Doping Effect on the Charge Ordering in LuFe2O4
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0966-T03-04
Doping Effect on the Charge Ordering in LuFe2O4 Yoji Matsuo1, Satoshi Shinohara1, Shigeo Mori1, Yoichi Horibe2, Kenji Yoshii3, and Naoshi Ikeda4 1 Department of Physics, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka, 599-8531, Japan 2 Dept. of Physics & Astronomy, Rutgers University, Piscataway, NJ, 08854 3 JAERI, Mikazuki, Hyougo, 679-5148, Japan 4 Department of Physics, Okayama University, Okayama, 700-8530, Japan
ABSTRACT Change of the charge ordered (CO) structure by substituting Cu2+ for Fe2+ in LuFe2O4 was investigated by means of the transmission electron microscopy. The CO structure in LuFe2O4 is characterized by the modulated structure with the wave vector of q=1/3[1-13/2] and the average size of the CO domains can be estimated to be about 10-20nm. On the contrary, the Cu2+ substitution in LuFe2O4 destroyed the CO structure drastically and induced characteristic local lattice distortion, which gives rise to characteristic diffuse scattering in the reciprocal space. High-resolution lattice images revealed that there exist nano-scale clusters, which are characterized as the short-range ordering of the Fe3+ and Cu2+ ions on the triangular lattice. In addition, the magnetic measurement revealed that LuFeCuO4 exhibits an antiferromagnetic transition around 50K, which is lower than the Neel temperature of 250K in LuFe2O4. INTRODUCTION Recently magnetic ferroelectric materials have attracted renewed interest because of their potential in novel magnetoelectric and magneto-optical devices.[1] In these materials, magnetism and ferroelectricity are involved with local spins and off-center structural distortion, respectively. These two seemingly unrelated phenomena can coexist in these materials. In addition, the mutual control of magnetization and electric polarization can be realized in some hexagonal manganites such as TbMnO3.[2,3] On the other hand, the coexisting state of the magnetic and electric ordering was found in insulating antiferromagnetic compounds such as RFe2O4 (R=Ho-Lu, Y) with the two-dimensional triangular lattice and with the strong geometrical frustration.[4] RFe2O4 (R=Ho-Lu,Y) crystallized a rhombohedral structure with the space group of R-3m. [5] In these compounds, the hexagonal double-layers of Fe ions are sandwiched by thick layered blocks composed of rare-earth and oxygen ions, as shown in Fig. 1. Because the average valence of Fe ions in RFe2O4 is Fe2.5+, Fe2+ and Fe3+ ions sit on the same crystallographic site on the triangular lattice with equal density. This coexistence of Fe2+ and Fe3+ on the triangular lattice should lead to a spin/charge frustration. Some peculiar properties such as the anomalous dielectric behavior were found in LuFe2O4. According to the previous work by Yamada et al. [6], it is revealed that the relaxation of the charge frustration results in the formation of the charge ordering (CO) on the triangular lattice below 350 K, which is characterized as the unique arrangement of Fe2+ and Fe3+ ions. In addition, Ikeda et al. found that the appearan
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