Browsing by Author "Chu, Y. -H."

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A Strain-Driven Morphotropic Phase Boundary in BiFeO(3)
(2009-11-13)
Zeches, R. J.
;
Rossell, M. D.
;
Zhang, J. X.
;
Hatt, A. J.
;
He, Q.
Piezoelectric materials, which convert mechanical to electrical energy and vice versa, are typically characterized by the intimate coexistence of two phases across a morphotropic phase boundary. Electrically switching one to the other yields large electromechanical coupling coefficients. Driven by global environmental concerns, there is currently a strong push to discover practical lead-free piezoelectrics for device engineering. Using a combination of epitaxial growth techniques in conjunction with theoretical approaches, we show the formation of a morphotropic phase boundary through epitaxial constraint in lead-free piezoelectric bismuth ferrite (BiFeO(3)) films. Electric field-dependent studies show that a tetragonal-like phase can be reversibly converted into a rhombohedral-like phase, accompanied by measurable displacements of the surface, making this new lead-free system of interest for probe-based data storage and actuator applications.
A Strain-Driven Morphotropic Phase Boundary in BiFeO3
(2009-11-13)
Zeches, R. J.
;
Rossell, M. D.
;
Zhang, J. X.
;
Hatt, A. J.
;
He, Q.
Piezoelectric materials, which convert mechanical to electrical energy and vice versa, are typically characterized by the intimate coexistence of two phases across a morphotropic phase boundary. Electrically switching one to the other yields large electromechanical coupling coefficients. Driven by global environmental concerns, there is currently a strong push to discover practical lead-free piezoelectrics for device engineering. Using a combination of epitaxial growth techniques in conjunction with theoretical approaches, we show the formation of a morphotropic phase boundary through epitaxial constraint in lead-free piezoelectric bismuth ferrite (BiFeO3) films. Electric field-dependent studies show that a tetragonal-like phase can be reversibly converted into a rhombohedral-like phase, accompanied by measurable displacements of the surface, making this new lead-free system of interest for probe-based data storage and actuator applications.
Above-bandgap voltages from ferroelectric photovoltaic devices
(2010-02-01)
Yang, S. Y.
;
Seidel, J.
;
Byrnes, S. J.
;
Shafer, P.
;
Yang, C. -H.
In conventional solid-state photovoltaics, electron-hole pairs are created by light absorption in a semiconductor and separated by the electric field spaning a micrometre-thick depletion region. The maximum voltage these devices can produce is equal to the semiconductor electronic bandgap. Here, we report the discovery of a fundamentally different mechanism for photovoltaic charge separation, which operates over a distance of 1-2 nm and produces voltages that are significantly higher than the bandgap. The separation happens at previously unobserved nanoscale steps of the electrostatic potential that naturally occur at ferroelectric domain walls in the complex oxide BiFeO(3). Electric-field control over domain structure allows the photovoltaic effect to be reversed in polarity or turned off. This new degree of control, and the high voltages produced, may find application in optoelectronic devices.
Conduction at domain walls in oxide multiferroics
(2009-03-01)
Seidel, J.
;
Martin, L. W.
;
He, Q.
;
Zhan, Q.
;
Chu, Y. -H.
Domain walls may play an important role in future electronic devices, given their small size as well as the fact that their location can be controlled. Here, we report the observation of room-temperature electronic conductivity at ferroelectric domain walls in the insulating multiferroic BiFeO(3). The origin and nature of the observed conductivity are probed using a combination of conductive atomic force microscopy, high-resolution transmission electron microscopy and first-principles density functional computations. Our analyses indicate that the conductivity correlates with structurally driven changes in both the electrostatic potential and the local electronic structure, which shows a decrease in the bandgap at the domain wall. Additionally, we demonstrate the potential for device applications of such conducting nanoscale features.
Critical thickness and orbital ordering in ultrathin La(0.7)Sr(0.3)MnO(3) films
(2008-09-01)
Huijben, M.
;
Martin, L. W.
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Chu, Y. -H.
;
Holcomb, M. B.
;
Yu, P.
Detailed analysis of transport, magnetism, and x-ray absorption spectroscopy measurements on ultrathin La(0.7)Sr(0.3)MnO(3) films with thicknesses from 3 to 70 unit cells resulted in the identification of a lower critical thickness for a nonmetallic nonferromagnetic layer at the interface with the SrTiO(3) (001) substrate of only three unit cells (similar to 12 angstrom). Furthermore, linear-dichroism measurements demonstrate the presence of a preferred (x(2)-y(2)) in-plane orbital ordering for all layer thicknesses without any orbital reconstruction at the interface. A crucial requirement for the accurate study of these ultrathin films is a controlled growth process, offering the coexistence of layer-by-layer growth and bulklike magnetic/transport properties.
Critical thickness and orbital ordering in ultrathin La0.7Sr0.3MnO3 films
(2008-09-01)
Huijben, M.
;
Martin, L. W.
;
Chu, Y. -H.
;
Holcomb, M. B.
;
Yu, P.
Detailed analysis of transport, magnetism, and x-ray absorption spectroscopy measurements on ultrathin La0.7Sr0.3MnO3 films with thicknesses from 3 to 70 unit cells resulted in the identification of a lower critical thickness for a nonmetallic nonferromagnetic layer at the interface with the SrTiO3 (001) substrate of only three unit cells (similar to 12 angstrom). Furthermore, linear-dichroism measurements demonstrate the presence of a preferred (x(2)-y(2)) in-plane orbital ordering for all layer thicknesses without any orbital reconstruction at the interface. A crucial requirement for the accurate study of these ultrathin films is a controlled growth process, offering the coexistence of layer-by-layer growth and bulklike magnetic/transport properties.
Domain Wall Conductivity in La-Doped BiFeO(3)
(2010-11-05)
Seidel, J.
;
Maksymovych, P.
;
Batra, Y.
;
Katan, A.
;
Yang, S. -Y.
The transport physics of domain wall conductivity in La-doped bismuth ferrite (BiFeO(3)) has been probed using variable temperature conducting atomic force microscopy and piezoresponse force microscopy in samples with arrays of domain walls in the as-grown state. Nanoscale current measurements are investigated as a function of bias and temperature and are shown to be consistent with distinct electronic properties at the domain walls leading to changes in the observed local conductivity. Our observation is well described within a band picture of the observed electronic conduction. Finally, we demonstrate an additional degree of control of the wall conductivity through chemical doping with oxygen vacancies, thus influencing the local conductive state.
Domain Wall Conductivity in La-Doped BiFeO3
(2010-11-05)
Seidel, J.
;
Maksymovych, P.
;
Batra, Y.
;
Katan, A.
;
Yang, S. -Y.
The transport physics of domain wall conductivity in La-doped bismuth ferrite (BiFeO3) has been probed using variable temperature conducting atomic force microscopy and piezoresponse force microscopy in samples with arrays of domain walls in the as-grown state. Nanoscale current measurements are investigated as a function of bias and temperature and are shown to be consistent with distinct electronic properties at the domain walls leading to changes in the observed local conductivity. Our observation is well described within a band picture of the observed electronic conduction. Finally, we demonstrate an additional degree of control of the wall conductivity through chemical doping with oxygen vacancies, thus influencing the local conductive state.
Domain Wall Geometry Controls Conduction in Ferroelectrics
(2012-11-01)
Vasudevan, R. K.
;
Morozovska, A. N.
;
Eliseev, E. A.
;
Britson, J.
;
Yang, J. -C.
A new paradigm of domain wall. nanoelectronics has emerged recently, in which the domain wall in a ferroic is itself an active device element The ability to spatially. modulate the ferroic order parameter within a single domain wall allows the physical properties to be tailored at will and hence opens vastly unexplored device possibilities: Here, we demonstrate via ambient and ultrahigh-vacuum (UHV) scanning probe microscopy (SPM) measurements in bismuth ferrite that the conductivity of the domain walls can be modulated by up to 500% in the spatial dimension as a function of domain wall curvature. Landau-Ginzburg-Devonshire calculations reveal the conduction is a.. result of carriers or vacancies migrating to neutralize the charge at the farmed interface. Phase field modeling indicates that anisotropic potential distributions can occur even for initially uncharged walls, from polarization dynamics mediated by elastic effects. These results are the first proof of concept for modulation of charge as a function of domain wall geometry by a proximal probe, thereby expanding potential applications for oxide ferroics in future nanoscale electronics.
Electrically controllable spontaneous magnetism in nanoscale mixed phase multiferroics
(2011-03-01)
He, Q.
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Chu, Y. -H.
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Heron, J. T.
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Yang, S. Y.
;
Liang, W. I.
Magnetoelectrics and multiferroics present exciting opportunities for electric-field control of magnetism. However, there are few room-temperature ferromagnetic-ferroelectrics. Among the various types of multiferroics the bismuth ferrite system has received much attention primarily because both the ferroelectric and the antiferromagnetic orders are quite robust at room temperature. Here we demonstrate the emergence of an enhanced spontaneous magnetization in a strain-driven rhombohedral and super-tetragonal mixed phase of BiFeO(3). Using X-ray magnetic circular dichroism-based photoemission electron microscopy coupled with macroscopic magnetic measurements, we find that the spontaneous magnetization of the rhombohedral phase is significantly enhanced above the canted antiferromagnetic moment in the bulk phase, as a consequence of a piezomagnetic coupling to the adjacent tetragonal-like phase and the epitaxial constraint. Reversible electric-field control and manipulation of this magnetic moment at room temperature is also shown.
Ferromagnetic Enhancement of CE-Type Spin Ordering in (Pr, Ca)MnO(3)
(2011-05-04)
Zhou, S. Y.
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Zhu, Y.
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Langner, M. C.
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Chuang, Y. -D.
;
Yu, P.
We present resonant soft x-ray scattering results from small bandwidth manganites (Pr, Ca)MnO(3), which show that the CE-type spin ordering (SO) at the phase boundary is stabilized only below the canted antiferromagnetic transition temperature and enhanced by ferromagnetism in the macroscopically insulating state (FM-I). Our results reveal the fragility of the CE-type ordering that underpins the colossal magnetoresistance effect in this system, as well as an unexpected cooperative interplay between FM-I and CE-type SO which is in contrast to the competitive interplay between the ferromagnetic metallic state and CE-type ordering.
Ferromagnetic Enhancement of CE-Type Spin Ordering in (Pr, Ca)MnO3
(2011-05-04)
Zhou, S. Y.
;
Zhu, Y.
;
Langner, M. C.
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Chuang, Y. -D.
;
Yu, P.
We present resonant soft x-ray scattering results from small bandwidth manganites (Pr, Ca)MnO3, which show that the CE-type spin ordering (SO) at the phase boundary is stabilized only below the canted antiferromagnetic transition temperature and enhanced by ferromagnetism in the macroscopically insulating state (FM-I). Our results reveal the fragility of the CE-type ordering that underpins the colossal magnetoresistance effect in this system, as well as an unexpected cooperative interplay between FM-I and CE-type SO which is in contrast to the competitive interplay between the ferromagnetic metallic state and CE-type ordering.
Hidden lattice instabilities as origin of the conductive interface between insulating LaAlO3 and SrTiO3
(2016-09-01)
Lee, P. W.
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Singh, V. N.
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Guo, G. Y.
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Liu, H. -J.
;
Lin, J. -C.
The metallic interface between insulating LaAlO3 and SrTiO3 opens up the field of oxide electronics. With more than a decade of researches on this heterostructure, the origin of the interfacial conductivity, however, remains unsettled. Here we resolve this long-standing puzzle by atomic-scale observation of electron-gas formation for screening hidden lattice instabilities, rejuvenated near the interface by epitaxial strain. Using atomic-resolution imaging and electron spectroscopy, the generally accepted notions of polar catastrophe and cation intermixing for the metallic interface are discounted. Instead, the conductivity onset at the critical thickness of 4-unit cell LaAlO3 on SrTiO3 substrate is accompanied with head-to-head ferroelectric-like polarizations across the interface due to strain-rejuvenated ferroelectric-like instabilities in the materials. The divergent depolarization fields of the head-to-head polarizations cast the interface into an electron reservoir, forming screening electron gas in SrTiO3 with LaAlO3 hosting complementary localized holes. The ferroelectriclike polarizations and electron-hole juxtaposition reveal the cooperative nature of metallic LaAlO3/SrTiO3.
Interface Ferromagnetism and Orbital Reconstruction in BiFeO(3)-La(0.7)Sr(0.3)MnO(3) Heterostructures
(2010-07-06)
Yu, P.
;
Lee, J. -S.
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Okamoto, S.
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Rossell, M. D.
;
Huijben, M.
We report the formation of a novel ferromagnetic state in the antiferromagnet BiFeO(3) at the interface with ferromagnet La(0.7)Sr(0.3)MnO(3). Using x-ray magnetic circular dichroism at Mn and Fe L(2,3) edges, we discovered that the development of this ferromagnetic spin structure is strongly associated with the onset of a significant exchange bias. Our results demonstrate that the magnetic state is directly related to an electronic orbital reconstruction at the interface, which is supported by the linearly polarized x-ray absorption measurement at the oxygen K edge.
Interface Ferromagnetism and Orbital Reconstruction in BiFeO3-La0.7Sr0.3MnO3 Heterostructures
(2010-07-06)
Yu, P.
;
Lee, J. -S.
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Okamoto, S.
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Rossell, M. D.
;
Huijben, M.
We report the formation of a novel ferromagnetic state in the antiferromagnet BiFeO3 at the interface with ferromagnet La0.7Sr0.3MnO3. Using x-ray magnetic circular dichroism at Mn and Fe L-2,L-3 edges, we discovered that the development of this ferromagnetic spin structure is strongly associated with the onset of a significant exchange bias. Our results demonstrate that the magnetic state is directly related to an electronic orbital reconstruction at the interface, which is supported by the linearly polarized x-ray absorption measurement at the oxygen K edge.
Mapping strain modulated electronic structure perturbations in mixed phase bismuth ferrite thin films
(2015-01-01)
Krishnan, P. S. Sankara Rama
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Aguiar, Jeffery A.
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Ramasse, Q. M.
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Kepaptsoglou, D. M.
;
Liang, W. -I.
Strain engineering of epitaxial ferroelectrics has emerged as a powerful method to tailor the electromechanical response of these materials, although the effect of strain at the atomic scale and the interplay between lattice displacements and electronic structure changes are not yet fully understood. Here, using a combination of scanning transmission electron microscopy (STEM) and density functional theory (DFT), we systematically probe the role of epitaxial strain in mixed phase bismuth ferrite thin films. Electron energy loss O K and Fe L-2,L-3 edge spectra acquired across the rhombohedral (R)-tetragonal (T) phase boundary reveal progressive, and systematic, changes in electronic structure going from one phase to the other. The comparison of the acquired spectra with theoretical simulations using DFT suggests a breakage in the structural symmetry across the boundary due to the simultaneous presence of increasing epitaxial strain and off-axial symmetry in the T phase. This implies that the imposed epitaxial strain plays a significant role in not only changing the crystal-field geometry, but also the bonding environment surrounding the central iron cation at the interface thus providing new insights and a possible link to understand how the imposed strain could perturb magnetic ordering in the T phase BFO.
Nanoscale characterization of emergent phenomena in multiferroics
(2012-10-01)
He, Q.
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Arenholz, E.
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Scholl, A.
;
Chu, Y. -H.
;
Ramesh, R.
Multiferroics exhibit intriguing physical properties and in turn promise new device applications as a result of the coupling between their order parameters. In this review article, we introduce photoemission electron microscopy (PEEM) as a powerful tool to study multiferroicity with the capability of probing the charge, spin and orbital states of a material simultaneously with nanoscale spatial resolution and element sensitivity. Several systematical studies of ferroelectricity, antiferromagnetism, and multiferroicity using PEEM are discussed. In the end, we outline several challenges remaining in multiferroic research, and how PEEM can be employed as an important characterization tool providing critical information to understand the emergent phenomena in multiferroics. Published by Elsevier Ltd.
Nanoscale phase boundaries: a new twist to novel functionalities
(2012)
Zhang, J. X.
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Zeches, R. J.
;
He, Q.
;
Chu, Y. -H.
;
Ramesh, R.
In functional materials, nanoscale phase boundaries exhibit exotic phenomena that are notably absent in their parent phases. Over the past two decades, much of the research into complex oxides (such as cuprate superconductors, CMR manganites and relaxor ferroelectrics) has demonstrated the key role that nanoscale inhomogeneities play in controlling the electronic and/or ionic structure of these materials. One of the key characteristics in such systems is the strong susceptibility to external perturbations, such as magnetic, electric and mechanical fields. A direct consequence of the accommodation of a large number of cationic substitutions in complex oxides is the emergence of a number of physical phenomena from essentially the same crystal framework. Recently, multiferroic behavior, which is characterized by the co-existence and potential coupling of multiple ferroic order parameters, has captured considerable worldwide research interest. The perovskite, BiFeO3, exhibits robust ferroelectricity coupled with antiferromagnetism at room temperature. A rather unique feature of this material system is its ability to "morph" its ground state when an external mechanical constraint is imposed on it. A particularly striking example is observed when a large (similar to 4 to 5%) compressive strain is imposed on a thin film through the epitaxial constraint from the underlying substrate. Under these conditions, the ground state rhombohedral phase transforms into a tetragonal-like (or a derivative thereof) phase with a rather large unit cell (c/a ratio of similar to 1.26). When the epitaxial constraint is partially relaxed by increasing the film thickness, this tetragonal-like phase evolves into a "mixed-phase" state, consisting of a nanoscale admixture of the rhombohedral-like phase embedded in the tetragonal-like phase. Such a system gives us a new pathway to explore a variety of mechanical, magnetic and transport phenomena in constrained dimensions. This article reviews our progress to date in this direction and also captures some possible areas of future research. We use the electromechanical response and the magnetic properties as examples to illustrate that its novel functionalities are intrinsically due to the phase boundaries and not the constituent phases. The possible origin of the giant piezoelectric response and enhanced magnetic moment across the boundaries is proposed based on the flexoelectric and flexomagnetic effects.
Near-threshold all-digital PLL with dynamic voltage scaling power management
(2016-01-21)
Chang, C. -W.
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Chang, K. -Y.
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Chu, Y. -H.
;
Jou, S. -J.
;
電子工程學系及電子研究所
A near-threshold all-digital phase-locked loop (ADPLL) with a power management unit (PMU) is presented to make the proposed ADPLL work reliably across variations and power consumption as well is reduced. When operated under near-threshold condition from 0.52 to 0.58 V V-DD, the gated digitally controlled oscillator frequency range is from 90.8 to 245.7 MHz. When the ADPLL is operated at 0.52 V V-DD, a lock-in time of 9.5 mu s at 100 MHz output clock frequency is measured with an rms period jitter of 0.17% UI. With the PMU, the ADPLL power reduction at 130 MHz output frequency is 39% and the buck converter power consumption is nearly 30 mu W. Consequently, the proposed ADPLL with PMU is suitable to event-driven or low-voltage applications.
Observation of a three-dimensional quasi-long-range electronic supermodulation in YBa2Cu3O7 (-) (x)/La0.7Ca0.3MnO3 heterostructures
(2016-03-01)
He, Junfeng
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Shafer, Padraic
;
Mion, Thomas R.
;
Vu Thanh Tra
;
He, Qing
Recent developments in high-temperature superconductivity highlight a generic tendency of the cuprates to develop competing electronic (charge) supermodulations. While coupled with the lattice and showing different characteristics in different materials, these supermodulations themselves are generally conceived to be quasi-two-dimensional, residing mainly in individual CuO2 planes, and poorly correlated along the c axis. Here we observed with resonant elastic X-ray scattering a distinct type of electronic supermodulation in YBa2Cu3O7 (- x) (YBCO) thin films grown epitaxially on La0.7Ca0.3MnO3 (LCMO). This supermodulation has a periodicity nearly commensurate with four lattice constants in-plane, eight out of plane, with long correlation lengths in three dimensions. It sets in far above the superconducting transition temperature and competes with superconductivity below this temperature for electronic states predominantly in the CuO2 plane. Our finding sheds light on the nature of charge ordering in cuprates as well as a reported long-range proximity effect between superconductivity and ferromagnetism in YBCO/LCMO heterostructures.