Ferroelectric and Structural Antiphase Domains in Hexagonal R MnO 3
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Ferroelectric and Structural Antiphase Domains in Hexagonal RMnO3 K. Kobayashi1, T. Koyama1, Y. Horibe2, T. Katsufuji3, S-W. Cheong2 and S. Mori1.* Department of Materials Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan 2 Department of Physics and Astronomy, Rutgers University, Piscataway, NJ, 08854, USA 3. Department of Physics, Waseda University, Tokyo 169-8555, Japan * [email protected] 1
ABSTRACT We have investigated characteristic ferroelectric and structural antiphase domain structures in single crystals of hexagonal RMnO3 (R=Y, Ho, Lu, and Yb) by obtaining various electron diffraction patterns, dark-filed images and high-resolution lattice images. In the ferroelectric phase of RMnO3 characteristic domain structures consisting of six ferroelectric and structural antiphase domains, which can be identified as the “cloverleaf” pattern, is found in the (110) plane, in addition to the (001) plane, and are inherent to the ferroelectric phase of hexagonal RMnO3. In domain configuration with the cloverleaf pattern in the (110) plane, the structural antiphase boundaries are inclined to be parallel to the [001] direction. INTRODUCTION Hexagonal manganites RMnO3 (R: rare earth) with R=Ho-Lu, Y and Sc are improper ferroelectric (FE) compounds with Curie temperature (Tc) considerably higher than 1000K [1]. These compounds have a hexagonal structure with the space group of P63cm in the FE phase below Tc. In the hexagonal structure of RMnO3, each Mn3+ ion is surrounded by three in-plane and two apical oxygen ions to form a MnO5 block. The MnO5 blocks are two-dimensionally connected with each other on their corners, and a triangular Mn3+ lattice is formed, as shown in Fig. 1 [1]. In particular, the Mn trimerization is formed on triangular lattice and, as a result, the modulated structure with the modulation vector of q=1/3a110 appears in the FE phase. The FE phase is characterized by a buckling of the layered MnO5 polyhedra, accompanied by displacements of the R ions along the [001] direction, which give rise to a net electric polarization [2,3,4]. In YMnO3, a structural phase transition from a paraelectric P63/mmc structure to a FE P63cm structure takes place at about Tc~933K and a spontaneous polarization (Ps) appears along the [001] direction. Subsequently, the antiferromagnetic transition occurs at approximately TN~70K on cooling. [5,6] These hexagonal manganites have a strong coupling between the FE and magnetic orderings and these two distinct orderings coexist in one compound, which are referred to as “multiferroic” compounds [7]. Recently transmission electron microscopic (TEM) studies, combined with conductive atomic force microscopy, revealed the formation of the intriguing conductive “cloverleaf” pattern compromising six antiphase domains emerging from one central point. These six domains is characterized as structural antiphase domains due to the Mn trimerization along the [110] direction, in which antiparallel polarizations along the [001] direction appear in the adjacent domains [8]. I
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