Features of the ferroelectric domain structure in the multiferroic material YbMnO 3
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Features of the ferroelectric domain structure in the multiferroic material YbMnO3 Takumi Inoshita,1 Yasuhide Inoue,2 Yoichi Horibe,3 and Yasumasa Koyama1,2 1 Department of Electronic and Physical system, Waseda University, 3-4-1, Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan 2 Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-826, Nishiwaseda, Shinjuku-ku, Tokyo, 169-0051, Japan 3 Department of Materials Science and Engineering, Kyushu Institute of Technology, 1-1, Sensui, Tobata-ku, Kitakyushu, Fukuoka, 804-8550, Japan ABSTRACT The multiferroic material YbMnO3 has been reported to exhibit both ferroelectric and antiferromagnetic orders in the ground state. Of these two orders, the ferroelectric order is associated with the P63/mmc-to-P63cm structural transition, which occurs around 1270 K. The interesting feature of the ferroelectric state is that a cloverleaf domain structure with a pseudosix-fold symmetry is observed in transmission electron microscopy images with the beam incidence parallel to the hexagonal axis. To understand the origin of the formation of the cloverleaf domain structure, we have examined the crystallographic features of the ferroelectric state in YbMnO3 by transmission electron microscopy. In this study, particularly, we adopted the experimental condition that electron beam incidences are perpendicular to the hexagonal axis. It was, as a result, found that there existed various ferroelectric domain structures including the cloverleaf domain structure under the present condition. The notable feature of domain structures found in this study is that each domain structure basically consists of six domains, whose domain boundaries are terminated at one point. Because this feature makes us reminiscent of a discommensurate structure in an incommensurate state, we took high-resolution electron micrographs of areas including domain boundaries. Their analysis indicated that a domain boundary could be identified as a discommensuration with a phase slip of π/3. It is thus understood that the cloverleaf domain structure should be one of domain morphologies for a discommensurate structure, which is related to the break of the translational symmetry. INTRODUCTION The hexagonal manganite YbMnO3 is one of multiferroic materials exhibiting both ferroelectric and antiferromagnetic orders in the ground state [1,2]. According to the previous studies on YbMnO3, the appearance of its ferroelectricity is associated with the P63/mmc-toP63cm structural transition, which occurs around 1270 K on cooling from the paraelectric P63/mmc state [3,4]. Because the transition is due to the condensation of the K3 mode at the zone-boundary K point, the ferroelectric state can be regarded as the 3k state characterized by the _
appearance of three modulation waves with k = h in the hexagonal notation. However, the K3 mode itself cannot produce a net electric polarization. The ferroelectricity is then understood to be due to a mode coupling between the K3 and zone-center polar Γ2- modes [4]. Another
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