Browsing by Author "Ramesh, Ramamoorthy"

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A nanoscale shape memory oxide
(2013-11-01)
Zhang, Jinxing
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Ke, Xiaoxing
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Gou, Gaoyang
;
Seidel, Jan
;
Xiang, Bin
Stimulus-responsive shape-memory materials have attracted tremendous research interests recently, with much effort focused on improving their mechanical actuation. Driven by the needs of nanoelectromechanical devices, materials with large mechanical strain, particularly at nanoscale level, are therefore desired. Here we report on the discovery of a large shape-memory effect in bismuth ferrite at the nanoscale. A maximum strain of up to similar to 14% and a large volumetric work density of similar to 600 +/- 90 J cm(-3) can be achieved in association with a martensitic-like phase transformation. With a single step, control of the phase transformation by thermal activation or electric field has been reversibly achieved without the assistance of external recovery stress. Although aspects such as hysteresis, microcracking and so on have to be taken into consideration for real devices, the large shape-memory effect in this oxide surpasses most alloys and, therefore, demonstrates itself as an extraordinary material for potential use in state-of-art nanosystems.
A Strain-Driven Antiferroelectric-to-Ferroelectric Phase Transition in La-Doped BiFeO3 Thin Films on Si
(2017-09-01)
Chen, Deyang
;
Nelson, Christopher T.
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Zhu, Xiaohong
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Serrao, Claudy R.
;
Clarkson, James D.
A strain-driven orthorhombic (O) to rhombohedral (R) phase transition is reported in La-doped BiFeO3 thin films on silicon substrates. Biaxial compressive epitaxial strain is found to stabilize the rhombohedral phase at La concentrations beyond the morphotropic phase boundary (MPB). By tailoring the residual strain with film thickness, we demonstrate a mixed O/R phase structure consisting of O phase domains measuring tens of nanometers wide within a predominant R phase matrix. A combination of piezoresponse force microscopy (PFM), transmission electron microscopy (TEM), polarization electric field hysteresis loop (P-E loop), and polarization maps reveal that the O-R structural change is an antiferroelectric to ferroelectric (AFE-FE) phase transition. Using scanning transmission electron microscopy (STEM), an atomically sharp O/R MPB is observed. Moreover, X-ray absorption spectra (XAS) and X-ray linear dichroism (XLD) measurements reveal a change in the antiferromagnetic axis orientation from out of plane (R-phaSe) to in plane (O-p hase). These findings provide direct evidence of spin-charge-lattice coupling in La-doped BiFeO3 thin films. Furthermore, this study opens a new pathway to drive the AFE-FE O-R phase transition and provides a route to study the O/R MPB in these films.
Atomic-Scale Evolution of Local Electronic Structure Across Multiferroic Domain Walls
(2011-04-05)
Chiu, Ya-Ping
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Chen, Yu-Ting
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Huang, Bo-Chao
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Shih, Min-Chuan
;
Yang, Jan-Chi
Direct evidence of the electronic configurations across domain walls in BiFeO(3) is quantitatively characterized by cross-sectional scanning tunneling microscopy. Atomic-scale band evolution and the asymmetrically built-in potential barrier at domain boundaries are demonstrated. The 109 degrees domain walls register a remarkable decrease in the bandgap, suggesting a new route to control the local conducting channels within 2 nm.
Atomically Resolved Electronic States and Correlated Magnetic Order at Termination Engineered Complex Oxide Heterointerfaces
(2018-02-01)
Huang, Bo-Chao
;
Yu, Pu
;
Chu, Y. H.
;
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.
Atomically Resolved Mapping of Polarization and Electric Fields Across Ferroelectric/Oxide Interfaces by Z-contrast Imaging
(2011-06-03)
Chang, Hye Jung
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Kalinin, Sergei V.
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Morozovska, Anna N.
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Huijben, Mark
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Chu, Ying-Hao
Direct atomic displacement mapping at ferroelectric interfaces by aberration corrected scanning transmission electron microscopy (STEM) (a-STEM image, b-corresponding displacement profile) is combined with the Landau-Ginsburg-Devonshire theory to obtain the complete interface electrostatics in real space, including separate estimates for the polarization and intrinsic interface charge contributions.
Complex strain evolution of polar and magnetic order in multiferroic BiFeO3 thin films
(2018-09-21)
Chen, Zuhuang
;
Chen, Zhanghui
;
Kuo, Chang-Yang
;
Tang, Yunlong
;
Dedon, Liv R.
Electric-field control of magnetism requires deterministic control of the magnetic order and understanding of the magnetoelectric coupling in multiferroics like BiFeO3 and EuTiO3. Despite this critical need, there are few studies on the strain evolution of magnetic order in BiFeO3 films. Here, in (110)-oriented BiFeO3 films, we reveal that while the polarization structure remains relatively unaffected, strain can continuously tune the orientation of the antiferromagnetic-spin axis across a wide angular space, resulting in an unexpected deviation of the classical perpendicular relationship between the antiferromagnetic axis and the polarization. Calculations suggest that this evolution arises from a competition between the Dzyaloshinskii-Moriya interaction and single-ion anisotropy wherein the former dominates at small strains and the two are comparable at large strains. Finally, strong coupling between the BiFeO3 and the ferromagnet Co0.9Fe0.1 exists such that the magnetic anisotropy of the ferromagnet can be effectively controlled by engineering the orientation of the antiferromagnetic-spin axis.
Concurrent transition of ferroelectric and magnetic ordering near room temperature
(2011-11-01)
Ko, Kyung-Tae
;
Jung, Min Hwa
;
He, Qing
;
Lee, Jin Hong
;
Woo, Chang Su
Strong spin-lattice coupling in condensed matter gives rise to intriguing physical phenomena such as colossal magnetoresistance and giant magnetoelectric effects. The phenomenological hallmark of such a strong spin-lattice coupling is the manifestation of a large anomaly in the crystal structure at the magnetic transition temperature. Here we report that the magnetic Neel temperature of the multiferroic compound BiFeO3 is suppressed to around room temperature by heteroepitaxial misfit strain. Remarkably, the ferroelectric state undergoes a first-order transition to another ferroelectric state simultaneously with the magnetic transition temperature. Our findings provide a unique example of a concurrent magnetic and ferroelectric transition at the same temperature among proper ferroelectrics, taking a step toward room temperature magnetoelectric applications.
Direct Observation of Capacitor Switching Using Planar Electrodes
(2010-10-22)
Balke, Nina
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Gajek, Martin
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Tagantsev, Alexander K.
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Martin, Lane W.
;
Chu, Ying-Hao
Ferroelectric polarization switching in epitaxial (110) BiFeO(3) films is studied using piezoresponse force microscopy of a model in-plane capacitor structure. The electrode orientation is chosen such that only two active domain variants exist. Studies of the kinetics of domain evolution allows clear visualization of nucleation sites, as well as forward and lateral growth stages of domain formation. It is found that the location of the reverse-domain nucleation is correlated with the direction of switching in a way that the polarization in the domains nucleated at an electrode is always directed away from it. The role of interface charge injection and surface screening charge on switching mechanisms is explored, and the nucleation is shown to be controllable by the bias history of the sample. Finally, the manipulation of domain nucleation through domain structure engineering is illustrated. These studies pave the way for the engineering and design of the ferroelectric device structures through control of individual steps of the switching process.
Domain wall functionality in BiFeO3
(2013)
Seidel, Jan
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Singh-Bhalla, Guneeta
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He, Qing
;
Yang, Seung-Yeul
;
Chu, Ying-Hao
In this article we provide an overview of phenomena involving domain walls in BiFeO3 as nanoscale functional elements. Because ferroelectric domain walls can be routinely modified using electric fields, our results suggest a new degree of flexibility for domain boundary engineering and oxide-based nanoelectronics.
Dynamic Conductivity of Ferroelectric Domain Walls in BiFeO(3)
(2011-05-01)
Maksymovych, Peter
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Seidel, Jan
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Chu, Ying Hao
;
Wu, Pingping
;
Baddorf, Arthur P.
Topological walls separating domains of continuous polarization, magnetization, and strain in ferroic materials hold promise of novel electronic properties, that are intrinsically localized on the nanoscale and that can be patterned on demand without change of material volume or elemental composition. We have revealed that ferroelectric domain walls in multiferroic BiFeO(3) are inherently dynamic electronic conductors, closely mimicking memristive behavior and contrary to the usual assumption of rigid conductivity. Applied electric field can cause a localized transition between insulating and conducting domain walls, tune domain wall conductance by over an order of magnitude, and create a quasicontinuous spectrum of metastable conductance states. Our measurements identified that subtle and microscopically reversible distortion of the polarization structure at the domain wall is at the origin of the dynamic conductivity. The latter is therefore likely to be a universal property of topological defects in ferroelectric semiconductors.
Dynamic Conductivity of Ferroelectric Domain Walls in BiFeO3
(2011-05-01)
Maksymovych, Peter
;
Seidel, Jan
;
Chu, Ying Hao
;
Wu, Pingping
;
Baddorf, Arthur P.
Topological walls separating domains of continuous polarization, magnetization, and strain in ferroic materials hold promise of novel electronic properties, that are intrinsically localized on the nanoscale and that can be patterned on demand without change of material volume or elemental composition. We have revealed that ferroelectric domain walls in multiferroic BiFeO3 are inherently dynamic electronic conductors, closely mimicking memristive behavior and contrary to the usual assumption of rigid conductivity. Applied electric field can cause a localized transition between insulating and conducting domain walls, tune domain wall conductance by over an order of magnitude, and create a quasicontinuous spectrum of metastable conductance states. Our measurements identified that subtle and microscopically reversible distortion of the polarization structure at the domain wall is at the origin of the dynamic conductivity. The latter is therefore likely to be a universal property of topological defects in ferroelectric semiconductors.
Enhanced electric conductivity at ferroelectric vortex cores in BiFeO(3)
(2012-01-01)
Balke, Nina
;
Winchester, Benjamin
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Ren, Wei
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Chu, Ying Hao
;
Morozovska, Anna N.
Topological defects in ferroic materials are attracting much attention both as a playground of unique physical phenomena and for potential applications in reconfigurable electronic devices. Here, we explore electronic transport at artificially created ferroelectric vortices in BiFeO(3) thin films. The creation of one-dimensional conductive channels activated at voltages as low as 1V is demonstrated. We study the electronic as well as the static and dynamic polarization structure of several topological defects using a combination of first-principles and phase-field modelling. The modelling predicts that the core structure can undergo a reversible transformation into a metastable twist structure, extending charged domain walls segments through the film thickness. The vortex core is therefore a dynamic conductor controlled by the coupled response of polarization and electron-mobile-vacancy subsystems with external bias. This controlled creation of conductive one-dimensional channels suggests a pathway for the design and implementation of integrated oxide electronic devices based on domain patterning.
Enhanced electric conductivity at ferroelectric vortex cores in BiFeO3
(2012-01-01)
Balke, Nina
;
Winchester, Benjamin
;
Ren, Wei
;
Chu, Ying Hao
;
Morozovska, Anna N.
Topological defects in ferroic materials are attracting much attention both as a playground of unique physical phenomena and for potential applications in reconfigurable electronic devices. Here, we explore electronic transport at artificially created ferroelectric vortices in BiFeO3 thin films. The creation of one-dimensional conductive channels activated at voltages as low as 1V is demonstrated. We study the electronic as well as the static and dynamic polarization structure of several topological defects using a combination of first-principles and phase-field modelling. The modelling predicts that the core structure can undergo a reversible transformation into a metastable twist structure, extending charged domain walls segments through the film thickness. The vortex core is therefore a dynamic conductor controlled by the coupled response of polarization and electron-mobile-vacancy subsystems with external bias. This controlled creation of conductive one-dimensional channels suggests a pathway for the design and implementation of integrated oxide electronic devices based on domain patterning.
Interface dipole between two metallic oxides caused by localized oxygen vacancies
(2012-10-11)
Borisevich, Albina Y.
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Lupini, Andrew R.
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He, Jun
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Eliseev, Eugene A.
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Morozovska, Anna N.
Oxygen vacancies are increasingly recognized to play a role in phenomena observed at transition-metal oxide interfaces. Here, we report a study of SrRuO3 and La0.7Sr0.3MnO3 interfaces using a combination of quantitative aberration-corrected scanning transmission electron microscopy, electron energy-loss spectroscopy, and density functional calculations. Cation displacements are observed at the interface, indicative of a dipolelike electric field even though both materials are nominally metallic. The observed displacements are reproduced by theory if O vacancies are present in the near-interface La0.7Sr0.3MnO3 layers. The results suggest that atomic-scale structural mapping can serve as a quantitative indicator of the presence of O vacancies at interfaces.
Manipulating magnetoelectric energy landscape in multiferroics
(2020-06-05)
Huang, Yen-Lin
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Nikonov, Dmitri
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Addiego, Christopher
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Chopdekar, Rajesh, V
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Prasad, Bhagwati
Magnetoelectric coupling at room temperature in multiferroic materials, such as BiFeO3, is one of the leading candidates to develop low-power spintronics and emerging memory technologies. Although extensive research activity has been devoted recently to exploring the physical properties, especially focusing on ferroelectricity and antiferromagnetism in chemically modified BiFeO3, a concrete understanding of the magnetoelectric coupling is yet to be fulfilled. We have discovered that La substitutions at the Bi-site lead to a progressive increase in the degeneracy of the potential energy landscape of the BiFeO3 system exemplified by a rotation of the polar axis away from the < 111 > (pc) towards the < 112 > (pc) discretion. This is accompanied by corresponding rotation of the antiferromagnetic axis as well, thus maintaining the right-handed vectorial relationship between ferroelectric polarization, antiferromagnetic vector and the Dzyaloshinskii-Moriya vector. As a consequence, La-BiFeO3 films exhibit a magnetoelectric coupling that is distinctly different from the undoped BiFeO3 films. BiFeO3 has a wide application but the impact of rare-earth substitution for the evolution of the coupling mechanism is unknown. Here, the authors reveal the correlation between ferroelectricity, antiferromagnetism, a weak ferromagnetic moment, and their switching pathways in La-substituted BiFeO3.
Oxide interfaces: pathways to novel phenomena
(2012-07-01)
Yu, Pu
;
Chu, Ying-Hao
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Ramesh, Ramamoorthy
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材料科學與工程學系
;
Department of Materials Science and Engineering
Novel phenomena and functionalities at artificial heterointerfaces have been attracting extensive scientific attention in both materials science and fundamental condensed matter physics. The interplay between degrees of freedom at interfaces of complex oxides could lead to exotic and unexpected states of matter. In this article, using the model system of BiFeO3 and La0.7Sr0.3MnO3, we review recent progress on our understanding of the novel states formed at this heterointerface. Furthermore, we discuss how emergent interfacial phenomena can be employed to influence the bulk properties of these materials. We summarize by highlighting several possible and promising directions for future study.
Self-assembled vertical heteroepitaxial nanostructures: from growth to functionalities
(2014-06-01)
Liu, Heng-Jui
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Liang, Wen-I
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Chu, Ying-Hao
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Zheng, Haimei
;
Ramesh, Ramamoorthy
Self-assembled vertical heteroepitaxial nanostructures (VHN) in the complex oxide field have fascinated scientists for decades because they provide degrees of freedom to explore in condensed matter physics and design-coupled multifunctionlities. Recently, of particular interest is the perovskite-spinel-based VHN, covering a wide spectrum of promising applications. In this review, fabrication of VHN, their growth mechanism, control, and resulting novel multifunctionalities are discussed thoroughly, providing researchers a comprehensive blueprint to construct promising VHN. Following the fabrication section, the state-of-the-art design concepts for multifunctionalities are proposed and reviewed by suitable examples. By summarizing the outlook of this field, we are excitedly expecting this field to rise with significant contributions ranging from scientific value to practical applications in the foreseeable future.
Switching kinetics in epitaxial BiFeO(3) thin films
(2010-04-15)
Pantel, Daniel
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Chu, Ying-Hao
;
Martin, Lane W.
;
Ramesh, Ramamoorthy
;
Hesse, Dietrich
The switching kinetics in epitaxial (001)-, (110)-, and (111)-oriented BiFeO(3) thin films were investigated as a function of applied field and time. It was found that the ferroelectric switching behavior obeys the Kolmogorov-Avrami-Ishibashi theory only in the high field range. The detailed behavior depends on the film orientation. A comparison with standard systems, such as epitaxial Pb(Zr(0.2)Ti(0.8))O(3) films, reveals some similarities as well as some differences. For instance, the presence of 109 degrees and 71 degrees ferroelastic domain walls might be ruled out as the source of the decrease in switched polarization at low applied fields, in contrast to what is the case for a/c domain walls in tetragonal Pb(Zr(0.2)Ti(0.8))O(3). (C) 2010 American Institute of Physics. [doi:10.1063/1.3392884]
Switching kinetics in epitaxial BiFeO3 thin films
(2010-04-15)
Pantel, Daniel
;
Chu, Ying-Hao
;
Martin, Lane W.
;
Ramesh, Ramamoorthy
;
Hesse, Dietrich
The switching kinetics in epitaxial (001)-, (110)-, and (111)-oriented BiFeO3 thin films were investigated as a function of applied field and time. It was found that the ferroelectric switching behavior obeys the Kolmogorov-Avrami-Ishibashi theory only in the high field range. The detailed behavior depends on the film orientation. A comparison with standard systems, such as epitaxial Pb(Zr0.2Ti0.8)O-3 films, reveals some similarities as well as some differences. For instance, the presence of 109 degrees and 71 degrees ferroelastic domain walls might be ruled out as the source of the decrease in switched polarization at low applied fields, in contrast to what is the case for a/c domain walls in tetragonal Pb(Zr0.2Ti0.8)O-3. (C) 2010 American Institute of Physics. [doi:10.1063/1.3392884]
Tailoring Magnetoelectric Coupling in BiFeO3/La0.7Sr0.3MnO3 Heterostructure through the Interface Engineering
(2019-03-15)
Yi, Di
;
Yu, Pu
;
Chen, Yi-Chun
;
Lee, Hsin-Hua
;
He, Qing
Electric field control of magnetism ultimately opens up the possibility of reducing energy consumption of memory and logic devices. Electric control of magnetization and exchange bias are demonstrated in all-oxide heterostructures of BiFeO3 (BFO) and La0.7Sr0.3MnO3 (LSMO). However, the role of the polar heterointerface on magnetoelectric (ME) coupling is not fully explored. Here, the ME coupling in BFO/LSMO heterostructures with two types of interfaces, achieved by exploiting the interface engineering at the atomic scale, is investigated. It is shown that both magnetization and exchange bias are reversibly controlled by switching the ferroelectric polarization of BFO. Intriguingly, distinctly different modulation behaviors that depend on the interfacial atomic sequence are observed. These results provide new insights into the underlying physics of ME coupling in the model system. This study highlights that designing interface at the atomic scale is of general importance for functional spintronic devices.