Browsing by Author "Munroe, P."

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Effect of processing kinetics on the structure of ferromagnetic-ferroelectric-ferromagnetic interfaces
(2012-11-15)
Krishnan, P. S. Sankara Rama
;
Ramasse, Q. M.
;
Liang, Wen-I
;
Chu, Ying-Hao
;
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]
Mapping strain modulated electronic structure perturbations in mixed phase bismuth ferrite thin films
(2015-01-01)
Krishnan, P. S. Sankara Rama
;
Aguiar, Jeffery A.
;
Ramasse, Q. M.
;
Kepaptsoglou, D. M.
;
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.