Browsing by Author "Wang, Zhiguang"

Now showing 1 - 14 of 14
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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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    Domain rotation induced strain effect on the magnetic and magneto-electric response in CoFe2O4/Pb(Mg,Nb)O-3-PbTiO3 heterostructures
    Wang, Zhiguang; Viswan, Ravindranath; Hu, Bolin; Li, Jiefang; Harris, Vincent G.; Viehland, Dwight D. (American Institute of Physics, 2012-02-01)
    The present work shows good control of both magnetic and electric properties with electric and magnetic fields, respectively, for epitaxial CoFe2O4 (CFO) films on Pb(Mg,Nb)O-3-PbTiO3 (PMN-PT). X-ray reciprocal space mapping revealed a transformation between a- and c-domains in the PMN-PT under electric field (E). Magnetic hysteresis loop and magnetic force microscopy (NM) measurements showed a considerable change in the magnetic properties in specific areas of CFO layers poled by MFM probe tips. Furthermore, a pulsed electric field applied to the substrate was found to switch the magnetization of CFO between high and low values, depending on the polarity of E. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3684546]
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    Effects of Precursor-Substrate Distances on the Growth of GaN Nanowires
    Cheng, Hongbin; Li, Jia; Wu, Dongxu; Li, Yanxi; Wang, Zhiguang; Wang, Xianying; Zheng, Xuejun (Hindawi, 2015-08-12)
    GaN nanowires were synthesized through the Ni-catalyzed chemical vapor deposition (CVD) method using Ga2O3/GaN mixtures as gallium sources, and precursor-substrate distances were investigated as the important factor for the growth of GaN nanowires. The microstructure, composition, and photoluminescence property were characterized by X-ray diffraction, field emission scanning electron microscopy, high-resolution transmission electron microscopy, and photoluminescence spectra. The results showed that single crystalline GaN nanowires with the diameter of about 90 nm and the length up to tens of micrometers had been grown thickly across Si (100) substrates with uniform density. Moreover, the variations of the GaN nanowire morphology, density, and size were largely attributed to substrate positions which would influence Ga precursor density in the carrier gas, the saturation degree of gaseous reactants, and the catalyst activity, respectively, in the fabrication of GaN nanowires by the vapour liquid solid mechanism.
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    Electric-field induced strain modulation of magnetization in Fe-Ga/Pb(Mg1/3Nb2/3)-PbTiO3 magnetoelectric heterostructures
    Zhang, Yue; Wang, Zhiguang; Wang, Yaojin; Luo, Chengtao; Li, Jiefang; Viehland, Dwight D. (American Institute of Physics, 2014-02-24)
    Magnetostrictive Fe-Ga thin layers were deposited on < 110 >-oriented Pb(Mg1/3Nb2/3)-30% PbTiO3 (PMN-30% PT) substrates by pulsed laser deposition. The as-prepared heterostructures showed columnar arrays aligned in the out-of-plane direction. Transmission electron microscopy revealed nanocrystalline regions within the columnar arrays of the Fe-Ga film. The heterostructure exhibited a strong converse magnetoelectric coupling effect of up to 4.55 x 10(-7) s m(-1), as well as an electric field tunability of the in-plane magnetic anisotropy. Furthermore, the remanent magnetization states of the Fe-Ga films can be reversibly and irreversibly changed by external electric fields, suggesting a promising and robust application in magnetic random access memories and spintronics. (C) 2014 AIP Publishing LLC.
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    Epitaxial growth of Pb(Zr0.53Ti0.47)O-3 films on Pt coated magnetostrictive amorphous metallic substrates toward next generation multiferroic heterostructures
    Hu, Bolin; Chen, Yajie; Yang, Aria F.; Gillette, Scott M.; Fitchorov, Trifon; Geiler, Anton L.; Daigle, Andrew; Su, X. D.; Wang, Zhiguang; Viehland, Dwight D.; Harris, Vincent G. (American Institute of Physics, 2012-03-15)
    Piezoelectric films of Pb(Zr0.53Ti0.47)O-3 (PZT) were deposited by pulsed laser deposition onto metallic magnetostrictive substrates. In order to optimize the growth of PZT films, a buffer layer of Pt was employed, as well as variation of deposition temperature, pressure, and laser energy. Room temperature 0-20 x-ray diffraction measurements indicate all diffraction features correspond to reflections indexed to a single PZT phase of space group P4mm. Scanning electron microscopy images reveal pinhole-free dense films of pyramidal shaped crystal arrangements whose orientation and size were controlled by variation of oxygen pressures during deposition. The resulting PZT films had a value of d(33) similar to 46 pm/V representing a 53% increase over previous efforts to realize a piezoelectric/Metglas (TM) film heterostructure. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3697605]
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    Giant electric field controlled magnetic anisotropy in epitaxial BiFeO3-CoFe2O4 thin film heterostructures on single crystal Pb(Mg1/3Nb2/3)(0.7)Ti0.3O3 substrate
    Wang, Zhiguang; Yang, Yaodong; Viswan, Ravindranath; Li, Jiefang; Viehland, Dwight D. (AIP Publishing, 2011-07-01)
    We have deposited self-assembled BiFeO3 (BFO)-CoFe2O4 (CFO) thin films on Pb(Mg1/3Nb2/3)(0.7)Ti0.3O3 (PMN-PT) substrates and studied the change in magnetic anisotropy under different strain conditions induced by an applied electric field. After electric field poling, we observed (i) giant magnetization change: magnetization of original CFO phase is three times larger than that of strained one and (ii) magnetic force microscopy line profiles that exhibited significant change in the CFO magnetic domain response in accordance to magnetization-field (M-H) loops. Together, these results demonstrate good control of the magnetic properties of CFO via an electric field induced strain. (C) 2011 American Institute of Physics. [doi:10.1063/1.3619836]
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    Giant magnetoelectric effect in self-biased laminates under zero magnetic field
    Li, Menghui; Wang, Zhiguang; Wang, Yaojin; Li, Jiefang; Viehland, Dwight D. (AIP Publishing, 2013-02-01)
    A giant magnetoelectric (ME) effect in self-biased annealed Metglas/Pb(Zr,Ti)O-3/Metglas laminates under zero magnetic bias is reported. The remanent magnetization was increased by annealing Metglas, which generated an internal bias field. This shifted the M-H hysteresis loops, yielding large values for the ME voltage coefficient of alpha(ME) = 12 V/cm.Oe and 380 V/cm.Oe at 1 kHz and electromechanical resonance under zero magnetic bias, respectively. This self-biased laminate is shown to have a high sensitivity to ac magnetic fields. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794056]
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    Magnetoelectric and multiferroic properties of variously oriented epitaxial BiFeO3-CoFe2O4 nanostructured thin films
    Yan, Li; Wang, Zhiguang; Xing, Zengping; Li, Jiefang; Viehland, Dwight D. (American Institute of Physics, 2010-03-15)
    We report the ferroelectric, ferromagnetic, and magnetoelectric (ME) properties of self-assembled epitaxial BiFeO3-CoFe2O4 (BFO-CFO) nanostructure composite thin films deposited on (001), (110), and (111) SrTiO3 (STO) single crystal substrates. These various properties are shown to depend on orientation. The maximum values of the relative dielectric constant, saturation polarization, longitudinal piezoelectric coefficient, saturation magnetization, and ME coefficient at room temperature were 143, 86 mu m/cm(2), 50 pm/V, 400 emu/cc, and 20 mV/cm Oe, respectively.
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    Magnetoelectric effect in crystallographically textured BaTiO3 films deposited on ferromagnetic metallic glass foils
    Wang, Zhiguang; Yan, Li; Yang, Yaodong; Li, Jiefang; Das, Jaydip; Geiler, Anton L.; Yang, Aria F.; Chen, Yajie; Harris, Vincent G.; Viehland, Dwight D. (American Institute of Physics, 2011-02-01)
    We demonstrate a significant control of the polarization response under an applied magnetic field for a magnetoelectric (ME) heterostructure. This structure was comprised of a 2 mu m thick ferroelectric BaTiO3 (BTO) film deposited on flexible ferromagnetic metallic glass foil (25 mu m thick). Au was used as a buffer layer to control BTO growth orientation, and to protect the metallic glass from oxidation. x-ray diffraction and scanning electron microscopy demonstrated the successful growth of well-crystallized BTO films with a high degree of (111) orientation on the amorphous metallic glass foils. Well-defined polarization (P-E) and magnetization (M-H) hysteresis loops confirmed the coexistence of ferroelectric and ferromagnetic properties. A ME voltage coefficient of about similar to 60 mV/cm Oe was measured. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3544352]
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    Magnetoelectric Effect in Ferroelectric-Ferromagnetic Heterostructures
    Wang, Zhiguang (Virginia Tech, 2014-05-28)
    The magnetoelectric (ME) effect, a coupling effect between magnetic and electric orders, has been widely investigated, both from a fundamental science perspective and an applications point of view. Magnetoelectric composites with one piezoelectric phase and one magnetostrictive phase can be magneto-electrically coupled via elastic strain mediation. Bulk magnetoelectric composites have been intensively studied as magnetic sensors due their significant magnetic-to-electric signal transforming efficiency, which promises high magnetic field sensitivity. In contrast, electric field-controlled magnetization in magnetoelectric thin films is more attractive for information recording and novel electrically-tunable microwave magnetic devices. For the present work, we prepared a series of magnetoelectric structures capable of modulating the magnetization with an electric field -- all of which display unprecedented magnetic coercive field tunability. These structures show promise for a number of applications, including magnetic memory and spintronics. First, we generated self-assembled BiFeO3-CoFe2O4 (BFO-CFO) nanostructures of varying architectural structures on differently-oriented perovskite substrates. We were able to control aspect ratio through both thickness control and by manipulating growth thermodynamics. The relationship between magnetic shape and strain anisotropy was systematically analyzed using both in-plane and out-of-plane magnetic easy axis data. The BFO-CFO self-assembled structures may be useful for applications, including longitudinal and perpendicular magnetic memory; additionally they can serve as a prototype for analyzing the magnetoelectric effect-based magnetoresistive random-access memory. BFO-CFO grown on piezoelectric Pb(Mg,Nb)O3-PbTiO3 (PMN-PT) shows a large magnetoelectric coupling coeffcient. Second, we sought to clarify the relationship between ferroelectric/ferroelastic phase transformation and the magnetoelectric effect in CFO films on PMN-PT heterostructures. Elastic strain is an essential component of electro-mechanical-magnetic coupling. Most prior studies that used piezoelectric materials as a strain source assumed that these materials shared a linear relationship (d31 or d33) with the electric field, which is true only with small electric field signals. In contrast, the largest strain is produced during phase transformation in piezoelectric single crystals. In this work, we systematically investigated electric field induced phase transformation in PMN-PT single crystals with different compositions. A signficant finding that emerged from this study is that a large in-plane uniaxial strain can be controlled by an electric field, and this strain can be used to control the magnetic easy axis distribution in the in-plane. The electric field is along the out-of-plane direction, which is perpendicular to the uniaxial strain and the surface of the sample, and thus can be easily incorporated into real device design. Finally, we identified very large magnetic coercive field tunability in the CFO/PMN-PT monolithic structures -- in fact, more than ten times larger than previously reported magnetoelectric heterostructures. We used a <011> oriented PMN-PT substrate, where a large uniaxial strain can be induced by an electric field. Importantly, since the two in-plane directions have the same dimensions, the uniaxial strain can induce a significant magnetic anisotropy distribution change in the two in-plane directions. A unprecedented magnetic coercive field change of up to 580 Oe has been observed, which shows great potential for applications in both magnetic memory and microwave magnetic devices.
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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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    Piezoelectric properties of epitaxial Pb(Zr-0.525, Ti-0.475)O-3 films on amorphous magnetic metal substrates
    Hu, Bolin; Chen, Yajie; Yang, Aria F.; Gillette, Scott M.; Fitchorov, Trifon; Geiler, Anton L.; Daigle, Andrew; Su, X. D.; Wang, Zhiguang; Viehland, Dwight D.; Vittoria, Carmine; Harris, Vincent G. (American Institute of Physics, 2012-04-01)
    Epitaxial growth of high piezoelectric constant Pb(Zr-0.525, Ti-0.475)O-3 (PZT) thin films deposited on amorphous magnetic Metglas (R) substrates by pulsed laser deposition (PLD) is reported. Particularly, Pt or Au buffer layers were employed to initiate epitaxial growth of the PZT films from atop of an amorphous surface. The optimization of deposition conditions for the PZT films with different buffer layers was systematically investigated. The crystal structure, texturing, and surface morphology of the samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Ferroelectric and piezoelectric properties were measured indicating high polarization 27 mu C/cm(2) and piezoelectric constant d(33), 46 pm/V for the Pt buffered films. The PZT thin films grown on a magnetostrictive material have demonstrated high quality crystallographic structure and piezoelectric response, having potential for use in emerging magnetoelectric sensors. (C) 2012 American Institute of Physics. [doi:10.1063/1.3677864]
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    Pulsed Laser Deposition of BaTiO3 Thin Films on Different Substrates
    Yang, Yaodong; Wang, Zhiguang; Li, Jiefang; Viehland, Dwight D. (Hindawi Publishing Corporation, 2010)
    We have studied the deposition of BaTiO3 (BTO) thin films on various substrates. Three representative substrates were selected from different types of material systems: (i) SrTiO3 single crystals as a typical oxide, (ii) Si wafers as a semiconductor, and (iii) Ni foils as a magnetostrictive metal. We have compared the ferroelectric properties of BTO thin films obtained by pulsed laser deposition on these diverse substrates.
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    Self-assembled NaNbO3-Nb2O5 (ferroelectric-semiconductor) heterostructures grown on LaAlO3 substrates
    Wang, Zhiguang; Li, Yanxi; Chen, Bo; Viswan, Ravindranath; Li, Jiefang; Viehland, Dwight D. (AIP Publishing, 2012-09-01)
    We deposited NaNbO3 (NNO)-Nb2O5 (NO) self-assembled heterostructures on LaAlO3 (LAO) to form ferroelectric-semiconductor vertically integrated nanostructures. The NNO component formed as nanorods embedded in a NO matrix. X-ray diffraction confirmed epitaxial growth of both NNO and NO phases. Phase distribution was detected by scanning electron microscopy. The NNO/NO volume ratio was strongly dependent on the deposition temperature due to the volatility of sodium. Piezoelectric force microscopy revealed a good piezoelectric response in the NNO component with a piezoelectric coefficient of D-33 approximate to 12 pm/V, with SrRuO3 (SRO) acting as bottom electrode. The current-voltage characterization of NNO-NO/SRO-LAO showed a typical diode rectifying behavior. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754713]