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Brownmillerite SrFeOx epitaxial thin films: Topotactic phase transformation, orientation control, and improper ferroelectricity

掲載日:
講演会
開催日:
日時 平成29年8月7日(月) 15:30 – 16:30
場所 電子科学研究所1階 セミナー室 1 – 2
講演者 Prof. Woo Seok Choi
所属等 Department of Physics, Sungkyunkwan University, Suwon, 440-746, Korea
タイトル Brownmillerite SrFeOx epitaxial thin films: Topotactic phase transformation, orientation control, and improper ferroelectricity
講演趣旨 Strontium ferrite (SrFeOx, SFO) exhibits a rich x-dependent electronic and magnetic phases, ranging from an antiferromagnetic insulating to a ferromagnetic metallic phase. By decreasing x (or increasing oxygen vacancy concentration) from nominal 3.0 (perovskite) to 2.5 (brownmillerite), the valence state of Fe changes from 4+ to 3+, which greatly influences the material’s crystal and electronic structures. A wide variety of physical and chemical behavior can be observed from the materials system, ranging from the topotactic phase transformation to improper ferroelectricity.
In the first part of the talk, we will discuss topotactic phase transformation in SrFeOx epitaxial thin films studied by real-time spectroscopic ellipsometry. The topotactic phase transformation illustrates strong correlation among defect concentration, atomic and electronic structure. A pressure-temperature phase diagram of topotactic phase transformation is constructed and discussed in terms of thermodynamic calculation. In the second part, we will present thermodynamic control of crystallographic orientation of the anisotropic brownmillerite SrFeO2.5 thin film on SrTiO3 substrate. The anisotropic electronic and phonon structures are discussed based on the layered crystal structure and orbital dependent optical transitions. The simple control of oxygen vacancy channel orientation has a large implication in designing functional properties such as oxygen ion conduction and hydrogen strogate. In the third part, the brownmillerite SrFeO2.5 thin film is examined in terms of improper ferroelectricity. The inversion symmetry of the crystal can be broken based on the specific ordering of rotated FeO4 tetrahedral units. This leads to a structural ferroelectricity in the material, which can be confirmed by different experimental approaches.
参考資料(論文) Amit Khare, Dongwon Shin, Taesup Yoo, Minu Kim, Tae Dong Kang, Jaekwang Lee, Seulki Roh, Jungseek Hwang, Sung Wng Kim, Tae Won Noh, Hiromichi Ohta, and Woo Seok Choi, “Topotactic Metal-Insulator Transition in Epitaxial SrFeOx Thin Films”, Adv. Mater. in press.
備考等 English (英語の講演です)
主催 電子科学研究所学術交流委員会
連絡先 電子科学研究所 薄膜機能材料研究分野 太田裕道(ex 9428)
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