) / La$_{0.7}$Sr$_{0.3}$MnO$_{3 }$(LSMO), electric field effect devices. The antiferromagnetic (AFM) ordering of the BFO dielectric layer is coupled to the ferromagnetic (FM) ordering of the LSMO channel layer and is observed as exchange bias --a shift of the LSMO magnetic hysteresis curve along the applied field axis. Because BFO has coupled AFM and FE order parameters, it acts as an electrically controllable AFM pinning layer. This allows for the electric control of the FM properties of LSMO. By switching the FE polarization of BFO we observe a change in conductivity in the channel of over 100{\%}, and a 71{\%} change in magnetic coercivity at 5.5 K. Furthermore, we can reversibly switch between two distinct exchange bias states corresponding to the different FE polarizations. The difference in exchange bias between the two states is approximately 20mT. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 12:39PM |
W37.00005: Development of Electrically Controlled Magnetism in all Oxide Multiferroic Systems David Kirkwood, Jiwei Lu, Stuart Wolf Multiferroic materials have been shown to provide electrical control of magnetism through several different mechanisms. The coupling of ferroic properties in these materials holds promise for new material functionalities that will strongly impact technologies in several fields such as data storage, sensors and switches. In this study we have examined the growth of the multiferroic bismuth iron oxide (BiFeO$_{3})$ and the ferromagnetic oxide Fe$_{3}$O$_{4}$ with a novel pulsed electron deposition (PED) tool. An experimental phase space was built using available deposition parameter manipulation. Conditions have been found yielding single phase epitaxial BiFeO$_{3}$ and Fe$_{3}$O$_{4}$ as well as mixed phase (BiFeO$_{3}$ + Fe$_{3}$O$_{4})$ thin films on STO (100) template substrates. The magnetic properties and electrical field control of magnetism have been studied using MOKE microscopy. [Preview Abstract] |
Thursday, March 18, 2010 12:39PM - 12:51PM |
W37.00006: Switching of Ferroelectric Domains in Multiferroic BiFeO$_{3}$ Thin Films Joon-Hyuk Yang, Lourdes Salamanca-Riba We are investigating the switching of ferroelectric domains under different residual strains. We are investigating the role of strain in BiFeO$_{3}$ films on their ferroelectric properties and on the formation of polymorph phases within the films. We are also investigating the role of grain boundaries on the switching mechanism in these films. In addition, we are investigating if there is a gradient in the concentration of Fe$^{3+}$/Fe$^{2+}$ across grain boundaries and domain boundaries that could affect the switching behavior of these multiferroic materials. The BiFeO$_{3}$ films are grown using pulsed laser deposition, and characterized by X-ray diffraction and TEM. The strain is correlated to the ferroelectric properties of the films, These results will be related to the ferroelectric and ferromagnetic properties of the films in an attempt to understand the switching mechanism of ferroelectric domains under different amounts of strain. [Preview Abstract] |
Thursday, March 18, 2010 12:51PM - 1:03PM |
W37.00007: ABSTRACT WITHDRAWN |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W37.00008: Femtosecond spectroscopy of multiferroic BiFeO$_{3}$ thin films J. Zhang, E. Abreu, J.R. Schneck, D.S. Rana, I. Kawayama, M. Tonouchi, L. Ziegler, R.D. Averitt BiFeO$_{3}$ is a multiferroic material characterized by a room temperature antiferromagnetic and ferroelectric phase and a 2.6eV bandgap. Terahertz emission from BiFeO$_{3}$ thin films following excitation above bandgap by UV femtosecond pulses has been observed [1]. The THz emission is related to ultrafast depolarization of the ferroelectric order in the film as observed from the hysteresis of the amplitude of the emitted THz signal as a function of a biasing electric field. Experiments probing the electronic and lattice dynamics with sub-picosecond resolution are an essential step towards identifying the intrinsic mechanism responsible for depolarization and THz emission in BiFeO$_{3}$. In this study we investigate the carrier dynamics of epitaxial BiFeO$_{3}$ thin films using degenerate pump-probe spectroscopy at 400 nm with sub-50 fs pulses. We have observed a 5 THz coherent optical phonon oscillation consistent with the A$_{1}$ phonon mode as observed in Raman spectroscopy [2]. In this talk, the relation of the observed dynamics to the THz emission will be discussed. [1] D.S. Rana, et al., Advanced Materials 21, 2881 (2009). [2] S. Kamba, et al, Phys. Rev. B 75, 024403 (2007). [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W37.00009: Properties of diluted antiferromagnets from atomistic simulations Sergey Prosandeev, Laurent Bellaiche An effective Hamiltonian technique is used to study the effect of substituting Fe ions by non-magnetic elements (in a specific sublattice) on the properties of the (mostly) antiferromagnetic BiFeO$_3$ multiferroic material. Interestingly, for small concentrations of the non-magnetic element and at low temperature, this substitution creates a net magnetic moment (per volume), {\bf M}, that is directed along the antiferromagnetic moment {\bf L} (unlike in pure BiFeO$_3$), and that increases in size with the non-magnetic concentration. On the other hand, above a percolation threshold, the net magnetic moment diminishes in magnitude with the non-magnetic concentration while the magnetoelectric coupling is dramatically enhanced. The origin of these unexpected phenomema are discussed in detail. In particular, it is proposed that the enhancement of the magnetoelectric coupling is mostly due to an increase of the magnetic susceptibility. [Preview Abstract] |
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