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Browsing by Author "Scott, J. F."

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    Phonon anomalies and phonon-spin coupling in oriented PbFe0.5Nb0.5O3 thin films
    Correa, Margarita; Kumar, Amit; Priya, Shashank; Katiyar, Ram S.; Scott, J. F. (American Physical Society, 2011-01-18)
    We present Raman data on both single-crystal and thin-film samples of the multiferroic PbFe0.5Nb0.5O3 (PFN). We show first that the number and selection rules of Raman lines are compatible with a face-centered-cubic Fm-3m structure, as is known in other ABO(3) relaxors, such as PbSc1/2Ta1/2O3. We then compare Raman data with anomalies in magnetization and the dielectric constant near the magnetic-phase-transition temperature (T-N), the diffuse ferroelectric-phase-transition temperature (T-m), and the pseudostructural-phase transition temperature (Burns temperature similar to T-B). The temperature evolution of the Raman spectra for the PFN film shows measurable changes in phonon positions, intensities, and full width at half maxima near 200, 410, and 650 K-temperatures that match well with experimentally observed T-N, T-m, and T-B, respectively. The increase in frequency with increasing temperature for the lowest-energy F-2g phonon mode is particularly unexpected. These changes suggest the transition of the crystal structure from an ordered phase to a disordered phase near T-B. The Raman study revealed phonon anomalies in the vicinity of T-m and T-N that are attributed to the dynamical behavior of polar nanoregions and spin-phonon coupling owing to its relaxor and multiferroic nature, respectively, which is well supported by dielectric and magnetic properties of the PFN thin film. Softening of the Fe-O mode was observed near the T-N. We correlate the anomalous shift of the Fe-O mode frequency with the normalized square of the magnetization sublattice; agreement with the experimental results suggests strong spin-phonon coupling near T-N owing to phonon modulation of the superexchange integral; however, the shifts in frequency with temperature are small (<3 cm(-1)).
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    Room temperature multiferroic effects in superlattice nanocapacitors
    Dussan, S.; Kumar, Amit; Scott, J. F.; Priya, Shashank; Katiyar, Ram S. (AIP Publishing, 2010-12-01)
    A composite nanocapacitor was fabricated based on ferroelectric PbZr(0.52)Ti(0.48)O(3)( PZT) and half-metallic oxide La(0.67)Sr(0.33)MnO(3)(LSMO) with 5 nm/l nm periodicity, respectively. X-ray theta-2 theta scan and Phi-scan revealed epitaxial growth of nanocapacitors. At room temperature, a microscopic polarization of 11 mu C/cm(2) and magnetization of 50 emu/cm(3) were measured for superlattice nanocapacitors. Local piezo force microscopy measurements revealed switching of polarization under external bias field confirming ferroelectric behavior. Zero field cooling measurements showed the existence of cusp in magnetization at low temperatures indicating spin-glass-like behavior contrary to Pb( Zr(x)Ti(1-x))O(3) /La(x)Sr(1-x)MnO(3) bilayer structure. Frequency dependent dielectric anomaly was observed near room temperature suggesting dynamic magneto (resistance)-dielectric coupling. (C) 2010 American Institute of Physics. [doi:10.1063/1.3528210]