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Journal Articles

Effect of grain boundary on the friction coefficient of pure Fe under the oil lubrication

Adachi, Nozomu*; Matsuo, Yasutaka*; Todaka, Yoshikazu*; Fujimoto, Mikiya*; Hino, Masahiro*; Mitsuhara, Masatoshi*; Oba, Yojiro; Shiihara, Yoshinori*; Umeno, Yoshitaka*; Nishida, Minoru*

Tribology International, 155, p.106781_1 - 106781_9, 2021/03

https://doi.org/10.1016/j.triboint.2020.106781
 Times Cited Count：11 Percentile：65.79(Engineering, Mechanical)

Journal Articles

Multiferroic character and magnetic phase of LuFe$$_{2}$$O$$_{4}$$

Oishi, Daisuke*; Hayakawa, Hironori*; Akahama, Hiroshi*; Ikeda, Naoshi*; Kambe, Takashi*; Matsuo, Yoji*; Kimizuka, Noboru*; Kano, Jun*; Yoshii, Kenji

Ferroelectrics, 415(1), p.51 - 56, 2011/10

https://doi.org/10.1080/00150193.2011.577371
 Times Cited Count：5 Percentile：24.39(Materials Science, Multidisciplinary)

The variation of AC dielectric constant in the function of the DC magnetic fields was found in low oxygen vacant LuFe$$_{2}$$O$$_{4}$$ single crystal in which the electric polarization arise from the polar charge ordering. These magneto-electric coupling should be explained by a novel mechanism where the boundary motion of the polar charge ordered domain is affected by the magnetic (spin) domain boundary motion.

Journal Articles

Doping effect on charge ordered structure in RFe$$_{2}$$O$$_{4}$$(R=Lu and Yb)

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

Journal of Physics; Conference Series, 200, p.012128_1 - 012128_4, 2010/02

https://doi.org/10.1088/1742-6596/200/1/012128
 Times Cited Count：3 Percentile：73.57(Materials Science, Multidisciplinary)

RFe$$_{2}$$O$$_{4}$$ (R= Ho-Lu,Y) is a novel ferroelectric material in which ferroelectric polarization is caused by the charge ordering (CO) of Fe$$^{2+}$$ and Fe$$^{3+}$$. In this work we investigated the CO structure and dielectric/magnetic properties in RFe$$_{2}$$O$$_{4}$$ and RFeCuO$$_{4}$$ (R=Lu,Yb, M=Cu,Co) by a transmission electron microscopy (TEM), combining with dielectric and magnetic measurements. LuFeCuO$$_{4}$$ is a dielectric compound, which shows the characteristic low-frequency dielectric dispersion. In addition, the Cu$$^{2+}$$ substitution in LuFe$$_{2}$$O$$_{4}$$ suppressed the formation of the CO structure and induced characteristic polar nanodomains with the 5-10nm size. Magnetic measurement revealed that LuFeCuO$$_{4}$$ shows the magnetic transition around 50K, which is much lower than the magnetictransition temperature of 250K in LuFe$$_{2}$$O$$_{4}$$. Our experimental results suggest that the polar nanodomains should be responsible for the magnetic/dielectric properties in RFeMO$$_{4}$$.

Journal Articles

Structural characterization and dielectric properties of hexagonal Lu(Fe,Ti)O$$_{3}$$

Matsuo, Yoji*; Hoshiyama, Takuya*; Mori, Shigeo*; Yoshii, Kenji; Michiue, Yuichi*; Kambe, Takashi*; Ikeda, Naoshi*; Brown, F.*; Kimizuka, Noboru*

Japanese Journal of Applied Physics, 48(9), p.09KB04_1 - 09KB04_3, 2009/10

https://doi.org/10.1143/JJAP.48.09KB04
 Times Cited Count：1 Percentile：5.21(Physics, Applied)

We have investigated an average crystal structure and microstructures in LuFe$$_{0.56}$$Ti$$_{0.44}$$O$$_{3}$$ by the transmission electron microscopy (TEM), in combination with the dielectric measurements. In this work, high-qualitypolycrystalline samples of LuFe$$_{0.56}$$Ti$$_{0.44}$$O$$_{3}$$ were successfully synthesized in air by the conventional solid state reaction. The average crystal structure at room temperature is characterized by the hexagonal structure (space group: $$P$$6$$_{3}$$ cm). Dielectric measurements revealed that LuFe$$_{0.56}$$Ti$$_{0.44}$$O$$_{3}$$ shows broad dielectric dispersion around 570 K, which should originate from the formation of the polar domain structures on the nanometer scale.

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

https://doi.org/10.1080/00150190902873204
 Times Cited Count：6 Percentile：28.67(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

https://doi.org/10.1016/j.jssc.2009.04.008
 Times Cited Count：15 Percentile：50.81(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

Local structure modulation in the electronic ferroelectric oxide LuFe$$_{2}$$O$$_{4}$$

Hayakawa, Hironori*; Morimoto, Masanori*; Ikeda, Naoshi*; Yoneda, Yasuhiro; Kohara, Shinji*; Yoshii, Kenji; Matsuo, Yoji*; Michiuchi, Takamasa*; Mori, Shigeo*

Transactions of the Materials Research Society of Japan, 34(1), p.51 - 54, 2009/05

https://doi.org/10.14723/tmrsj.34.51

We have recently reported that LuFe$$_{2}$$O$$_{4}$$ becomes ferroelectric below 330 K, owing to a charge-ordering transition of Fe ions. As details of the properties of this material have not been reported so far, we have investigated its local structure in the ferromagnetic phase by means of synchrotron X-ray diffraction. The analysis of pair-distribution function showed the existence of displacement of Lu atoms; this structure is different from the average structure of this materials known thus far. This result encourages a detailed reinvestigation of crystal structure of LuFe$$_{2}$$O$$_{4}$$. We also show the results obtained from some other experimental methods, such as dielectric measurement and electron diffraction.

Journal Articles

Ferroelectricity from ordering of Fe valences in the rare-earth iron oxides RFe$$_{2}$$O$$_{4}$$(R=Y, Ho-Lu)

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

Nihon Kessho Gakkai-Shi, 51(2), p.162 - 168, 2009/04

https://doi.org/10.5940/jcrsj.51.162

The rare-earth iron oxides RFe$$_{2}$$O$$_{4}$$ (R=Y, Ho-Lu) have a rhombohedral crystal structure, consisting of an alternating stacking of triangular lattices of R, Fe and O ions. Recently, we have proposed that this system shows a new mechanism of ferroelectricity; the ferroelectric state originates from a polar charge-ordered structure of Fe$$^{2+}$$ and Fe$$^{3+}$$ on a triangular lattice. In this article, a review is given on the details of this ferroelectricity, which were revealed by measurements of synchrotron X-ray diffraction and dielectric properties of one of the RFe$$_{2}$$O$$_{4}$$ oxides, LuFe$$_{2}$$O$$_{4}$$.

Journal Articles

Stoichiometric study of the dielectric and magnetic properties in charge frustrated system LuFe$$_{2}$$O$$_{4}$$

Michiuchi, Takamasa*; Yokota, Yusuke*; Komatsu, Takuma*; Hayakawa, Hironori*; Kuroda, Tomoko*; Maeda, Daisuke*; Matsuo, Yoji*; Mori, Shigeo*; Yoshii, Kenji; Hanasaki, Noriaki*; et al.

Ferroelectrics, 378(1), p.175 - 180, 2009/00

https://doi.org/10.1080/00150190902859187
 Times Cited Count：18 Percentile：58.73(Materials Science, Multidisciplinary)

We have synthesized the samples of LuFe$$_{2}$$O$$_{4}$$, which shows the ferroelectricity due to charge ordering of Fe ions, under several different reducing conditions using a CO$$_{2}$$-CO flow. The reducing condition was changed by changing a flow ratio of CO$$_{2}$$ and CO. It was found that the flow ratio of CO$$_{2}$$/CO of about 5 provided the sample with the highest magnetic transition temperature of 240-250 K. This sample showed the dielectric constant of 10000 at room temperature. The imaginary part of the dielectric response offered the activation energy of 0.4-0.5 eV, which is slightly larger than that reported previously (about 0.3 eV). We will perform further investigation of the physical properties of this sample to clarify full details and their origin of LuFe$$_{2}$$O$$_{4}$$.

Journal Articles

Effects of oxygen annealing on dielectric properties of LuFeCuO$$_{4}$$

Matsuo, Yoji*; Suzuki, Muneyasu*; Noguchi, Yuji*; Yoshimura, Takeshi*; Fujimura, Norifumi*; Yoshii, Kenji; Ikeda, Naoshi*; Mori, Shigeo*

Japanese Journal of Applied Physics, 47(11), p.8464 - 8467, 2008/11

https://doi.org/10.1143/JJAP.47.8464
 Times Cited Count：11 Percentile：41.64(Physics, Applied)

We have investigated the physical properties of LuFeCuO$$_{4}$$, which is a derivative material of the electronic ferroelectric LuFe$$_{2}$$O$$_{4}$$. From electron diffraction measurements at room temperature, ionic ordering of Fe$$^{3+}$$ and Cu$$^{2+}$$ in the a-b plane was observed. This ordering is similar to that in LuFe$$_{2}$$O$$_{4}$$ and has an electric dipole. The observation of small domains (5-10 nm) indicates the presence of polar regions. Dielectric measurements showed that a peak of dielectric constant appeared at around 500 K, and the peak value was about 1000, indicating that LuFeCuO$$_{4}$$ is a dielectric material. We will present the experimental data on the samples prepared under high-pressure oxygen.

Journal Articles

Magnetic and dielectric properties of HoFe$$_{2}$$O$$_{4}$$ and R$$_{1-x}$$R'$$_{x}$$Fe$$_{2}$$O$$_{4}$$ (R, R': rare earths)

Yoshii, Kenji; Yoneda, Yasuhiro; Maeda, Daisuke*; Yokota, Yusuke*; Michiuchi, Takamasa*; Komatsu, Takuma*; Ikeda, Naoshi*; Matsuo, Yoji*; Mori, Shigeo*

Japanese Journal of Applied Physics, 47(9), p.7599 - 7602, 2008/09

https://doi.org/10.1143/JJAP.47.7599
 Times Cited Count：8 Percentile：32.87(Physics, Applied)

We have investigated the physical properties of HoFe$$_{2}$$O$$_{4}$$ and R$$_{1-x}$$R'$$_{x}$$Fe$$_{2}$$O$$_{4}$$ (R, R': rare earths). Dielectric measurements showed their dielectric constants of 1000 at around room temperature, which is characteristic of the RFe$$_{2}$$O$$_{4}$$ series (R: rare earths). However, the magnetic transition temperatures and the coherency in dielectric response were found to be different from those of RFe$$_{2}$$O$$_{4}$$ reported so far. Interestingly, these materials suggested higher ferroelectric temperatures than those reported so far, suggesting a possibility of application of these materials.

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

https://doi.org/10.1143/JJAP.47.7595
 Times Cited Count：12 Percentile：44.11(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

https://doi.org/10.1021/ic702365k
 Times Cited Count：28 Percentile：73.11(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

Charge ordered structure and dielectric properties in LuFe$$_{2}$$O$$_{4}$$

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

Nihon Kessho Gakkai-Shi, 50(2), p.150 - 154, 2008/06

https://doi.org/10.5940/jcrsj.50.150

LuFe$$_{2}$$O$$_{4}$$ shows a new mechanism of ferroelectricity; the charge-ordered arrangement of Fe$$^{2+}$$ and Fe$$^{3+}$$ is the origin of ferroelectricity. In this work, we have investigated this material mainly by means of the transmission electron microscopy (TEM). It was found that both the charge-ordered structure and charge ordering processes depend strongly on the amount of the oxygen deficiency ($$delta$$) in LuFe$$_{2}$$O$$_{4-delta}$$. For proper understanding of the properties in LuFe$$_{2}$$O$$_{4}$$ and related materials, it should be essential to prepare LuFe$$_{2}$$O$$_{4-delta}$$ samples with properly controlled oxygen-concentration and elucidate the physical properties in LuFe$$_{2}$$O$$_{4-delta}$$ with $$delta$$$$sim$$0.

Journal Articles

Electronic ferroelectricity from charge ordering in RFe$$_{2}$$O$$_{4}$$

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

IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 55(5), p.1043 - 1045, 2008/05

We report our recent discovery of novel ferroelectricity arising from the polar ordering of Fe$$^{3+}$$ and Fe$$^{2+}$$ in a mixed valence triangular lattice oxide LuFe$$_{2}$$O$$_{4}$$, where the electric polarization is not a result of ionic displacement. The polar ordering of Fe$$^{3+}$$ and Fe$$^{2+}$$ was confirmed with a resonant X-ray scattering study in SPring-8. The origin of such ordering is the competitive interaction between Fe$$^{3+}$$ and Fe$$^{2+}$$ in the triangular lattice, i.e., the charge frustration. The polar superlattice of Fe$$^{3+}$$ and Fe$$^{2+}$$ develops below 350 K, where the electric polarization appears. The ferroelectricity arising from the polar charge ordering or the polar electron distribution may have great potential for the future application of ferroelectrics.

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

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

https://doi.org/10.1103/PhysRevB.76.024423
 Times Cited Count：99 Percentile：93.50(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 electron ordering

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

Materials Research Society Symposium Proceedings, Vol.966 (Internet), 9 Pages, 2007/03

We report ferroelectricity in the triangular mixed valence material RFe$$_{2}$$O$$_{4}$$ (R=Y, Dy, Ho, Er, Tm, Yb, Lu). This ferroelectricity originates from the polar ordering of Fe$$^{3+}$$ and Fe$$^{2+}$$ or the polar ordering of electrons on Fe$$^{3+}$$. The electric polarization arising from such electron ordering may not couple with the lattice modulation. The ordering process of Fe$$^{3+}$$ and Fe$$^{2+}$$ is explained by the frustrated interaction of charges in a triangular lattice. The ferroelectric property of this material is present at room temperature.

36 (Records 1-20 displayed on this page)