Department of Physics, Applied Physics and Astronomy

Binghamton University

Department of Physics, Applied Physics and Astronomy

PHYSICS COLLOQUIUM

“Evidence of rhombohedral structure within hetero-epitaxial BiFeO3 thin films”

Dr. In Tae Bae

S3IP Analytical and Diagnostics Laboratory, Binghamton University

Abstract:

BiFeO3 (BFO) is a multiferroic material that shows ferroelectricity and antiferromagnetism. It was reported that when BFO grows thin film, its spontaneous polarization becomes ~60mC/cm2, which is one order of magnitude higher than its bulk form.[i] In order to understand the mechanism of the substantial increase in spontaneous polarization, the knowledge about detailed crystal structure of the hetero-epitaxially grown BFO film so important that a number of studies have attempted to reveal its crystal structure. Some studies reported thin film BFO has cubic structure with monoclinic tilting,[ii],[iii] while others reported monoclinically distorted rhombohedral,[iv] and monoclinic structures with domains.[v],[vi] Despite these previous efforts, the crystal structure of thin film BFO remains debated. It may be worth noting that most of the previous studies have implemented x-ray scattering based techniques. While x-ray scattering has strength for investigating subtle changes in single crystal structure by focusing on a local reciprocal area, transmission electron microscopy (TEM) technique has advantage to investigate overall crystal structure in that it readily yields information on two-dimensional reciprocal lattices up to a scattering vector of ~220 nm-1.[vii]

In this work, cross-sectional TEM study was performed on BFO layers grown on SrTiO3 (STO) substrates from multiple BFO zone axes to acquire multiple cross-sections of three-dimensional reciprocal lattice of BFO to precisely evaluate its crystal structure and epitaxial growth mechanism.

Nano-beam electron diffraction (NBED) patterns combined with structure factor calculation and high-resolution TEM images unambiguously revealed that BFO thin layer grew with a rhombohedral structure identical to its bulk form. To the best of my knowledge, this is the first to provide evidence of rhombohedral structure within hetero-epitaxially grown BFO film. No evidence of monoclinic and/or tetragonal distortion was found. The rhombohedral BFO thin layer was found to grow onto STO substrate by maintaining an epitaxial relationship in a manner that minimizes the lattice mismatch at the BFO/STO interface. High resolution TEM images compared with multi-slice simulation also proved rhombohedral structure within the BFO films. In addition, electron energy loss spectrum obtained from O K-edge of BFO thin layer shows highly similar characteristics with its bulk form, providing additional evidence of rhombohedral structure of BFO thin layer. NBED combined with SF calculation and high-resolution TEM analysis combined with multi-slice simulation turned out highly effective to properly interpret crystal structure of BFO thin films and to study its growth mechanism.

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Monday, September 21, 2015

Science Library Room 212

PRESENTATION 11:00 PM– 12:00PM

ALL WELCOME – COFFEE AND REFRESHMENTS AT 10:50am