Browsing by Author "Nagarajan, V."

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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.
Controlling magnetoelectric coupling by nanoscale phase transformation in strain engineered bismuth ferrite
(2012)
Liu, Y. Y.
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Vasudevan, R. K.
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Pan, K.
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Xie, S. H.
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Liang, W-I
The magnetoelectric coupling in multiferroic materials is promising for a wide range of applications, yet manipulating magnetic ordering by electric field proves elusive to obtain and difficult to control. In this paper, we explore the prospect of controlling magnetic ordering in misfit strained bismuth ferrite (BiFeO3, BFO) films, combining theoretical analysis, numerical simulations, and experimental characterizations. Electric field induced transformation from a tetragonal phase to a distorted rhombohedral one in strain engineered BFO films has been identified by thermodynamic analysis, and realized by scanning probe microscopy (SPM) experiment. By breaking the rotational symmetry of a tip-induced electric field as suggested by phase field simulation, the morphology of distorted rhombohedral variants has been delicately controlled and regulated. Such capabilities enable nanoscale control of magnetoelectric coupling in strain engineered BFO films that is difficult to achieve otherwise, as demonstrated by phase field simulations.
Domain Wall Geometry Controls Conduction in Ferroelectrics
(2012-11-01)
Vasudevan, R. K.
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Morozovska, A. N.
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Eliseev, E. A.
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Britson, J.
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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.
Effect of processing kinetics on the structure of ferromagnetic-ferroelectric-ferromagnetic interfaces
(2012-11-15)
Krishnan, P. S. Sankara Rama
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Ramasse, Q. M.
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Liang, Wen-I
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Chu, Ying-Hao
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Nagarajan, V.
Trilayer heterostructures consisting of a ferroelectric bismuth ferrite (BFO) film sandwiched between ferromagnetic lanthanum strontium manganese oxide (LSMO) films were fabricated using pulsed laser deposition. Both BFO thicknesses (20 nm, 5 nm) and cooling rates were varied to investigate the role of processing parameters on the chemistry of the interfaces. The interfaces were investigated using a dedicated aberration corrected scanning transmission electron microscope (STEM) operated at 100 kV via STEM-high angle annular dark field (STEM-HAADF) and STEM-electron energy loss spectroscopy (STEM-EELS) modes. Combined analysis through STEM-HAADF and STEM-EELS revealed the formation of lattice distortion in certain regions of the BFO layer for the similar to 5 nm film. Piezoresponse force microscopy (PFM) studies of the similar to 5 nm BFO sample revealed weak ferroelectric domain switching. Stacking fault defects with mixed valence manganese (Mn-B site cation) were formed in the top LSMO layer when the heterostructure was cooled at a slower rate irrespective of BFO thickness, thereby demonstrating the effect of processing kinetics on the physical integrity of the heterostructure. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4765045]
Impact of material properties and device architecture on the device performance for a gate all around nanowire tunneling FET
(2017-11-01)
Singh, S. K.
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Gupta, A.
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Yu, H. W.
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Nagarajan, V.
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Anandan, D.
This paper systematically investigates the impact of gate dielectric, channel dimensional profile and the interface trap charge density on a homojunction indium-arsenide (InAs) gate all around nanowire tunneling FET (HJ-GAA-TFET). Device models were calibrated against the experimental data and simulations were performed to investigate the underlying physics. Device on-off (I-on/I-off) ratio was considered as key figure-of-merit (FOM) to improve. It is observed that the off current (I-off) is a weak function of dielectric constant, however, the on current (Ion) increases from 1.51 x 10(-7) A mu m(-1) to 1.79 x 10(-6) A mu m(-1) as the dielectric constant increases from SiO2 to La2O3. It was also observed that as the diameter increases, both Ion and Ioff increases. I-on/I-off ratio is independent for higher channel lengths but as the channel length is reduced below 30 nm, I-off increases causing degradation in Ion/Ioff ratio. Finally, the effect of interface traps was realised on the I-on/I-off ratio. Interface traps impact the flat-band voltage causing a shift in the device performance. It is observed that as the trap density increases, Ioff degrades rapidly by similar to 3 orders in magnitude.
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.
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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.
Misfit strain driven cation inter-diffusion across an epitaxial multiferroic thin film interface
(2014-02-07)
Krishnan, P. S. Sankara Rama
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Morozovska, Anna N.
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Eliseev, Eugene A.
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Ramasse, Quentin M.
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Kepaptsoglou, Demie
Cation intermixing at functional oxide interfaces remains a highly controversial area directly relevant to interface-driven nanoelectronic device properties. Here, we systematically explore the cation intermixing in epitaxial (001) oriented multiferroic bismuth ferrite (BFO) grown on a (001) lanthanum aluminate (LAO) substrate. Aberration corrected dedicated scanning transmission electron microscopy and electron energy loss spectroscopy reveal that the interface is not chemically sharp, but with an intermixing of similar to 2 nm. The driving force for this process is identified as misfit-driven elastic strain. Landau-Ginzburg-Devonshire-based phenomenological theory was combined with the Sheldon and Shenoy formula in order to understand the influence of boundary conditions and depolarizing fields arising from misfit strain between the LAO substrate and BFO film. The theory predicts the presence of a strong potential gradient at the interface, which decays on moving into the bulk of the film. This potential gradient is significant enough to drive the cation migration across the interface, thereby mitigating the misfit strain. Our results offer new insights on how chemical roughening at oxide interfaces can be effective in stabilizing the structural integrity of the interface without the need for misfit dislocations. These findings offer a general formalism for understanding cation intermixing at highly strained oxide interfaces that are used in nanoelectronic devices. (C) 2014 AIP Publishing LLC.
Structural, magnetic, and ferroelectric properties of T-like cobalt-doped BiFeO3 thin films
(2018-02-01)
Young, T.
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Sharma, P.
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Kim, D. H.
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Ha, Thai Duy
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Juang, Jenh-Yih
We present a comprehensive study of the physical properties of epitaxial cobalt-doped BiFeO3 films similar to 50 nm thick grown on (001) LaAlO3 substrates. X-ray diffraction and magnetic characterization demonstrate high quality purely tetragonal-like (T') phase films with no parasitic impurities. Remarkably, the step-and-terrace film surface morphology can be fully recovered following a local electric-field-induced rhombohedrallike to T' phase transformation. Local switching spectroscopy experiments confirm the ferroelectric switching to follow previously reported transition pathways. Critically, we show unequivocal evidence for conduction at domain walls between polarization variants in T'-like BFO, making this material system an attractive candidate for domain wall-based nanoelectronics. (c) 2018 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license.
Unraveling the origins of electromechanical response in mixed-phase bismuth ferrite
(2013-07-08)
Vasudevan, R. K.
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Okatan, M. B.
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Liu, Y. Y.
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Jesse, S.
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Yang, J. -C.
The origin of giant electromechanical response in a mixed-phase rhombohedral-tetragonal BiFeO3 thin film is probed using subcoercive scanning probe microscopy based multiple-harmonic measurements. Significant contributions to the strain arise from a second-order harmonic response localized at the phase boundaries. Strain and dissipation data, backed by thermodynamic calculations, suggest that the source of the enhanced electromechanical response is the motion of phase boundaries. These findings elucidate the key role of labile phase boundaries, both natural and artificial, in achieving thin films with giant electromechanical properties.