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Journal Articles

Crystallographical and morphological changes in charge-ordering transition of RFe$$_{2}$$O$$_{4}$$ (R: Y, Lu) investigated by transmission electron microscopy

Horibe, Yoichi*; Mori, Shigeo*; Ikeda, Naoshi*; Yoshii, Kenji; Maeno, Hiroshi*; Murakami, Yasukazu*

Ferroelectrics, 584(1), p.20 - 30, 2021/00

Temperature dependence of charge-ordered crystal structures and domain structures in RFe$$_{2}$$O$$_{4}$$ (R: Y and Lu) was investigated by energy-filtered transmission electron microscopy, combined with conventional transmission electron microscopy. The presence of three-dimensional to two-dimensional charge ordering transition were observed in both RFe$$_{2}$$O$$_{4}$$ on heating. Furthermore, real-space images obtained with the energy-filtered transmission electron microscopy revealed that YRFe$$_{2}$$O$$_{4}$$ has less anisotropic nanometer-scale charge-ordered domains than LuRFe$$_{2}$$O$$_{4}$$. These findings in RFe$$_{2}$$O$$_{4}$$ indicate the importance of the interchange interactions between Fe-O bilayers in addition to those within bilayers in the structural phase transitions associated with charge ordering in this system.

Journal Articles

Exchange bias in multiferroic $$R$$Fe$$_{2}$$O$$_{4}$$ ($$R$$=Y, Er, Tm, Yb, Lu and In)

Yoshii, Kenji; Ikeda, Naoshi*; Nishihata, Yasuo; Maeda, Daisuke*; Fukuyama, Ryota*; Nagata, Tomoko*; Kano, Jun*; Kambe, Takashi*; Horibe, Yoichi*; Mori, Shigeo*

Journal of the Physical Society of Japan, 81(3), p.033704_1 - 033704_4, 2012/03

 Times Cited Count:10 Percentile:58.4(Physics, Multidisciplinary)

Exchange bias (EB) behavior has been observed for multiferroic $$R$$Fe$$_{2}$$O$$_{4}$$ ($$R$$=Y, Er, Tm, Yb, Lu and In). The materials with the small $$R$$$$^{3+}$$ ions ($$R$$=Tm, Yb, Lu and In) exhibit large EB fields ($$_sim^{>}$$ 1 kOe) below $$sim$$100-150 K. This property is rooted in a magnetically glassy state, arising from the competition between ferro- and antiferromagnetic domain interactions. In addition, the exchange bias field tends to be more enhanced for smaller $$R$$$$^{3+}$$ ions. Hence, the EB is controlled by the substitution at the $$R$$-site in this series.

Journal Articles

Direct observation of low-temperature superstructure in spin- and charge-frustrated ferrite YFe$$_{2}$$O$$_{4-delta}$$

Horibe, Yoichi*; Ikeda, Naoshi*; Yoshii, Kenji; Mori, Shigeo*

Physical Review B, 82(18), p.184119_1 - 184119_5, 2010/11

 Times Cited Count:8 Percentile:40.08(Materials Science, Multidisciplinary)

The low-temperature superstructure of YFe$$_{2}$$O$$_{4-delta}$$ was carefully investigated by transmission electron microscopy. We found that the unique superstructure at about 100 K is characterized by $$bar{1}$$/14 2/7 1/14-type superlattice reflection spots, suggesting the presence of charge reordering process. The low-temperature high-resolution images clearly show superlattice modulations in Y-O layers as well as Fe-O layers. The modification of intra-bilayer charge interactions due to lattice distortions in Y-O layers is discussed to play a crucial role in the stabilization of long-periodic superstructures at low temperatures.

Journal Articles

Oxygen-deficient effect on charge ordering in spin- and charge-frustrated ferrite YFe$$_{2}$$O$$_{4-delta}$$

Horibe, Yoichi*; Yoshii, Kenji; Ikeda, Naoshi*; Mori, Shigeo*

Physical Review B, 80(9), p.092104_1 - 092104_4, 2009/09

 Times Cited Count:15 Percentile:56.86(Materials Science, Multidisciplinary)

The effects of oxygen deficiency on charge ordering were investigated by transmission electron microscopy at various deficiencies and temperatures of YFe$$_{2}$$O$$_{4-delta}$$. We found that the detailed nature of the charge ordering structures is associated with changes in the superlattice reflections and diffuse scattering. They go from (1/3 1/3 1/2)-type superlattice reflection to (1/3 1/3 ${it l}$)-type straight-type diffuse scattering, then to zigzag-type diffuse scattering. With the oxygen deficiencies increased, the interchange correlation of Fe-O bilayers obviously decreases, yet the triple super periodicity along [110]$$^{*}$$ remains unchanged. These findings on frustrated ferritesindicate the stability of in-plane threefold charge ordering and the importance of the interchange interaction between bilayers in the structural phase transitions.

Journal Articles

Nanometer-sized domain structures in LuFe$$M$$O$$_{4}$$ ($$M$$=Cu, Co) revealed by energy-filtered transmission electron microscopy

Matsuo, Yoji*; Hirata, Akihiko*; Horibe, Yoichi*; Yoshii, Kenji; Ikeda, Naoshi*; Mori, Shigeo*

Ferroelectrics, 380(1), p.56 - 62, 2009/06

 Times Cited Count:6 Percentile:31.42(Materials Science, Multidisciplinary)

Diluted effect on the charge ordered (CO) structure in dielectric materials LuFe$$M$$O$$_{4}$$ ($$M$$=Cu, Co) were investigated by an energy-filtered transmission electron microscope (TEM), in combination with a conventional TEM. It is revealed that partial substitution of Cu$$^{2+}$$ and Co$$^{2+}$$ for Fe$$^{2+}$$ in LuFe$$_{2}$$O$$_{4}$$ destabilized the CO structure. In LuFeCuO$$_{4}$$, there exist characteristic zigzag-shaped diffuse streaks and diffuse spots at the h/3-$$delta$$ h/3-$$delta$$ 0-type positions (h: integer, $$delta$$=0.06) in reciprocal space, which originate from static cationic ordering of Fe$$^{3+}$$ and Cu$$^{2+}$$ ions in the triangular lattice. Moreover, real-space images by the energy-filtered TEM revealed that nanometer-sized domain structures with the 5$$sim$$10 nm size are formed. In contrast, honeycomb-shaped diffuse streaks were found in LuFeCoO$$_{4}$$, in which no domain structures are observed in real-space images.

Journal Articles

Magnetic and dielectric properties of YbFe$$_{2-x}$$Mn$$_{x}$$O$$_{4}$$ (0$$underline{<}$$x$$underline{<}$$1)

Yoshii, Kenji; Ikeda, Naoshi*; Michiuchi, Takamasa*; Yokota, Yusuke*; Okajima, Yuka; Yoneda, Yasuhiro; Matsuo, Yoji*; Horibe, Yoichi*; Mori, Shigeo*

Journal of Solid State Chemistry, 182(7), p.1611 - 1618, 2009/06

 Times Cited Count:12 Percentile:46.6(Chemistry, Inorganic & Nuclear)

We have investigated the magnetic and dielectric properties of YbFe$$_{2-x}$$Mn$$_{x}$$O$$_{4}$$ (0$$underline{<}$$x$$underline{<}$$1), which is an Fe-site-substituted system of new multiferroic oxides RFe$$_{2}$$O$$_{4}$$ (R=Y, Ho-Lu). X-ray diffraction measurements show that a solid solution is formed between YbFe$$_{2}$$O$$_{4}$$ (x=0) and YbFeMnO$$_{4}$$ (x=1) for 0$$underline{<}$$x$$underline{<}$$1. The valence of the Mn ion is determined to be $$sim$$2+, consistently with the suppression of low-temperature magnetization by the Mn substitution. The magnetic transition temperature and the dielectric constant decrease monotonically with increasing x. This result plausibly confirms that the magnetic and dielectric properties in oxides isostructural with RFe$$_{2}$$O$$_{4}$$ are governed by the electron transfer between Fe-site ions, unlike ordinary ferroelectrics and dielectrics, in which the ionic displacement plays a key role. The possibility for application is briefly discussed.

Journal Articles

Effect of oxygen vacancies on charge ordered structure in YFe$$_{2}$$O$$_{4-delta}$$

Mori, Shigeo*; Shinohara, Satoshi*; Matsuo, Yoji*; Horibe, Yoichi*; Yoshii, Kenji; Ikeda, Naoshi*

Japanese Journal of Applied Physics, 47(9), p.7595 - 7598, 2008/09

 Times Cited Count:12 Percentile:46.79(Physics, Applied)

We have investigated the electronic ferroelectric material YFe$$_{2}$$O$$_{4}$$by means of electron transmission microscopy. The measurement at room temperature showed the charge-ordered structure of Fe ions in the a-b plane, which is the origin of ferroelectricity. An enlargement of unit cell along the c-axis was also observed. These phenomena were not observed at high temperatures around 600 K. That is, the charge ordering in the a-b plane appeared below about 500 K, whreas the enlargement of unit cell along the c-axis was found at room temperature. Below 100 K, an additional enlargement of unit cell along [119] was observed. This behavior is characteristic of YFe$$_{2}$$O$$_{4}$$. These results are discussed in connection with the dielectric properties.

Journal Articles

Magnetic and dielectric properties of InFe$$_{2}$$O$$_{4}$$, InFeCuO$$_{4}$$, and InGaCuO$$_{4}$$

Yoshii, Kenji; Ikeda, Naoshi*; Okajima, Yuka; Yoneda, Yasuhiro; Matsuo, Yoji*; Horibe, Yoichi*; Mori, Shigeo*

Inorganic Chemistry, 47(14), p.6493 - 6501, 2008/07

 Times Cited Count:24 Percentile:71.15(Chemistry, Inorganic & Nuclear)

The magnetic and dielectric properties of InFe$$_{2}$$O$$_{4}$$, InFeCuO$$_{4}$$and InGaCuO$$_{4}$$ have been investigated. All these materials areisostructural with RFe$$_{2}$$O$$_{4}$$ (R=Y, Ho-Lu), which shows ferroelectricity due to iron-valence ordering. InFe$$_{2}$$O$$_{4}$$ exhibits similar properties to those of RFe$$_{2}$$O$$_{4}$$; the origins of the magnetic and dielectric phenomena are likely common in InFe$$_{2}$$O$$_{4}$$ and RFe$$_{2}$$O$$_{4}$$. From measurements of the other two materials, we found that both the magnetic transition temperature and the dielectric constant are decreased in the order of InFe$$_{2}$$O$$_{4}$$, InFeCuO$$_{4}$$ and InGaCuO$$_{4}$$. This result strongly supports the previously reported explanation based on an electron transfer between the Fe-site ions for the corresponding rare-earth systems. Therefore, we propose that the dielectric properties of the oxides isostructural with RFe$$_{2}$$O$$_{4}$$ are plausibly governed by electron transfer; this situation is different from that of ordinary ferroelectrics and dielectrics, in which the displacement factions and anions is important. In addition, InFeCuO$$_{4}$$ and InGaCuO$$_{4}$$ exhibit large dielectric constants (larger than about 1500). Inconsideration of this property, the possible applications of these oxides are discussed.

Journal Articles

Dilution effect on microstructures with respect to magnetic and dielectric properties in charge-ordered ferroelectric LuFe$$_{2}$$O$$_{4}$$

Matsuo, Yoji*; Horibe, Yoichi*; Yoshii, Kenji; Ikeda, Naoshi*; Mori, Shigeo*

Transactions of the Materials Research Society of Japan, 33(1), p.31 - 34, 2008/03

We investigated magnetic/dielectric properties and related microstructures in LuFeCuO$$_{4}$$, which was obtained by substituting Cu$$^{2+}$$ ion for Fe$$^{2+}$$ one in LuFe$$_{2}$$O$$_{4}$$. It was found that LuFeCuO$$_{4}$$ shows a characteristic low-frequency dielectric dispersion around 300 K. We analyzed it by using the Debye model and found that LuFeCuO$$_{4}$$ is adielectric compound with the orientational polarization. Electron diffraction experiments revealed that there exists characteristic zigzag-shaped diffuse scattering in LuFeCuO$$_{4}$$. In addition, we found the presence of nano-sized domains, which consist of the short-range ordering of Fe$$^{3+}$$ and Cu$$^{2+}$$ on the triangular lattice. The present results suggest that the short-range ordering gives rise to some anomalous dielectric properties in LuFeCuO$$_{4}$$.

Journal Articles

Microstructures related to the ferroelectric properties in BiFeO$$_{3}$$-BaTiO$$_{3}$$

Kitagawa, Shuji*; Horibe, Yoichi*; Yoshii, Kenji; Suzuki, Muneyasu*; Noguchi, Yuji*; Nishihara, Sadafumi*; Hosokoshi, Yuko*; Mori, Shigeo*

Transactions of the Materials Research Society of Japan, 33(1), p.27 - 30, 2008/03

Microstructures related to the ferroelectric (FE) properties in(1-x)BiFeO$$_{3}$$-xBaTiO$$_{3}$$ were examined mainly by a transmission electron microscopy, in combination with conventional magnetic and dielectric measurements. It was found that macroscopic-sized FE domain structures in BiFeO$$_{3}$$changed into fine FE microstructures with the 20-30nm size in the x=0.25compound, as the BaTiO$$_{3}$$ concentration (x) was increased. In addition, we found characteristic tweed-like contrast due to the strain field in the x=0.33 compound at room temperature. We carefully investigated the spatial distribution of the FE microdomains in the x=0.25 compound by obtaining real-space images and determined spatial configuration of the spontaneous polarization in each FE microdomains.

Journal Articles

Ferroelectric domain structures in BiFeO$$_{3}$$-BaTiO$$_{3}$$

Kitagawa, Shuji*; Ozaki, Tomoatsu*; Horibe, Yoichi*; Yoshii, Kenji; Mori, Shigeo*

Ferroelectrics, 376(1), p.122 - 128, 2008/00

 Times Cited Count:10 Percentile:45.51(Materials Science, Multidisciplinary)

Microstructures associated with the ferroelectric (FE) properties in (1-x)BiFeO$$_{3}$$-xBaTiO$$_{3}$$ were investigated mainly by a transmission electron microscopy. It was found that large FE domains in BiFeO$$_{3}$$ changed into complicated FE microdomains with the 20-30 nm width in the x = 0.25 compound. The directions of the spontaneous polarization in each FE microdomains were determined by obtaining dark-field images. Changes of the FE microdomains for x = 0.28 composition as a function of the temperature were also investigated.

Journal Articles

Magnetic and dielectric properties of $textit{R}$Fe$$_2$$O$$_4$$, $textit{R}$Fe$textit{M}$O$$_4$$, and $textit{R}$GaCuO$$_4$$ ($textit{R}$=Yb and Lu, $textit{M}$=Co and Cu)

Yoshii, Kenji; Ikeda, Naoshi*; Matsuo, Yoji*; Horibe, Yoichi*; Mori, Shigeo*

Physical Review B, 76(2), p.024423_1 - 024423_12, 2007/07

 Times Cited Count:96 Percentile:94.21(Materials Science, Multidisciplinary)

The magnetic and dielectric properties have been investigated for the isostructural oxides of ${it R}$Fe$$_{2}$$O$$_{4}$$, ${it R}$Fe${it M}$O$$_{4}$$, and ${it R}$GaCuO$$_{4}$$ (${it R}$=Yb and Lu, ${it M}$=Co and Cu). The magnetization measurements for ${it R}$Fe$$_{2}$$O$$_{4}$$ showed ferrimagnetic ordering at $$sim$$250 K. This system also exhibited large dielectric constants of $$sim$$10,000-30,000 at around room temperature, which is attributable to the charge-ordering-induced ferroelectricity, as was proposed in our recent report. The magnetic transition temperatures are lowered to $$sim$$45-90 K for ${it R}$Fe${it M}$O$$_{4}$$. Magnetic ordering is not found for ${it R}$GaCuO$$_{4}$$. AC magnetic susceptibility measurements indicate that magnetic ordering becomes short-ranged by the substitution at the Fe site. The overall characteristic behavior of the magnetic properties is explained in terms of the change of a spin value as well as the dilution of magnetic interactions. Although the AC dielectric measurements show the existence of polar regions in each material, the dielectric constants below 300 K become smaller in the order of ${it R}$Fe$$_{2}$$O$$_{4}$$, ${it R}$FeCoO$$_{4}$$, ${it R}$FeCuO$$_{4}$$, and ${it R}$GaCuO$$_{4}$$. From the analysis of the dielectric dispersion, the distribution of the fluctuation time of polar regions is wider in ${it R}$FeCoO$$_{4}$$, ${it R}$FeCuO$$_{4}$$, and ${it R}$CuGaO$$_{4}$$ than in ${it R}$Fe$$_{2}$$O$$_{4}$$; a coherent motion of polar regions is suppressed in the substituted systems. By comparison to the results from the magnetic measurements, the dielectric properties are discussed in connection with a charge transfer between the transition-metal 3${it d}$ orbitals, consistently with the proposed novel mechanism of the ferroelectricity in ${it R}$Fe$$_{2}$$O$$_{4}$$.

Journal Articles

Doping effect on the charge ordering in LuFe$$_{2}$$O$$_{4}$$

Matsuo, Yoji*; Horibe, Yoichi*; Mori, Shigeo*; Yoshii, Kenji; Ikeda, Naoshi*

Journal of Magnetism and Magnetic Materials, 310(2, Part2), p.e349 - e351, 2007/03

The magnetic and dielectric properties are investigated forTmFe$$_{2}$$O$$_{4}$$ and TmFeCuO$$_{4}$$. The properties of TmFe$$_{2}$$O$$_{4}$$, which shows the ferroelectricity from iron-valence ordering, are essentially the same as those of the other RFe$$_{2}$$O$$_{4}$$ oxides, i.e., ferrimagnetic ordering at around 250 K and dielectric constants larger than 3000 at around room temperature. On the other hand, the Cu-substituted oxide TmFeCuO$$_{4}$$ shows no magnetic transition and considerably smaller dielectric constants. The change of the properties is likely attributable to a reduction of a charge transfer between the transition-metal ions.

Journal Articles

Doping effect on the charge ordering in LuFe2O4

Matsuo, Yoji*; Shinohara, Satoshi*; Mori, Shigeo*; Horibe, Yoichi*; Yoshii, Kenji; Ikeda, Naoshi*

Materials Research Society Symposium Proceedings, Vol.966 (Internet), 6 Pages, 2007/03

Change of the charge ordered (CO) structure by substituting Cu$$^{2+}$$ for Fe$$^{2+}$$ in LuFe$$_{2}$$O$$_{4}$$ was investigated by means of the transmission electron microscopy. The CO structure in LuFe$$_{2}$$O$$_{4}$$ is characterized by the modulated structure with the wave vector of q=1/3[1 -1 3/2] and the average size of the CO domains can be estimated to be about 10-20nm. On the contrary, the Cu$$^{2+}$$ substitution 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 Cu$$^{2+}$$ and Fe$$^{3+}$$ ions on the triangular lattice. In addition, the magnetic measurement revealed that LuFeCuO$$_{4}$$ exhibits an antiferromagnetic transition around 50K, which is lower than the Neel temperature of 250K in LuFe$$_{2}$$O$$_{4}$$.

Journal Articles

Ferroelectricity from iron valence ordering in the charge-frustrated system LuFe$$_{2}$$O$$_{4}$$

Ikeda, Naoshi*; Osumi, Hiroyuki*; Owada, Kenji; Ishii, Kenji; Inami, Toshiya; Kakurai, Kazuhisa; Murakami, Yoichi*; Yoshii, Kenji; Mori, Shigeo*; Horibe, Yoichi*; et al.

Nature, 436(7054), p.1136 - 1138, 2005/08

 Times Cited Count:763 Percentile:99.79(Multidisciplinary Sciences)

By means of resonant X-ray scattering experiments using synchrotron radiation of SPring-8, it was found that a mixed-valence system LuFe$$_{2}$$O$$_{4}$$ is a ferroelectric material owing to a real-space charge-ordering of Fe$$^{2+}$$ and Fe$$^{3+}$$. This ordering is rooted in a charge frustration between the Fe ions on a triangular lattice. The new mechanism of ferroelectricity shown in the present paper offers great potential for designing future ferroelectric devices to be coupled or controlled with the degrees of freedom of electron: charge, spin and orbital.

Oral presentation

Magnetic and dielectric properties of electronic ferroelectric RFe$$_{2}$$O$$_{4}$$ and related oxides (R: rare earths)

Yoshii, Kenji; Ikeda, Naoshi*; Okajima, Yuka; Yoneda, Yasuhiro; Matsuo, Yoji*; Horibe, Yoichi*; Mori, Shigeo*

no journal, , 

We have investigated the magnetic and dielectric properties of electronic ferroelectric oxides RFe$$_{2}$$O$$_{4}$$ (R: rare earths) and their derivative materials. It was found that the magnetic transition temperature and the dielectric constant showed a possible relationship with electrical resistivity. In consideration of the mechanism of the ferroelectricity in RFe$$_{2}$$O$$_{4}$$, this result suggests that the magnetic and dielectric properties are likely rooted in the electron transfer at the Fe-site ions.

Oral presentation

Elemental substitution effects in the electronic ferroelectric RFe$$_{2}$$O$$_{4}$$ (R=rare earths)

Yoshii, Kenji; Yoneda, Yasuhiro; Ikeda, Naoshi*; Matsuo, Yoji*; Hoshiyama, Takuya*; Horibe, Yoichi*; Mori, Shigeo*

no journal, , 

We have investigated the Fe-site substitution effects in the electronic ferroelectric RFe$$_{2}$$O$$_{4}$$ (R=rare earths). It was found that continuous solid solutions were formed for the Mn substitution; RFe$$_{2-x}$$Mn$$_{x}$$O$$_{4}$$ was synthesized for 0$$underline{<}$$x$$underline{<}$$1. On the other hand, the substitution of other cations (such as Cu and Ga) was possible in limited x regions; e.g., Ga could be substituted for 0$$underline{<}$$x$$underline{<}$$0.4-0.5 It was also found that the low-temperature magnetization and the dielectric constant were reduced by the substitution. Considering the mechanism of dielectric response in RFe$$_{2}$$O$$_{4}$$, the change of the physical properties likely arises from a change of electron transfer between the Fe-site ions.

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