Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session N21: Focus Session: BiFeO3 and Domain Wall Conductance |
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Sponsoring Units: DMP Chair: Alexei Belik, NIMS Japan Room: 323 |
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N21.00001: Current at domain walls, roughly speaking: nanoscales studies of disorder roughening and conduction Invited Speaker: Patrycja Paruch Domain walls in (multi)ferroic materials are the thin elastic interfaces separating regions with different orientations of magnetisation, electric polarisation, or spontaneous strain. Understanding their behaviour, and controlling domain size and stability, is key for their integration into applications, while fundamentally, domain walls provide an excellent model system in which the rich physics of disordered elastic interfaces can be accesses. In addition, domain walls can present novel properties, quite different from those of their parent materials, making them potentially useful as active components in future nano-devices. Here, we present our atomic force microscopy studies of ferroelectric domain walls in epitaxial Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_3$ and BiFeO$_3$ thin films, in which we use piezorespose force microscopy to show unusual domain wall roughening behaviour, with very localised disorder regions in the sample leading to a complex, multi-affine scaling of the domain wall shape [1]. We also show the effects of temperature, environmental conditions, and defects on switching dynamics and domain wall roughness [2]. We combine these observations with parallel conductive-tip atomic force microscopy current measurements, which also show highly localised variations in conduction, and highlight the key role played by oxygen vacancies in the observed domain wall conduction [3]. \\[4pt] [1] Guyonnet et al., PRL 109, 147601 (2012)\\[0pt] [2] Paruch et al, PRB 85, 214115 (2012); Blaser et al, APL. 101, 142906 (2012)\\[0pt] [3] Guyonnet et al., Adv. Mat. 25, 5377 (2011) [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N21.00002: Nanoscale spatial control of domain wall conductivity in BiFeO$_3$ thin films Brian Smith, Rama Vasudevan, Bouwe Kuiper, Andre ten Elshof, Art Baddorf, Peter Maksymovich, Sergei Kalinin, Mark Hujben, Guus Rijnders, Gertjan Koster Use of ferroelectric domain walls for applications is an attractive prospect. Domain walls can have properties not found in bulk allowing added functionality. The 1D nature of a ferroelectric domain wall could be exploited to create devices with dimensions on the order of a single unit cell. Intensive research on domain wall conductivity in BiFeO$_3$ is ongoing since the first report in 2009 [1]. Here we report on the spatial control of domain wall conductivity in an epitaxial grown BiFeO$_3$ film 25nm thick on self-assembled SrRuO$_3$ nanowires using an ordered mixed terminated DyScO$_3$ substrate as a growth template [2]. The SrRuO$_3$ nanowires (5nm high, 100nm wide separated by 200nm) run across the substrate and are contacted at the sample edge creating alternating insulating/conducting surfaces. Using PFM/cAFM the domains, switching and domain wall conductivity is explored. Domain wall conductivity is only present in over the nanowires. In addition to providing spatial control of the conductivity this result provides evidence that the conduction is confined to a single domain wall throughout the thickness of the film and is not the results of network of interconnected domains.\\[4pt] [1] Seidel J, et. al. Nat. Mat. 2009, 8, 229\\[0pt] [2] Kuiper et al., MRS Communications, Doi:10.1557/mrc.2011.8 [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N21.00003: Domain walls in a perovskite oxide with two primary structural order parameters: first-principles study of BiFeO$_3$ Oswaldo Di\'eguez, Pablo Aguado-Puente, Javier Junquera, Jorge \'I\~niguez We present a first-principles study of ferroelectric domain walls (FE-DWs) in multiferroic BiFeO$_3$ (BFO), a material in which the FE order parameter coexists with anti-ferrodistortive (AFD) modes involving rotations of the O$_6$ octahedra. We find that the energetics of the DWs are dominated by the capability of the domains to match their O$_6$ octahedra rotation patterns at the plane of the wall, so that the distortion of the oxygen groups is minimized. Our results thus indicate that, in essence, it is the discontinuity in the AFD order parameter, and not the change in the electric polarization, what decides which crystallographic planes are most likely to {\em host} BFO's FE-DWs. Such a result clearly suggests that the O$_6$ rotational patterns play a primary role in the FE phase of this compound, in contrast with the usual (implicit) assumption that they are subordinated to the FE order parameter. Interestingly, we find that the structure of BFO at the most stable DWs resembles the atomic arrangements that are characteristic of low-lying (meta)stable phases of the material. Our work thus contributes to shape a coherent picture of the structural variants that BFO can present and the way in which they are related. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N21.00004: Ferroelectric Tunnel Junctions Based on Pseudotetragonal BiFeO$_{3}$ Flavio Y. Bruno, S. Boyn, V. Garcia, S. Fusil, H. Yamada, C. Carretero, C. Deranlot, E. Jacquet, K. Bouzehouane, S. Xavier, J. Grollier, M. Bibes, A. Barthelemy The concept of a ferroelectric tunnel junction (FTJ) was formulated in the early 70s by Esaki et al. It took more than 30 years to realize this idea experimentally in a reliable and reproducible manner[1]. FTJs have shown to be versatile devices and the possibility to use them as memories [2] and memristors [3] have been recently demonstrated on BaTiO$_{3}$ based junctions. With the aim of expanding its functionalities we have realized FTJ with multiferroic pseudotetragonal BiFeO$_{3}$ (T-BFO) tunnel barriers. In order to fabricate junctions we deposited fully epitaxial bilayers consisting of a LaNiO$_{3}$ or doped CaMnO$_{3}$ bottom electrodes and the T-BFO tunnel barriers. On top of this bilayers, Co/Au electrodes as small as 200 nm in diameter were defined by e-beam lithography and lift-off. We have measured ON/OFF ratios as large as 10000 on these junctions, much larger than that observed in FTJs with BaTiO$_{3}$ tunnel barriers. We will show that the resistance of the FTJ in its high, low and intermediate states is related with the polarization state of the barrier as observed by PFM. [1]Nature 460,81(2009). [2]Nat. Nanotech. 7, 101 (2011). [3] Nature Mat. 11, 860 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N21.00005: THz spectroscopy of spin waves in multiferroic BiFeO$_3$ in high magnetic fields Urmas Nagel, T. R\~o\~om, H. Engelkamp, D. Talbayev, H.T. Yi, S.-W. Cheong BiFeO$_3$ is both antiferromagnetic and ferroelectric with high N\`eel and Curie temperatures, about 640\,K and 110\,K, respectively. In low magnetic field Fe$^{3+}$ spins order cycloidally, inducing an additional electric polarization, which interacts with the feeroelctric polarization of the lattice and produces a magneto-electric term in the total energy. We have measured the magnetic field dependence of infrared active magnon modes in an untwinned BiFeO$_3$ single crystal at 4K. The magnon modes soften close to the critical field of about 18.8T along the [001] cubic axis, where the cycloid is destroyed and the low field magnon modes disappear. A new strong mode with linear magmetic field dependence appears above 19T and persists at least up to 31T. The dramatic change of the THz spectrum at 19T allows us to assign all the low field modes as excitations of the cycloid. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N21.00006: Strain dependence of transition temperatures, structural symmetry, and phase coexistence of BiFeO3 within the tetragonal-like structure Wolter Siemons, Christianne Beekman, Gregory MacDougall, Adam Aczel, Michael Biegalski, Jerel Zarestky, Shuhua Liang, Elbio Dagotto, Steve Nagler, Hans Christen We have investigated the influence of strain-imposed in-plane lattice symmetry on the structural and magnetic properties of tetragonal-like BiFeO$_{3}$. We find that an increase in the in-plane distortion results in an increase of the N\'{e}el temperature from 313 K to 324 K for films grown on YAlO$_{3}$ and LaAlO$_{3}$ respectively. The change in magnetic ordering temperature is reproduced in 3D Heisenberg Monte-Carlo simulations. The structural transition temperatures, from M$_{C}$ to M$_{A}$ monoclinic around 100 $^{\circ}$C and to a true tetragonal phase at higher temperature, are also found to depend on strain. Some of the strain is relieved through the creation of an additional polymorph, which causes stripe patterns in the surface morphology. We present how the abundance and shape of these patterns changes with the amount and symmetry of strain. These results show strain cannot be treated as a single scalar number or simply as a direct consequence of the lattice mismatch between the film material and the substrate. Research supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division, and performed in part at ORNL's Spallation Neutron Source and Center for Nanophase Materials Sciences (sponsored by DOE-BES). [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N21.00007: Local conductivity in supertetragonal and rhombohedral-like BiFeO$_{3}$ films Saeedeh Farokhipoor, Christianne Beekman, Wolter Siemons, Hans M. Christen, Beatriz Noheda Materials in which structural polymorphs coexist are of great interest in the design of magnetoelectric devices and piezoactuators at the nanoscale. In BiFeO$_{3}$, coexisting polymorphs are stabilized in thin film form by strain resulting from film/substrate lattice mismatch and/or thermal expansion differences. In films on LaAlO$_{3}$ substrates, these polymorphic phases give rise to stripe patterns; they are formed by the coexistence of the highly-strained (T') phase with an intermediary polymorph (S') in samples devoid of the rhombohedral-like relaxed (R') structure. Here, we investigate the local properties of the stripe patterns by piezoresponse force microscopy and conductive atomic force microscopy. This makes it possible to investigate the local conductivity both of specific domains and of different domain walls, and to compare the results to those obtained for R'-BiFeO$_{3}$ films (on SrTiO$_{3}$ substrates). We show that patterns of locally varying polarization and conductivity can be reversibly written and erased at length scales determined by the phase stability of the strain-induced structural polymorphs, and illustrate similarities and differences between R' and T' BiFeO$_{3}$. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N21.00008: Temperature dependent dielectric and ferroelectric studies of BiFeO3 thin film Anand P.S. Gaur, Sujit K. Barik, Ram S. Katiyar Although BiFeO$_{3}$ (BFO) has received a lot of interest due to its good multiferroic properties at room temperature, high leakage current limit its usage for practical applications. Recently, it is found that these properties in thin films can be different due to strain effect induced by substrate, preparation conditions and electrode effects, etc. In this context, we have studied the temperature dependence of polarization and dielectric properties of BFO thin film by varying the bottom electrode thickness and using different electrodes. The strain dependent ferroelectric switching behaviors have also been investigated with a traditional ferroelectric tester and switching spectroscopy piezoresponse force microscopy (SS-PFM), respectively. We used pulsed laser deposition to fabricate thin films of BFO using Si (100) substrate and SrTiO$_{3\, }$(STO) as buffer layer with different bottom electrodes such as SrRuO$_{3\, }$(SRO), LaNiO$_{3\, }$(LNO) and Pt/Si. The thickness of STO layer is kept fixed around 70 nm and the thicknesses of BFO and electrode layer were varied from 70 nm to 200nm. The layers were grown under optimized conditions and polycrystalline nature is found from room temperature XRD. A large enhancement of polarization is found while using LNO electrode and also with reducing the thickness of BFO layer. The remnant polarization and cohesivity also shows large increase with increaisng temperature, although leakage current increases significantly. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N21.00009: Temperature dependence of acoustic and low-energy optic phonons in the multiferroic BiFeO3 studied by inelastic neutron scattering Guangyong Xu, Zhijun Xu, John Shneelock, Peter Gehring, Chris Stock, Masaaki Matsuda, Genda Gu, T. Ito, Jinsheng Wen, R.J. Birgeneau, Stephen Shapiro We report inelastic neutron scattering measurements on the acoustic and low-energy phonons in the multiferroic material BiFeO$_3$. The phonon dispersion in the (200) and (111) zones have been mapped out for temperatures between 300K to 750K. The temperature dependence of the dispersion and phonon intensities will be discussed. Possible connections between the the antiferromagnetic phase transition at 640K and anomalies in the phonon modes are observed. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N21.00010: Phase coexistence, phase transitions, and piezoelectric switching in highly-strained BiFeO$_3$ Hans M. Christen, C. Beekman, W. Siemons, M. Chi, J.Y. Howe, M.D. Biegalski, N. Balke, P. Maksymovych, T.Z. Ward, A.K. Farrar, J.B. Romero, D. Tenne Highly strained ($T'$) BiFeO3 films are investigated as function of temperature by x-ray diffraction in combination with atomic-force, piezo-response force, and transmission electron microscopies. In these films on LaAlO$_3$ substrates, the coexistence of the $T'$ majority phase (c/a $\sim$ 1.25) with an intermediary $S'$ polymorph (c/a $\sim$ 1.09) leads to the formation of stripe patterns in samples where the bulk-like, nearly rhombohedral $R'$ polymorph is absent. While $T'$ films at 300K are monoclinic, our results reveal a true tetragonal high-temperature phase (at T$\geq$700K) for which Raman spectroscopy demonstrates a polar nature. However, piezoelectric switching of the $T'$ phase is possible only in the presence of the $S'$ polymorph. This polymorph, and the stripe patterns that result from its coexistence with the $T'$ form appear after growth upon cooling below $\sim$570K. This shows that the $S'$ polymorph is formed by additional stress resulting from the differences in thermal expansion between film and substrate. These results point to new approaches for tuning functional properties in materials exhibiting strain-induced polymorphic phase transitions. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N21.00011: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N21.00012: Symmetry of Epitaxial BiFeO$_3$ Films in the Ultrathin Regime Yongsoo Yang, Christian Schlep\"{u}tz, Carolina Adamo, Darrell Schlom, Roy Clarke BiFeO$_3$ (BFO) films grown on SrTiO$_3$ (STO) with a SrRuO$_3$ buffer layer exhibit a monoclinic structure at thicknesses greater than 40 nm, but higher structural symmetry can be observed for thinner films [Phys. Rev. B 81, 144115 (2010)]. We report a structural phase transition from monoclinic to tetragonal in ultra-thin BFO films grown directly on (100)-oriented STO. X-ray diffraction measurements of 3-dimensional reciprocal space maps reveal half-integer order peaks due to oxygen octahedral tilting. When the film thickness is decreased below 20 unit cells, the integer-order Bragg peak splitting associated with the presence of multiple domains of the monoclinic phase disappears. Instead, a single peak that is commensurate with the STO substrate lattice appears. The diffraction pattern has four-fold symmetry, ruling out the presence of a single monoclinic domain in favor of a tetragonal film structure. The evolution of the oxygen octahedra tilt pattern inferred from the intensities of half-order peaks suggests that this transition originates from the corner-connectivity of oxygen atoms at the interface between BFO and STO, and also strongly supports this monoclinic to tetragonal transition. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N21.00013: First-Principles Calculation of the Bulk Photovoltaic Effect in Bismuth Ferrite Steve Young, Fan Zheng, Andrew Rappe Bismuth ferrite is a multiferroic material with a large bulk polarization and a band gap in the visible spectrum. Significant anomalous photovoltaic effects have been observed in the material; however, the origins of this effect are unclear. While some investigations indicate that observed photovoltages and photocurrents are due to the bulk photovoltaic effect, in striped polydomain samples there is no evidence of this, and the observed response is attributed to a domain-wall-driven mechanism. We have computed the bulk photovoltaic response from first principles using shift current theory and compared it to the available experimental data, finding good agreement. By accounting for the geometry of the polydomain samples, we are able to explain the lack of observed bulk photovoltaic response. Furthermore, we show that these two mechanisms act antagonistically, suggesting that enhanced efficiency may be found in materials where these two effects interact cooperatively. [Preview Abstract] |
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