Browsing by Author "Chu, Y. H."

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Anisotropic strain, magnetic properties, and lattice dynamics in self-assembled multiferroic CoFe2O4-PbTiO3 nanostructures
(2014-04-07)
Tsai, C. Y.
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Chen, H. R.
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Chang, F. C.
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Kuo, H. H.
;
Cheng, H. M.
We investigate the anisotropic stress dependent magnetic and phonon behaviors in self-assembled CoFe2O4-PbTiO3 (CFO-PTO) nanostructures deposited on SrRuO3 buffered SrTiO3 substrates of various thickness. The increased vertical compressed stress with increasing thickness enhances the vertical magnetic anisotropy of CFO while gradually reducing the vertical polarization of PTO. By applying the magnetic-field dependent Raman scattering, the CFO-A(1g) and T-2g(1) phonon frequencies shift oppositely because of the magnetostriction. Moreover, the PTO-A(1) mode intensities anomalously enhance and the A(1) and E mode frequencies, respectively, increase and decrease, which prove the existence of the stress-mediated magnetoelectric (ME) effect. This study shows that the ME transmission process is dominated by interfacial stress, which is critical for enhancing the ME efficiency. (C) 2014 AIP Publishing LLC.
Atomic-scale evolution of modulated phases at the ferroelectric-antiferroelectric morphotropic phase boundary controlled by flexoelectric interaction
(2012-04-01)
Borisevich, A. Y.
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Eliseev, E. A.
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Morozovska, A. N.
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Cheng, C. -J.
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Lin, J. -Y.
Physical and structural origins of morphotropic phase boundaries (MPBs) in ferroics remain elusive despite decades of study. The leading competing theories employ either low-symmetry bridging phases or adaptive phases with nanoscale textures to describe different subsets of the macroscopic data, while the decisive atomic-scale information has so far been missing. Here we report direct atomically resolved mapping of polarization and structure order parameter fields in a Sm-doped BiFeO3 system and their evolution as the system approaches a MPB. We further show that both the experimental phase diagram and the observed phase evolution can be explained by taking into account the flexoelectric interaction, which renders the effective domain wall energy negative, thus stabilizing modulated phases in the vicinity of the MPB. Our study highlights the importance of local order-parameter mapping at the atomic scale and establishes a hitherto unobserved physical origin of spatially modulated phases existing in the vicinity of the MPB.
Atomically Resolved Electronic States and Correlated Magnetic Order at Termination Engineered Complex Oxide Heterointerfaces
(2018-02-01)
Huang, Bo-Chao
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Yu, Pu
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Chu, Y. H.
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Chang, Chia-Seng
;
Ramesh, Ramamoorthy
We map electronic states, band gaps, and interface-bound charges at termination-engineered BiFeO3/La0.7Sr0.3MnO3 interfaces using atomically resolved cross-sectional scanning tunneling microscopy. We identify a delicate interplay of different correlated physical effects and relate these to the ferroelectric and magnetic interface properties tuned by engineering the atomic layer stacking sequence at the interfaces. This study highlights the importance of a direct atomically resolved access to electronic interface states for understanding the intriguing interface properties in complex oxides.
Conduction control at ferroic domain walls via external stimuli
(2014)
Yang, J. C.
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Yeh, C. H.
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Chen, Y. T.
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Liao, S. C.
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Huang, R.
Intriguing functionalities at nano-sized domain walls have recently spawned a new paradigmfor developing novel nanoelectronics due to versatile characteristics. In this study, we explore a new scenario to modulate the local conduction of ferroic domain walls. Three controlling parameters, i.e., external electrical field, magnetic field and light, are introduced to the 90 degrees domain walls (90 degrees DWs) of BiFeO3. Electrical modulation is realized by electrical transport, where the mobility of 90 degrees DWs can be altered by gating voltage. We further use the ferromagnetic/antiferromagnetic coupling to reveal the inherent magnetism at the DWs. With an established magnetic nature, magneto-transport has been conducted to introduce magnetic controlling parameter, where a giant positive magnetoresistance change can be observed up to 200%. In addition, light modulated conduction, a core factor for multifunctional applications, is successfully demonstrated (current enhancement by a factor of 2 with 11 W white lamp). These results offer new insights to discover the tunability of domain wall nanoelectronics.
Correlation between nanoscale and nanosecond resolved ferroelectric domain dynamics and local mechanical compliance
(2011-05-01)
Polomoff, N. A.
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Rakin, A.
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Lee, S.
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Palumbo, V.
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Yu, P.
The local dynamics of ferroelectric domain polarization are uniquely investigated with sub-20-nm resolved maps of switching times, growth velocities, and growth directions. This is achieved by analyzing movies of hundreds of consecutive high speed piezo force microscopy images, which record domain switching dynamics through repeatedly alternating between high speed domain imaging and the application of 20-nanosecond voltage pulses. Recurrent switching patterns are revealed, and domain wall velocities for nascent domains are uniquely reported to be up to four times faster than for mature domains with radii greater than approximately 100 nm. Switching times, speeds, and directions are also shown to correlate with local mechanical compliance, with domains preferentially nucleating and growing in compliant sample regions while clearly shunting around locations with higher stiffness. This deterministic switching behavior strongly supports a defect-mediated energy landscape which controls polarization reversal, and that can therefore be predicted, modeled, and even manipulated through composition, processing, and geometry. Such results have important implications for the practical performance of ferroelectric devices by enabling guided optimization of switching times and feature densities, while the methods employed provide a new means to investigate and correlate dynamic functionality with mechanical properties at the nanoscale. (C) 2011 American Institute of Physics. [doi:10.1063/1.3581205]
Deterministic control of ferroelastic switching in multiferroic materials
(2009-12-01)
Balke, N.
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Choudhury, S.
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Jesse, S.
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Huijben, M.
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Chu, Y. H.
Multiferroic materials showing coupled electric, magnetic and elastic orderings provide a platform to explore complexity and new paradigms for memory and logic devices. Until now, the deterministic control of non-ferroelectric order parameters in multiferroics has been elusive. Here, we demonstrate deterministic ferroelastic switching in rhombohedral BiFeO(3) by domain nucleation with a scanning probe. We are able to select among final states that have the same electrostatic energy, but differ dramatically in elastic or magnetic order, by applying voltage to the probe while it is in lateral motion. We also demonstrate the controlled creation of a ferrotoroidal order parameter. The ability to control local elastic, magnetic and torroidal order parameters with an electric field will make it possible to probe local strain and magnetic ordering, and engineer various magnetoelectric, domain-wall-based and strain-coupled devices.
Direct observation of ferroelectric polarization-modulated band bending at oxide interfaces
(2012-03-19)
Huang, B. C.
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Chen, Y. T.
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Chiu, Y. P.
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Huang, Y. C.
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Yang, J. C.
This study presents a direct visualization of the influences of ferroelectric polarization on the electronic properties of the Schottky contact at the Nb-SrTiO3/BiFeO3 hetero-interface using scanning tunneling microscopy and spectroscopy (STM/S). The evolution of the local density of states across the Nb-SrTiO3/BiFeO3 interface reveals the interfacial band alignment and the characteristic quantities of the metal/ferroelectric contact. The unique combination of STM and STS in this study delivers an approach to obtain critical information on the interfacial electronic configurations of ferroelectric oxide interfaces and also their variation with ferroelectric polarization switching. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3691615]
Distribution of electronic reconstruction at the n-type LaAlO(3)/SrTiO(3) interface revealed by hard x-ray photoemission spectroscopy
(2011-12-26)
Chu, Y. Y.
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Liao, Y. F.
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Tra, V. T.
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Yang, J. C.
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Liu, W. Z.
We investigated the electronic reconstruction at the n-type LaAlO(3)/SrTiO(3) interface with hard x-ray photoelectron spectroscopy (HAXPES) under grazing incidence. By exploiting the collapse of evanescent x-ray waves and the abrupt increase of x-ray absorption at the critical incidence angle, our HAXPES study reveals a 2% electronic reconstruction from Ti(4+) to Ti(3+) occurring near the interface. Such an electronic reconstruction also extends from the interface into SrTiO(3) with a depth of about 48 angstrom (similar to 12 unit cells) and an estimated total charge transfer of similar to 0.24 electrons per two-dimensional unit cell. (C) 2011 American Institute of Physics. [doi:10.1063/1.3672099]
Distribution of electronic reconstruction at the n-type LaAlO3/SrTiO3 interface revealed by hard x-ray photoemission spectroscopy
(2011-12-26)
Chu, Y. Y.
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Liao, Y. F.
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Tra, V. T.
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Yang, J. C.
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Liu, W. Z.
We investigated the electronic reconstruction at the n-type LaAlO3/SrTiO3 interface with hard x-ray photoelectron spectroscopy (HAXPES) under grazing incidence. By exploiting the collapse of evanescent x-ray waves and the abrupt increase of x-ray absorption at the critical incidence angle, our HAXPES study reveals a 2% electronic reconstruction from Ti4+ to Ti3+ occurring near the interface. Such an electronic reconstruction also extends from the interface into SrTiO3 with a depth of about 48 angstrom (similar to 12 unit cells) and an estimated total charge transfer of similar to 0.24 electrons per two-dimensional unit cell. (C) 2011 American Institute of Physics. [doi:10.1063/1.3672099]
Domain growth dynamics in single-domain-like BiFeO(3) thin films
(2009-03-23)
Chen, Y. C.
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Lin, Q. R.
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Chu, Y. H.
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材料科學與工程學系
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Department of Materials Science and Engineering
We present a quantitative study of 180 degrees domain wall motion in epitaxial BiFeO(3) (111) films, which can be treated as a nearly ideal single-domain environment. The domains were dynamically written by applying voltage pulses and examined by the piezoresponse force microscope technique. A transition of domain growth behaviors from the activated type to the nonactivated type was observed when increasing the pulse voltages. The obtained activation field was close to the ideally thermodynamic switching field of BiFeO(3). The asymmetry of activated fields showed the preference of the downward polarization in the BiFeO(3)/SrRuO(3) films.
Domain growth dynamics in single-domain-like BiFeO3 thin films
(2009-03-23)
Chen, Y. C.
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Lin, Q. R.
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Chu, Y. H.
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材料科學與工程學系
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Department of Materials Science and Engineering
We present a quantitative study of 180 degrees domain wall motion in epitaxial BiFeO3 (111) films, which can be treated as a nearly ideal single-domain environment. The domains were dynamically written by applying voltage pulses and examined by the piezoresponse force microscope technique. A transition of domain growth behaviors from the activated type to the nonactivated type was observed when increasing the pulse voltages. The obtained activation field was close to the ideally thermodynamic switching field of BiFeO3. The asymmetry of activated fields showed the preference of the downward polarization in the BiFeO3/SrRuO3 films.
Domain pattern and piezoelectric response across polymorphic phase transition in strained bismuth ferrite films
(2010-12-13)
Kuo, H. Y.
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Shu, Y. C.
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Chen, H. Z.
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Hsueh, C. J.
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Wang, C. H.
A model is developed to investigate the domain pattern and piezoelectric response across the polymorphic phase transition in strained epitaxial bismuth ferrite films. The orientations of stripelike pattern of the mixed rhombohedral and tetragonal phases are predicted as the consequence of competition between elastic and depolarization energies. The abnormally large piezoelectric response is attributed to the strain-driven softening in dielectric stiffness. The results are in good agreement with recent experimental observations and provide a guidance for developing similar strain-driven polymorphic phase transition in other related perovskite material systems. (C) 2010 American Institute of Physics. [doi:10.1063/1.3525926]
Dynamic magnetic interaction in La2/3Sr1/3MnO3/BiFeO3 heterostructure
(2014-09-15)
Guo, H. Y.
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Lin, J. G.
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Yang, Jan Chi
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Chu, Y. H.
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材料科學與工程學系
The La2/3Sr1/3MnO3/BiFeO3 (LSMO/BFO) heterostructure is studied as probed by temperature-dependent ferromagnetic resonance (FMR). Consistent with the structure analysis, two FMR modes are found for both a single LSMO layer and a LSMO/BFO bilayer grown on DyScO3, corresponding to the strained and relaxed phases of LSMO. The FMR linewidth intensity vs temperature curve shows two maxima at 200 K and 140 K in the strained phase of LSMO/BFO bilayer, which is related to the magnetic excitations in BFO. Our results demonstrate a dynamic spin interaction between LSMO and BFO, suggesting an effective route to manipulate the magnetic properties of oxide heterostructures. (C) 2014 AIP Publishing LLC.
Electric modulation of conduction in multiferroic Ca-doped BiFeO(3) films
(2009-06-01)
Yang, C. -H.
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Seidel, J.
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Kim, S. Y.
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Rossen, P. B.
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Yu, P.
Many interesting materials phenomena such as the emergence of high-T(c) superconductivity in the cuprates and colossal magnetoresistance in the manganites arise out of a doping-driven competition between energetically similar ground states. Doped multiferroics present a tantalizing evolution of this generic concept of phase competition. Here, we present the observation of an electronic conductor-insulator transition by control of band-filling in the model antiferromagnetic ferroelectric BiFeO(3) through Ca doping. Application of electric field enables us to control and manipulate this electronic transition to the extent that a p-n junction can be created, erased and inverted in this material. A 'dome-like' feature in the doping dependence of the ferroelectric transition is observed around a Ca concentration of similar to 1/8, where a new pseudo-tetragonal phase appears and the electric modulation of conduction is optimized. Possible mechanisms for the observed effects are discussed on the basis of the interplay of ionic and electronic conduction. This observation opens the door to merging magnetoelectrics and magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom already present in the multiferroic BiFeO(3).
Electric modulation of conduction in multiferroic Ca-doped BiFeO3 films
(2009-06-01)
Yang, C. -H.
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Seidel, J.
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Kim, S. Y.
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Rossen, P. B.
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Yu, P.
Many interesting materials phenomena such as the emergence of high-T-c superconductivity in the cuprates and colossal magnetoresistance in the manganites arise out of a doping-driven competition between energetically similar ground states. Doped multiferroics present a tantalizing evolution of this generic concept of phase competition. Here, we present the observation of an electronic conductor-insulator transition by control of band-filling in the model antiferromagnetic ferroelectric BiFeO3 through Ca doping. Application of electric field enables us to control and manipulate this electronic transition to the extent that a p-n junction can be created, erased and inverted in this material. A 'dome-like' feature in the doping dependence of the ferroelectric transition is observed around a Ca concentration of similar to 1/8, where a new pseudo-tetragonal phase appears and the electric modulation of conduction is optimized. Possible mechanisms for the observed effects are discussed on the basis of the interplay of ionic and electronic conduction. This observation opens the door to merging magnetoelectrics and magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom already present in the multiferroic BiFeO3.
Electromechanical Coupling among Edge Dislocations, Domain Walls, and Nanodomains in BiFeO3 Revealed by Unit-Cell-Wise Strain and Polarization Maps
(2013-04-01)
Lubk, A.
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Rossell, M. D.
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Seidel, J.
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Chu, Y. H.
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Ramesh, R.
The performance of ferroelectric devices, for example, the ferroelectric field effect transistor, is reduced by the presence of crystal defects such as edge dislocations. For example, it is well-known that edge dislocations play a crucial role in the formation of ferroelectric dead-layers at interfaces and hence finite size effects in ferroelectric thin films. The detailed lattice structure including the relevant electromechanical coupling mechanisms in close vicinity of the edge dislocations is, however, not well-understood, which hampers device optimization. Here, we investigate edge dislocations in ferroelectric BiFeO3 by means of spherical aberration-corrected scanning transmission electron microscopy, a dedicated model-based structure analysis, and phase field simulations. Unit-cell-wise resolved strain and polarization profiles around edge dislocation reveal a wealth of material states including polymorph nanodomains and multiple domain walls characteristically pinned to the dislocation. We locally determine the piezoelectric tensor and identify piezoelectric coupling as the driving force for the observed phenomena, explaining, for example, the orientation of the domain wall with respect to the edge dislocation. Furthermore, an atomic model for the dislocation core is derived.
Emergent phenomena at multiferroic heterointerfaces
(2012-10-28)
Yu, P.
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Chu, Y. H.
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Ramesh, R.
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材料科學與工程學系
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Department of Materials Science and Engineering
The coupling and reconstruction of electronic degrees of freedom (such as charge, spin and orbital) at a heterointerface can lead to unexpected and exotic states of matter. In this study, using model systems consisting of multiferroic BiFeO3 and ferromagnetic La0.7Sr0.3MnO3, we review the current understanding of a novel interfacial magnetic state formed at the interface, and highlight some possible mechanisms responsible for this interesting phenomenon and identify open questions for future studies.
Evidence of Sharp and Diffuse Domain Walls in BiFeO3 by Means of Unit-Cell-Wise Strain and Polarization Maps Obtained with High Resolution Scanning Transmission Electron Microscopy
(2012-07-25)
Lubk, A.
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Rossell, M. D.
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Seidel, J.
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He, Q.
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Yang, S. Y.
Domain walls (DWs) substantially influence a large number of applications involving ferroelectric materials due to their limited mobility when shifted during polarization switching. The discovery of greatly enhanced conduction at BiFeO3 DWs has highlighted yet another role of DWs as a local material state with unique properties. However, the lack of precise information on the local atomic structure is still hampering microscopical understanding of DW properties. Here, we examine the atomic structure of BiFeO3 109 degrees DWs with pm precision by a combination of high-angle annular dark-field scanning transmission electron microscopy and a dedicated structural analysis. By measuring simultaneously local polarization and strain, we provide direct experimental proof for the straight DW structure predicted by ab initio calculations as well as the recently proposed theory of diffuse DWs, thus resolving a long-standing discrepancy between experimentally measured and theoretically predicted DW mobilities.
Ferroelectric domain wall pinning at a bicrystal grain boundary in bismuth ferrite
(2008-10-06)
Rodriguez, Brian J.
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Chu, Y. H.
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Ramesh, R.
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Kalinin, Sergei V.
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材料科學與工程學系
The ferroelectric polarization switching behavior at the 24 degrees (100) tilt grain boundary (GB) in an epitaxial multiferroic BiFeO(3) bicrystal film is studied using piezoresponse force microscopy (PFM). The PFM amplitudes across positively and negatively poled GB regions suggest the presence of a frozen polarization component at the interface. The switching experiments demonstrate that the GB attracts the domain wall and acts as a pinning center. The PFM results are compared with conductive atomic force microscopy and spectroscopy, which suggest domain wall pinning at the GB can be partially attributed to increased conductance at the GB. (C) 2008 American Institute of Physics.
Interface control of bulk ferroelectric polarization
(2012-06-19)
Yu, P.
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Luo, W.
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Yi, D.
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Zhang, J. X.
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Rossell, M. D.
The control of material interfaces at the atomic level has led to novel interfacial properties and functionalities. In particular, the study of polar discontinuities at interfaces between complex oxides lies at the frontier of modern condensed matter research. Here we employ a combination of experimental measurements and theoretical calculations to demonstrate the control of a bulk property, namely ferroelectric polarization, of a heteroepitaxial bilayer by precise atomic-scale interface engineering. More specifically, the control is achieved by exploiting the interfacial valence mismatch to influence the electrostatic potential step across the interface, which manifests itself as the biased-voltage in ferroelectric hysteresis loops and determines the ferroelectric state. A broad study of diverse systems comprising different ferroelectrics and conducting perovskite underlayers extends the generality of this phenomenon.