Structural and Ferroelectric Properties of Large c/a Phase Bismuth Ferrite Thin Films Prepared by Ion Beam Sputtering
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Structural and Ferroelectric Properties of Large c/a Phase Bismuth Ferrite Thin Films Prepared by Ion Beam Sputtering Seiji Nakashima1, Yosuke Tsujita1, Hironori Fujisawa1, Jung Min Park2, Takeshi Kanashima2, Masanori Okuyama3, and Masaru Shimizu1 1 Depattment of Electrical Engineering and Computer Sciences, Graduate School of Engineering, University of Hyogo, 2167, Shosya, Himeji, Hyogo, 671-2280, Japan. 2 Department of Systems Innovation, Graduate School of Engineering Science, Osaka University, 1-3, Machikaneyama-cho, Toyonaka, Osaka, 560-8531, Japan. 3 Institute for NanoScience Design, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan ABSTRACT BiFeO3 (BFO) thin films have been deposited on SrRuO3/SrTiO3 (001) substrate by using ion beam sputtering process. At low oxygen partial pressure of 11 m Pa, rhombohedral and large c/a mixed phase thin film have been obtained in spite of rhombohedral BFO single phase formation at high oxygen partial pressure of 73 mPa. From wide area 2T-< mappings, diffraction peaks from large c/a phase BFO thin film were obtained with the same extinction rule as those of rhombohedral BFO. Reciprocal space mappings around BFO (003) and BFO (103) spots indicate that lattice parameters of large c/a phase BFO were a = 0.381 nm and c = 0.461 nm (c/a =1.22), respectively. Moreover ferroelectric domain switching could be observed in both of rhombohedral BFO and mixed phase BFO thin films. INTRODUCTION Most of multiferroic materials are Pb-free and are expected as alternative lead-free ferroelectrics or piezoelectrics from the view point of environmental ecology although PbZr1xTixO3 (PZT) is used as the most popular ferroelectric and piezoelectric materials. Among multiferroic materials, BiFeO3 (BFO) is the most investigated material from view points of material design of multiferroics and lead-free ferroelectrics because of its attractive properties of simultaneous coexistence of excellent ferroelectricity and antiferromagnetism at room temperature. [1, 2]. Crystal structure of bulk BFO is rhombohedrally-distorted perovskite structure belonging to space group of R3c with lattice parameter of ahex = bhex = 0.571(5) nm and chex = 1.3858(5) nm [3]. The noncentrosymmetric-distorted structure which takes an important role for ferroelectricity is stabilized by Bi – O bond with strong covalent characteristics of the 6s2 lone pair in Bi3+. In a recent report, BFO single crystal shows remanent polarization (Pr) of 60 PC/cm2 along [012]hex direction [3, 4]. It is known that BFO has large c/a ratio (~1.25) phase, and morphotropic phase boundary of normal and large c/a phases driven by strain in thin film form.[5-7] However, detailed properties of the large c/a ratio BFO have not been clarified yet. In this study, BFO thin films have been deposited on SrRuO3-buffered SrTiO3 (001) substrate by ion beam sputtering (IBS) process. IBS can be generate sputtered particles having
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TABLE I Sputtering conditions for BFO deposition Rhombohedral and large c/a Rhombohedral single mixed phase
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