Browsing by Author "Yao, Jianjun"

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    Aging associated domain evolution in the orthorhombic phase of < 001 > textured (K0.5Na0.5)Nb0.97Sb0.03O3 ceramics
    Yao, Jianjun; Li, Jiefang; Viehland, Dwight D.; Chang, Y. F.; Messing, G. L. (AIP Publishing, 2012-03-01)
    Aging effect due to domain evolution in (K-0.5,Na-0.5)Nb0.97Sb0.03O3 < 001 > textured ceramics was investigated by piezoresponse force microscopy. We find that aging effect is pronounced in the orthorhombic single phase field. A more uniform and finer domain structure on the order of several hundred nanometers was observed after aging and is believed to originate from defect-migration. After poling, large domains (similar to 10 mu m) with smooth boundaries were found in the aged condition due to the more readily redistribution of uniform and finer domain structures after aging. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3698154]
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    Anatomy of vertical heteroepitaxial interfaces reveals the memristive mechanism in Nb2O5-NaNbO3 thin films
    Li, Linglong; Lu, Lu; Wang, Zhiguang; Li, Yanxi; Yao, Yonggang; Zhang, Dawei; Yang, Guang; Yao, Jianjun; Viehland, Dwight D.; Yang, Yaodong (Springer Nature, 2015-03-18)
    Dynamic oxygen vacancies play a significant role in memristive switching materials and memristors can be realized via well controlled doping. Based on this idea we deposite Nb2O5-NaNbO3 nanocomposite thin films on SrRuO3-buffered LaAlO3 substrates. Through the spontaneous phase separation and self-assembly growth, two phases form clear vertical heteroepitaxial nanostructures. The interfaces between niobium oxide and sodium niobate full of ion vacancies form the conductive channels. Alternative I-V behavior attributed to dynamic ion migration reveals the memristive switching mechanism under the external bias. We believe that this phenomenon has a great potential in future device applications.
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    Controlled growth of epitaxial BiFeO3 films using self-assembled BiFeO3-CoFe2O4 multiferroic heterostructures as a template
    Li, Yanxi; Yang, Yaodong; Yao, Jianjun; Viswan, Ravindranath; Wang, Z. G.; Li, Jiefang; Viehland, Dwight D. (AIP Publishing, 2012-07-01)
    The growth mechanism of a BiFeO3 layer deposited on self assembled (0.65) BiFeO3-(0.35) CoFe2O4 (BFO-CFO) composite thin films was studied. Epitaxial and self-assembled BFO-CFO thin films were deposited on SrTiO3 (111) substrates by pulsed laser deposition and were subsequently used as a seed layer for the deposition of an additional BFO layer. x-ray line scans showed the heterostructures were highly epitaxial. Cross-sectional scanning electron microscopy and focused ion beam images revealed the top BFO layer grew preferentially from BFO nanopillars in the BFO-CFO thin films, thus, demonstrating controlled growth. The multiferroic properties of this new nanostructure were then studied. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4734508]
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    Effect of Mn substituents on the domain and local structures of Na1/2Bi1/2TiO3-BaTiO3 single crystals near a morphotropic phase boundary
    Yao, Jianjun; Yang, Yaodong; Monsegue, Niven; Li, Yanxi; Li, Jiefang; Zhang, Qinhui; Ge, Wenwei; Luo, Haosu; Viehland, Dwight D. (AIP Publishing, 2011-03-01)
    The ferroelectric domain and local structures of Na1/2Bi1/2TiO3-x% BaTiO3 (NBT-BT) and 0.14 at. % Mn substituted Na1/2Bi1/2TiO3-x% BaTiO3 (Mn:NBT-BT) single crystals with x=5.5 near a morphotropic phase boundary have been investigated by transmission electron microscopy. Increased ferroelectric ordering and enhanced in-plane octahedral tilting were observed for Mn: NBT-BT compared with NBT-BT. Bragg-filtered lattice images revealed that the size of the in-phase tilt domains of Mn:NBT-BT were on the order of 2 to 8 nm, with a tendency of alignment along {110}. (C) 2011 American Institute of Physics. [doi:10.1063/1.3573801]
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    Evolution of domain structures in Na1/2Bi1/2TiO3 single crystals with BaTiO3
    Yao, Jianjun; Yan, Li; Ge, Wenwei; Luo, L. A.; Li, Jiefang; Viehland, Dwight D.; Zhang, Qinhui; Luo, Haosu (American Physical Society, 2011-02-16)
    The domain structures of Na1/2Bi1/2TiO3-BaxTiO3 (NBT-x%BT) crystals for x = 0, 4.5, and 5.5 have been investigated by polarized light and piezoresponse force microscopies. The results show that BaTiO3 (i) refines the size of polar nanoregions and enhances their self-organization, and (ii) suppresses the formation of proper ferroelastic domains at high temperatures in the paraelectric state, and it rather favors the formation of improper ones that form below the ferroelectric Curie temperature and that elastically accommodate the ferroelectric ones.
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    Hierarchical domains in Na1/2Bi1/2TiO3 single crystals: Ferroelectric phase transformations within the geometrical restrictions of a ferroelastic inheritance
    Yao, Jianjun; Ge, Wenwei; Luo, L.; Li, Jiefang; Viehland, Dwight D.; Luo, Haosu (AIP Publishing, 2010-05-01)
    We have found unique hierarchical domains in Na1/2Bi1/2TiO3 single crystals. A tetragonal ferroelastic domain structure is unchanged on cooling from 520 to 25 degrees C. Polar microdomains then nucleated on cooling in the rhombohedral ferroelectric phase within the geometrical restrictions imposed by the inherited ferroelastic domains. (C) 2010 American Institute of Physics. [doi:10.1063/1.3443717]
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    The influence of Mn substitution on the local structure of Na0.5Bi0.5TiO3 crystals: Increased ferroelectric ordering and coexisting octahedral tilts
    Yao, Jianjun; Ge, Wenwei; Yan, Li; Reynolds, William T. Jr.; Li, Jiefang; Viehland, Dwight D.; Kiselev, Dmitry A.; Kholkin, Andrei L.; Zhang, Qinhui; Luo, Haosu (American Institute of Physics, 2012-03-15)
    The ferroelectric domain structure of pure Na1/2Bi1/2TiO3 (NBT) and 1 at.% Mn-doped NBT (Mn-NBT) crystals was investigated by piezoresponse force microscopy. The correlation length of the polar regions was found to increase upon Mn substitution. High resolution transmission electron microscopy revealed that the coherency of the lattice across the domain boundaries between polar regions was also enhanced. Selected area electron diffraction showed that Mn favored coexisting 1/2 (ooo) and 1/2 (ooe) oxygen octahedral tiltings, over only 1/2 (ooo) for pure NBT. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3699010]
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    Magnetoelectric quasi-(0-3) nanocomposite heterostructures
    Li, Yanxi; Wang, Zhongchang; Yao, Jianjun; Yang, Tiannan; Wang, Zhiguang; Hu, Jia-Mian; Chen, Chunlin; Sun, Rong; Tian, Zhipeng; Li, Jiefang; Chen, Long-Qing; Viehland, Dwight D. (Nature Publishing Group, 2015-12-01)
    Magnetoelectric composites of magnetic and ferroelectric components are promising for their use in applications such as information storage. Here, the authors find that magnetic quasiparticles embedded in a ferroelectric film matrix show promising properties compared to the usual thin-film architectures.
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    Monoclinic M-B phase and phase instability in 110 field cooled Pb(Zn1/3Nb2/3)O-3-4.5%PbTiO3 single crystals
    Yao, Jianjun; Cao, Hu; Ge, Wenwei; Li, Jiefang; Viehland, Dwight D. (AIP Publishing, 2009-08-01)
    We report the finding of a monoclinic M-B phase in Pb(Zn1/3Nb2/3)O-3-4.5%PbTiO3 single crystals. High precision x-ray diffraction investigations of [110] field cooled crystals have shown a transformation sequence of cubic(C)-> tetragonal(T)-> orthorhombic(O)-> monoclinic(M-B), which is different from that previously reported [A.-E. Renault , J. Appl. Phys. 97, 044105 (2005)]. Beginning in the zero-field-cooled condition at 383 K, a rhombohedral (R)-> M-B -> O sequence was observed with increasing field. Coexisting M-B and O phases were then found upon removal of field, which fully transformed to M-B on cooling to room temperature.
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    Observation of partially incoherent <110> boundaries between polar nanodomains in Na1/2Bi1/2TiO3 single crystals
    Yao, Jianjun; Ge, Wenwei; Yang, Yaodong; Luo, Liang; Li, Jiefang; Viehland, Dwight D.; Bhattacharyya, Somnath; Zhang, Qinhui; Luo, Haosu (American Institute of Physics, 2010-09-15)
    The ferroelectric domain structure of Na1/2Bi1/2TiO3 single crystals was investigated by transmission electron microscopy (TEM) and piezoforce microscopy (PFM). Bright-field TEM and PFM images revealed the presence of polar nanodomains, whose boundaries had a tendency to align along the < 110 >. High resolution TEM images revealed planar defects along these < 110 > boundaries, which partially relaxed the coherency of the lattice. (C) 2010 American Institute of Physics. [doi:10.1063/1.3488879]
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    Role of coexisting tetragonal regions in the rhombohedral phase of Na0.5Bi0.5TiO3-xat.%BaTiO3 crystals on enhanced piezoelectric properties on approaching the morphotropic phase boundary
    Yao, Jianjun; Monsegue, Niven; Murayama, Mitsuhiro; Leng, W. N.; Reynolds, William T. Jr.; Zhang, Qinhui; Luo, Haosu; Li, Jiefang; Ge, Wenwei; Viehland, Dwight D. (AIP Publishing, 2012-01-01)
    The ferroelectric domain and local structures of Na0.5Bi0.5TiO3-xat.%BaTiO3 (NBT-x%BT) crystals for x = 0, 4.5, and 5.5 have been investigated by transmission electron microscopy. The results show that the size of polar nano-regions was refined with increasing xat. %BT. The tetragonal phase volume fraction, as identified by in-phase octahedral tilting, was found to be increased with BT. The findings indicate that the large electric field induced strains in morphotropic phase boundary compositions of NBT-x%BT originate not only from polarization rotation but also polarization extension. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3673832]
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    Structural Investigations of Highly Strictive Materials
    Yao, Jianjun (Virginia Tech, 2012-04-25)
    Ferroelectric (piezoelectric) and ferromagnetic materials have extensively permeated in modern industry. (Na1/2Bi1/2)TiO3-BaTiO3 (NBT-x%BT) single crystals and K1/2Na1/2NbO3 (KNN) textured ceramics are top environment-friendly candidates which have potential to replace the commercial lead zirconate titanate or PZT. High magnetostrictive strain (up to 400 ppm) of Fe-xat.%Ga makes this alloys promising alternatives to existing magnetostrictive materials, which commonly either contain costly rare-earth elements or have undesirable mechanical properties for device applications. These systems have common characteristics: compositional/thermal/ electrical dependent structural heterogeneity and chemical disorder on sub-micron or nano scale, resulting in diverse local structures and different physical properties. In this work, I have investigated domain and local structures of NBT-x%BT crystals, KNN ceramics and Fe-xat.%Ga alloys under various conditions, mainly by scanning probe and electron transmission techniques. In NBT-x%BT single crystals, polarized light, piezo-response force (PFM) and transmission electron (TEM) microscopies were used to study domain structures and oxygen octahedral tiltings. Hierarchical domain structures were found in NBT: a high-temperature tetragonal ferroelastic domain structure is elastically inherited into a lower temperature rhombohedral ferroelectric phase. Nanoscale domain engineering mechanism was found to still work in NBT-x%BT system and a modified phase diagram was proposed based on domain observations. An increased intensity of octahedral in-phase tilted reflections and a decrease in the anti-phase ones was observed, with increasing x as the morphotropic phase boundary (MPB) is approached. It was also found that Mn substituents favor the formation of long range ordered micro-sized ferroelectric domains and octahedral in-phase tilted regions near the MPB. Nano-size heterogeneous regions were observed within submicron domain structure, indicating that the nanoscale polarization dynamics are not confined by domain boundaries, and the high piezoelectricity of NBT-x%BT is due to a polarization dynamics with high sensitivity to electric field and a broadened relaxation time distribution. In KNN textured ceramics, an aging effect was found to exist in the orthorhombic single phase field, not only in the orthorhombic and tetragonal two-phase field as previously reported. No variation of phase structure was revealed between before and after aging states. However, pronounced changes in domain morphology were observed by both PFM and TEM: more uniform and finer domain structures were then found with aging. These changes were even more pronounced after poling the aged state. A large number of sub-micron lamellar domains within micron-domains were observed: suggesting a domain origin for improved piezoelectric properties. In Fe-xat.%Ga alloys, an underlying inhomogeneity from Ga atoms embedded into the α-Fe matrix was believed to be the origin of giant magneostrictive properties. I have systematically investigated the phase structure and nano-size heterogeneity of Fe-xat.%Ga alloys subjected to different thermal treatments using standard TEM and high resolution TEM for 10