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Date and time: 25th Feb. (Wed.) 15:00`
êŠ: SPring-8, –GŒõŠÙ
Place: SPring-8, "HOUKOUKAN" seminar room
Title: Ferroelectric materials with geometrical frustration
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Speaker: Prof. S. Mori (Osaka Prefecture University)@
Abstract:

RFe2O4 (R= Ho-Lu, Y) is a type of multiferroic material exhibiting the coexisting state of magnetic and ferroelectric orderings. Crystal structure of RFe2O4 consists of paired Fe-O triangular-lattice layers and R-O blocks stacked along the c axis. The average valence of Fe ions is +2.5, implying that equal amounts of Fe3+ and Fe2+ ions exists in the triangular lattice. This coexisting state of Fe3+ and Fe2+ should lead to the spin/charge frustration on the triangular lattice. Electron and neutron diffraction experiments revealed that the relaxation of the charge frustration results in the formation of a charge ordering (CO) in the triangular lattice below 350K in LuFe2O4. In addition, the emergence of spontaneous polarization (Ps) is strongly correlated with the formation of the CO structure and the magnitude of Ps increases discontinuously at the ferrimagnetic transition temperature of 250K. Here we report the CO structure and dielectric/magnetic properties in RFe2O4 and RFeMO4 (R=Lu,Yb, M=Cu,Co) by a transmission electron microscopy (TEM), combining with the conventional dielectric and magnetic measurements.

It was found that there appear two different types of characteristic diffuse scatterings in YFe2O4-ƒÂ at RT. One is characteristic diffuse streak elongated along the [001] direction through the 1/3 1/3 0 reciprocal position and the other is diffusive spot at 1/3-ƒÂ 1/3+ƒÂ 0-type incommensurate positions (ƒÂ ≈ 0.11). We examined what gives rise to the former characteristic diffuse streak by obtaining both dark-field images and high-resolution lattice images. We found the presence of nano-sized domains related to the CO. These CO nano-sized domains should be related to the ferroelectric domains with different orientation of the spontaneous polarization.

On the other hand, LuFeCuO4 is a dielectric compound with orientational polarization, which shows the characteristic low-frequency dielectric dispersion. In addition, it was found that the Cu2+ substitution in LuFe2O4 suppressed the formation of the CO structure and induced characteristic nanodomains with the 5∼10nm size, which give rise to unique diffuse scattering in the reciprocal space. In addition, magnetic measurement revealed that LuFeCuO4 shows the magnetic transition around 50K, which is much lower than the magnetic transition temperature of 250K in LuFe2O4. Our experimental results suggest that the nanodomains should be responsible for the magnetic/dielectric properties in RFeMO4.

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