Browsing by Author "Pyatakov, Alexander P."

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    Destruction of spin cycloid in (111)(c)-oriented BiFeO3 thin films by epitiaxial constraint: Enhanced polarization and release of latent magnetization
    Bai, Feiming; Wang, Junling; Wuttig, Manfred; Li, Jiefang; Wang, Naigang; Pyatakov, Alexander P.; Zvezdin, Anatoly Konstantinovich; Cross, L. E.; Viehland, Dwight D. (AIP Publishing, 2005-01-01)
    In BiFeO3 films, it has been found that epitaxial constraint results in the destruction of a space modulated spin structure. For (111)c films, relative to corresponding bulk crystals, it is shown (i) that the induced magnetization is enhanced at low applied fields; (ii) that the polarization is dramatically enhanced; whereas, (iii) the lattice structure for (111)(c) films and crystals is nearly identical. Our results evidence that eptiaxial constraint induces a transition between cycloidal and homogeneous anti ferromagnetic spin states, releasing a latent antiferromagnetic component locked within the cycloid. (C) 2005 American Institute of Physics.
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    Dramatically enhanced polarization in (001), (101), and (111) BiFeO3 thin films due to epitiaxial-induced transitions
    Li, Jiefang; Wang, Junling; Wuttig, Manfred; Ramesh, R.; Wang, Naigang; Ruette, Benjamin; Pyatakov, Alexander P.; Zvezdin, Anatoly Konstantinovich; Viehland, Dwight D. (AIP Publishing, 2004-06-01)
    Dramatically enhanced polarization has been found for (001), (101), and (111) films, relative to that of BiFeO3 crystals. The easy axis of spontaneous polarization lies close to (111), for the various oriented films. BiFeO3 films grown on (111) have a rhombohedral structure, identical to that of single crystals; whereas films grown on (101) or (001) are monoclinically distorted from the rhombohedral structure, due to the epitaxial constraint. (C) 2004 American Institute of Physics.
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    Magnetic-field-induced phase transition in BiFeO3 observed by high-field electron spin resonance: Cycloidal to homogeneous spin order
    Ruette, Benjamin; Zvyagin, Sergei; Pyatakov, Alexander P.; Bush, A.; Li, Jiefang; Belotelov, V. I.; Zvezdin, Anatoly Konstantinovich; Viehland, Dwight D. (American Physical Society, 2004-02-27)
    Bismuth ferrite is a magnetoelectric material, which simultaneously has polarization and spin orders. We have used electron spin resonance (ESR) as a local probe of the magnetic order in the magnetic-field range of 0-25 T, in the frequency domain of 115-360 GHz, and at a temperature of 4.2 K. The data reveal significant changes in the ESR spectra with increasing field, which have been analyzed by taking into account the magnetic anisotropy of the crystal and a magnetoelectric Dzyaloshinsky-Moria-like interaction. The results demonstrate an induced phase transition from an incommensurately cycloidal modulated state to one with homogeneous spin order.
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    Multiferroic properties of modified BiFeO3-PbTiO3-based ceramics: Random-field induced release of latent magnetization and polarization
    Wang, Naigang; Cheng, J.; Pyatakov, Alexander P.; Zvezdin, Anatoly Konstantinovich; Li, Jiefang; Cross, L. E.; Viehland, Dwight D. (American Physical Society, 2005-09-28)
    It has been found that aliovalent-substituted BiFeO3-PbTiO3-based polycrystalline materials have significantly enhanced multiferroic properties. Relative to unmodified BiFeO3, our results for modified BiFeO3-PbTiO3-based ceramics reveal: (i) a dramatic increase in the electric-field-induced polarization; and (ii) the establishment of a remanent magnetization. The results evidence the destruction of a space-modulated spin structure in bulk materials, via substituent effects, releasing a latent magnetization locked within the cycloid.
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    Nanogrowth twins and abnormal magnetic behavior in CoFe2O4 epitaxial thin films
    Yan, Li; Wang, Yu; Li, Jiefang; Pyatakov, Alexander P.; Viehland, Dwight D. (American Institute of Physics, 2008-12-15)
    Nanogrowth twins (GTs) have been observed in CoFe2O4 (CFO) epitaxial thin films deposited on (111) oriented SrTiO3 substrates by pulsed laser deposition. The GTs form during nucleation and growth and consist of CFO growth regions that have a mirror relationship with respect to each other. We show that the films with GTs (i) are better crystallized than the ones without them and (ii) have higher saturation magnetizations due to the presence of twin boundaries.
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    Two-phonon coupling to the antiferromagnetic phase transition in multiferroic BiFeO(3)
    Ramirez, Mariola O.; Krishnamurthi, M.; Denev, S.; Kumar, Amit; Yang, Seung-Yeul; Chu, Ying-Hao; Saiz, Eduardo; Seidel, Jan; Pyatakov, Alexander P.; Bush, A.; Viehland, Dwight D.; Orenstein, J.; Ramesh, R.; Gopalan, Venkatraman (AIP Publishing, 2008-01-01)
    A prominent band centered at similar to 1000-1300 cm(-1) and associated with resonant enhancement of two-phonon Raman scattering is reported in multiferroic BiFeO(3) thin films and single crystals. A strong anomaly in this band occurs at the antiferromagnetic Neel temperature, T(N)similar to 375 degrees C. This band is composed of three peaks, assigned to 2A(4), 2E(8), and 2E(9) Raman modes. While all three peaks were found to be sensitive to the antiferromagnetic phase transition, the 2E(8) mode, in particular, nearly disappears at T(N) on heating, indicating a strong spin-two-phonon coupling in BiFeO(3). (c) 2008 American Institute of Physics.