研究者詳細 - 池田　直

論文 - 池田　直

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Temperature and polarization dependence of Fe L-3-edge X-ray absorption spectra of LuFe2O4

A. Agui, M. Mizumaki, T. Kuroda, M. Kawai, T. Nagata, N. Ikeda, T. Uozumi

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 197 13 - 16 2014年12月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE BV

The charge ordering of Fe2+ and Fe3+ ions in LuFe2O4 induces ferroelectricity below 330 K. In this study, the X-ray absorption spectra (XAS) at the Fe L-2,L-3-edge are investigated to study charge ordered- and disordered-states. The polarization dependence of XAS revealed that the energy level of (d(xy), d(x2-y2)) is lower than that of (d(yz), d(zx)). In the charge-ordered state, the Fe L-3-XAS shows a double-peak structure, which is a characteristic of Fe2+ and Fe3+ ions. In the high-temperature region, the double-peak structure becomes a broad single peak, because of overlap with the thermally excited state. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2014.08.008

Current-Induced Enhancement of Magnetic Anisotropy in Spin-Charge-Coupled Multiferroic YbFe2O4

Kenji Yoshii, Daiju Matsumura, Hiroyuki Saitoh, Takashi Kambe, Mamoru Fukunaga, Yuji Muraoka, Naoshi Ikeda, Shigeo Mori

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 83 ( 6 ) 2014年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：PHYSICAL SOC JAPAN

We report the change in the magnetic properties of multiferroic YbFe2O4 caused by cooling in the presence of an electrical current. The magnetization measured after the cooling is similar to 10-20% smaller than that after cooling without a current. The current also affects the magnetic anisotropy below similar to 100 K; the magnetic coercivity is increased by similar to 10-20%. These phenomena may arise from the coexistence of ferromagnetic and antiferromagnetic domains or a disturbance in the formation of charge-ordered domains. From the viewpoint of application, the enhancement of anisotropy by an electrical current provides a method for stabilizing magnetization other than the well-known method of exchange bias.

DOI： 10.7566/JPSJ.83.063708

Valence instability of iron oxide ultrafine particles on ferroelectrics studied by Mossbauer spectroscopy

Jun Kano, Takumi Okamoto, Shin Nakamura, Akio Fuwa, Takafumi Otoyama, Yoshiaki Nakazaki, Hideki Hashimoto, Jun Takada, Norihiro Oshime, Miho Ito, Naoshi Ikeda

JAPANESE JOURNAL OF APPLIED PHYSICS 53 ( 5 ) 2014年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：IOP PUBLISHING LTD

Various types of iron oxides loaded on ferroelectric BaTiO3 and nonferroelectric ZrO2 supports have been studied by Fe-57 Mossbauer spectroscopy to investigate the valence instability of iron oxides affected by ferroelectric polarization fluctuation. Although the iron oxide loaded on ZrO2 shows a typical reduction process at high vacuum and high temperature, the iron oxide loaded on BaTiO3 shows an opposite process, i.e., iron oxide oxidization. We propose the dynamics of band structures of ferroelectrics in the paraelectric phase to understand this phenomenon. The oscillation of band bending causes electron transfer from an iron oxide to a ferroelectric support. This result suggests that the surface charges of ferroelectrics are useful in developing new functional materials with metal compounds. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.53.05FB24

Specific Heat Study of Multiferroic LuFe2O4 Single Crystal

N. Hasegawa, T. Mitsumura, M. Takesada, A. Onodera, J. Kano, N. Ikeda

FERROELECTRICS 462 ( 1 ) 145 - 150 2014年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：TAYLOR & FRANCIS LTD

Precise measurements of specific heat were performed on high quality single crystals of multiferroic LuFe2O4 by using an ac calorimeter. The thermal anomalies corresponding to four known phase transitions were observed at 220K (T-g), 260K (T-N), 380K (T-3CO) and 590K (T-2CO). In addition to them, a new clear anomaly was detected at 305K. No other distinct anomaly could be observed below 220K down to 100K. It is clarified that the temperature dependence of the order parameter associated with this new phase transition at 305K is quite similar to those of spontaneous polarization reported previously by Ikeda etal.

DOI： 10.1080/00150193.2014.890505

Preparation and electronic properties YbFe2O4 thin films by polymerized complex method

N. Okamura, T. Fujii, J. Takada, M. Nakanishi, J. Kano, N. Ikeda

Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy 61 S327 - S329 2014年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Japan Society of Powder and Powder Metallurgy

Rare-earth iron oxides (RFe2O4) have attracting attention as new electronic device materials because of their abundant functionalities such as ferroelectricity, ferrimagnetism, and high infrared absorption. These properties are strongly related to the mixed iron valance states between Fe2+ and Fe3+. We tried to prepare RFe2O4 films by using soft chemical method. The films were prepared by spin-coating of viscous aqueous solutions of Yb(CH3COO)3 and Fe(NO3)3 with citric acid. The molar ratio between Yb:Fe was 1:2 and the citric acid: metal ratio was set to 3.75:1. After the coating, the films dried at 100ï¿½C for 10 min in air were, heat-treated at 900ï¿½C for 0.2 h in H2/CO2 gases to control oxygen partial pressure, and quenched. The obtained films were characterized by using XRD, SEM, EDS, Mï¿½ssbauer, and optical-absorption spectra. The XRD pattern clearly indicated the well-crystallized and nearly monophasic YbFe2O4 formed on the substrates above 900ï¿½C to control the oxygen partial pressure. Equal amount of Fe2+ and Fe3+ ions was present in the films. The films showed small band-gap and large optical absorbance in infrared region.

DOI： 10.2497/jjspm.61.S327

Carrier mobility of iron oxide nanoparticles supported on ferroelectrics studied by Mössbauer Spectroscopy

Takumi Okamoto, Jun Kano, Shin Nakamura, Akio Fuwa, T. Otoyama, Y. Nakazaki, Hideki Hashimoto, J. Takada, Miho Ito, Naoshi Ikeda

Hyperfine Interactions 219 ( 1-3 ) 147 - 152 2013年4月

Magnetoelectric Effect Driven by Magnetic Domain Modification in LuFe2O4

Takashi Kambe, Yukimasa Fukada, Jun Kano, Tomoko Nagata, Hiroyuki Okazaki, Takayoshi Yokoya, Shuichi Wakimoto, Kazuhisa Kakurai, Naoshi Ikeda

PHYSICAL REVIEW LETTERS 110 ( 11 ) 117602 2013年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

The magnetocapacitance effect was investigated using impedance spectroscopy on single crystals of LuFe2O4. The intrinsic impedance response could be separated from the interfacial response and showed a clear hysteresis loop below T-Ferri similar to 240 K under the magnetic field. The neutron diffraction experiment under the magnetic field proves the origin of the dielectric property related to the motion of the nanosized ferromagnetic domain boundary. These results imply that the modification of the microscopic domain structure is responsible for the magnetoelectric effect in LuFe2O4. DOI: 10.1103/PhysRevLett.110.117602

DOI： 10.1103/PhysRevLett.110.117602

Magnetic properties of single crystalline YbFe2O4

K. Yoshii, M. Mizumaki, K. Matsumoto, S. Mori, N. Endo, H. Saitoh, D. Matsumura, T. Kambe, N. Ikeda

XXIST INTERNATIONAL SYMPOSIUM ON THE JAHN-TELLER EFFECT 2012 428 012032 2013年

記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）出版者・発行元：IOP PUBLISHING LTD

We have investigated the magnetic properties of a single crystal of multiferroic YbFe2O4. Ferrimagnetic ordering of Fe spins is observed at similar to 250 K. The magnetization measured during field cooling becomes negative below similar to 10 K owing to the antiparallel coupling between the Fe and Yb sublattices. The magnetization below similar to 10 K is flipped by changing the applied magnetic field in a positive direction, a property which may be used to the development of magnetic devices. Magnetocaloric measurements show a broad peak of the entropy change, likely owing to a multiple magnetic transition.

DOI： 10.1088/1742-6596/428/1/012032

Nonlinear Electric Conductivity of Charge Ordered System RFe2O4 (R = Lu, Yb)

T. Nagata, Y. Fukada, M. Kawai, J. Kano, T. Kambe, E. Dudzik, R. Feyerherm, P. E. Janolin, J. M. Kiat, N. Ikeda

FERROELECTRICS 442 ( 1 ) 45 - 49 2013年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：TAYLOR & FRANCIS LTD

We report nonlinear conductivity associated with a phase transition in polar charge ordered material YbFe2O4. To investigate the existence of nonlinear conductivity, we measured temperature dependence of conductivity as a function of applied electric current and estimated the sample heating precisely. Then we found the nonlinear conductivity below three dimensional charge ordering temperature 340 K. The result is consistent with diffraction observation.

DOI： 10.1080/00150193.2013.773874

Magnetic properties of R2Fe3O7 (R=Yb and Lu)

K. Yoshii, N. Ikeda, R. Fukuyama, T. Nagata, T. Kambe, Y. Yoneda, T. Fukuda, S. Mori

Solid State Communications 173 34 - 37 2013年

記述言語：英語掲載種別：研究論文（学術雑誌）

We have investigated the magnetic properties of R2Fe 3O7 (R=Yb and Lu), which belongs to the homologous family of electronic ferroelectric RFe2O4. Magnetization measurements show ferrimagnetic ordering at the Néel temperature (T N) of ∼270 K, which is slightly higher than that of RFe 2O4 (TN∼230-250 K). Observation of spin glass and exchange bias shows a coexistence of ferromagnetic and antiferromagnetic interactions, similarly to RFe2O4. This situation gives rise to a complex magnetic change and a broad peak of magnetic entropy change. Some characteristics, such as refrigeration temperatures higher than in RFe2O4, seem to offer a possibility of this system to applications. © 2013 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ssc.2013.08.030

Exchange Bias in Multiferroic RFe2O4 (R = Y, Er, Tm, Yb, Lu, and In)

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

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 81 ( 3 ) 33704 2012年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：PHYSICAL SOC JAPAN

An exchange bias (EB) behavior has been observed for multiferroic RFe2O4 (R Y, Er, Tm, Yb, Lu, and In). The materials with small R3+ ions (R = Tm, Yb, Lu, and In) exhibit large EB fields (greater than or similar to 1 kOe) below similar to 100-150 K. This property is rooted in a magnetically glassy state, arising from the competition between ferromagnetic and antiferromagnetic domain interactions. In addition, the EB field tends to be more enhanced for smaller R3+ ions. Hence, the EB is controlled by the substitution at the R-site in this series.

DOI： 10.1143/JPSJ.81.033704

Ferroelectricity and Ferrimagnetism of Hexagonal YbFeO3 Thin Films

Hayato Iida, Takuro Koizumi, Yoshiaki Uesu, Kay Kohn, Naoshi Ikeda, Shigeo Mori, Raphael Haumont, Pierre-Eumeric Janolin, Jean-Michel Kiat, Mamoru Fukunaga, Yukio Noda

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 81 ( 2 ) 24719 2012年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：PHYSICAL SOC JAPAN

The coexistence of ferroelectricity and magnetic order in epitaxial hexagonal YbFeO3 (h-YbFeO3) thin films deposited on yttria stabilized zirconia substrates is examined using an impedance analyzer, a second harmonic generation (SHG) microscope, an X-ray diffractometer, a transmission electron microscope and a superconducting quantum interference device magnetometer. The results show that h-YbFeO3 films exhibit ferroelectricity below 350K with threefold translational periodicity along the [110] direction similar to rare earth manganese oxides, and ferrimagnetism with a four-up and two-down spin configuration of Fe+3 and Yb+3 ions along the c-axis at low temperatures. In addition, a possibility of new ferroelectric phases with an electronic origin induced by the magnetic ordering is discussed based upon SHG and D-E hysteresis measurements.

DOI： 10.1143/JPSJ.81.024719

Magnetic and dielectric study of Bi2CuO4

K. Yoshii, T. Fukuda, H. Akahama, J. Kano, T. Kambe, N. Ikeda

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 471 ( 21-22 ) 766 - 769 2011年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE BV

Magnetic and dielectric properties have been investigated for Bi2CuO4, which has the same chemical formula as that of the parent materials of cuprate superconductors R2CuO4 (R: rare earths). Magnetization measurements show the antiferromagnetic transition of the Cu2+ spins at similar to 42 K, as reported previously. Dielectric measurements for the frequencies of 1 kHz to 1 MHz show that the dielectric constants are 100-500 at room temperature. The dielectric dispersion reveals that the dielectric response lacks spatial coherence, a property which indicates the possible existence of phase separation as suggested for La2CuO4. The imaginary part of dielectric response gives the activation energy of 0.22 eV, suggesting that the dielectric response is governed by the electron hopping between the Cu ions. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2011.05.049

電荷及びスピンフラストレーション系YbFe$_{2}$O$_{4}$における電荷秩序崩壊過程査読

松本圭右*, 小山司*, 森茂生*, 吉井賢資, 池田直*

Journal of Physics; Conference Series 320 012085\_1 - 012085\_5 2011年9月

記述言語：英語

表記酸化物はFe$^{3+}$とFe$^{2+}$が電荷秩序配列することによる新規なタイプの強誘電体である。本物質につき、Feの一部をMnに置き換えた場合の電荷秩序構造の変化を透過電子顕微鏡により観察した。10\%ほどの置換により、電荷秩序構造は破壊され、Fe$^{3+}$とMn$^{2+}$の短距離イオン秩序が発生することがわかった。この系は室温で比誘電率が1000ほどの値を示すが、この性質は短距離イオン秩序による電気双極子を起源とすると考えられる。

DOI： 10.1088/1742-6596/320/1/012085

Electronic phase transition of the valence-fluctuating fulleride Eu2.75C60

Yusuke Yamanari, Yuta Suzuki, Kumiko Imai, Eiji Shikoh, Akihiko Fujiwara, Naoko Kawasaki, Naoshi Ikeda, Yoshihiro Kubozono, Takashi Kambe

PHYSICAL REVIEW B 83 ( 24 ) 2011年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

The electronic properties of Eu2.75C60 are studied using magnetic susceptibility and electron spin resonance (ESR) from 2 to 300 K. Both the magnetic susceptibility and the ESR parameters clearly show an anomaly around the valence transition temperature, T-V = 70 K. The magnetic susceptibility shows weak temperature dependence above T-V, while it changes drastically to Curie-Weiss behavior below T-V. The low-temperature susceptibility can be reproduced by assuming the moment of free Eu2+ ions. This result reveals that Eu2.75C60 changes from the intermediate valence state to the divalent state below T-V. Although ESR signals above T-V should be attributed to conduction electrons, the ESR intensity below T-V follows the Curie-Weiss law with a distinct increase in the g-factor. This should be associated with a strong localization of pi electrons. We also found that, below similar to 17 K, the isothermal magnetization exhibits a weak hysteresis and thermoremanent magnetization appears. These results suggest that valence-ordered Eu2.75C60 undergoes antiferromagnetic ordering with a weak ferromagnetic component at the Neel temperature, T-N = 17 K.

DOI： 10.1103/PhysRevB.83.245103

Multiferroic Character and Magnetic Phase of LuFe2O4

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

FERROELECTRICS 415 51 - 56 2011年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：TAYLOR & FRANCIS LTD

The variation of AC dielectric constant in the function of the DC magnetic fields was found in low oxygen vacant LuFe2O4 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.

DOI： 10.1080/00150193.2011.577371

Doping effect on charge ordered structure in Mn-doped YbFe2O4

K. Matsumoto, T. Koyama, S. Mori, K. Yoshii, T. Kambe, N. Ikeda

3RD INTERNATIONAL CONGRESS ON CERAMICS (ICC3): ADVANCES IN ELECTRO CERAMICS 18 2011年

記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）出版者・発行元：IOP PUBLISHING LTD

Changes of charge ordered (CO) structure by partial substitution of Mn for Fe in YbFe2O4 were investigated by a transmission electron microscope (TEM), in combination with conventional dielectric measurement. It is revealed that partial substitution of Mn2+ for Fe2+ in YbFe2O4 destroyed drastically the CO structure with the wave vector of q=<1/3 1/3 1/2>. Consequently polar clustering structure giving rise to honeycomb-shaped diffuse streaks was found in YbFeMnO4. The random distribution of polar clustering structure gives rise to characteristic broad dielectric dispersion.

DOI： 10.1088/1757-899X/18/9/092047

Ultrafast melting of charge ordering in LuFe2O4 probed by terahertz spectroscopy

H. Itoh, K. Itoh, K. Anjyo, H. Nakaya, H. Akahama, D. Ohishi, S. Saito, T. Kambe, S. Ishihara, N. Ikeda, S. Iwai

Journal of Luminescence 2011年

Terahertz Spectroscopy of Electronic Ferroelectric LuFe2O4 Crystal 査読

Kitahara Hideaki, Kano Jun, Fukada Yukimasa, Kambe Takashi, Ikeda Naoshi, Hangyo Masanori

2011 36TH INTERNATIONAL CONFERENCE ON INFRARED, MILLIMETER, AND TERAHERTZ WAVES (IRMMW-THZ) 2011年

Direct observation of low-temperature superstructure in spin- and charge-frustrated ferrite YFe<inf>2</inf> O<inf>4-δ</inf>

Y. Horibe, N. Ikeda, K. Yoshii, S. Mori

Physical Review B - Condensed Matter and Materials Physics 82 2010年11月

The low-temperature superstructure of YFe2 O4-δ was carefully investigated by transmission electron microscopy. We found that the unique superstructure at about 100 K is characterized by 1̄ /142/71/14 -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 intrabilayer 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. © 2010 The American Physical Society.

DOI： 10.1103/PhysRevB.82.184119

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