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論文

Multi-spin-state dynamics during insulator-metal crossover in LaCoO$$_3$$

土居 篤典*; 藤岡 淳*; 福田 竜生; 筒井 智嗣*; 奥山 大輔*; 田口 康二郎*; 有馬 孝尚*; Baron, A. Q. R.*; 十倉 好紀*

Physical Review B, 90(8), p.081109_1 - 081109_4, 2014/01

 被引用回数:12 パーセンタイル:35.48(Materials Science, Multidisciplinary)

The phonon dynamics of spin-state crossover for perovskite LaCoO$$_3$$ was investigated using infrared and inelastic X-ray spectroscopy. We observed that the Co-O bond stretching phonons couples to the thermally excited spin-state. The momentum dispersion of the optical mode is anomalously dispersionless (less than 3 meV), and this supports the spin-state disproportionation scenario. The steep enhancement of spin-state fluctuation compared to the charge gap suggests the density dependent spin state disproportionation; a simple low spin (LS)/high spin (HS)- or LS/intermediate spin (IS)-state disproportionation is realized at the initial stage with dilute HS- (or IS-) state density, while the complex spin-state disproportionation involving all of the LS-, IS- and HS-state is formed when the density of HS- (or IS-) state is enhanced.

論文

Soft phonon mode coupled with antiferromagnetic order in incipient-ferroelectric Mott insulators Sr$$_{1-x}$$Ba$$_x$$MnO$$_3$$

酒井 英明*; 藤岡 淳*; 福田 竜生; Bahramy, M. S.*; 奥山 大輔*; 有田 亮太郎*; 有馬 孝尚*; Baron, A. Q. R.*; 田口 康二郎*; 十倉 好紀*

Physical Review B, 86(10), p.104407_1 - 104407_11, 2012/09

 被引用回数:17 パーセンタイル:30.41(Materials Science, Multidisciplinary)

Infrared optical and inelastic X-ray scattering spectra have been systematically investigated in combination with first-principles calculations for paraelectric and antiferromagnetic perovskite Sr$$_{1-x}$$Ba$$_x$$MnO$$_3$$ single crystals, which are close to a ferroelectric transition arising from off-center displacement of magnetic Mn$$^{4+}$$ ions. One optical phonon rapidly softens toward zero frequency at room temperature with increasing $$x$$. This soft-mode behavior is also reproduced by the first-principles calculations, from which we have predicted the vibration mode of all the optical phonons. Below the antiferromagnetic-transition temperature, the soft mode hardens with decreasing temperature and then resoftens toward the lowest temperature.

論文

Emergent phenomena in perovskite-type manganites

田口 康二郎*; 酒井 英明*; 奥山 大輔*; 石渡 晋太郎*; 藤岡 淳*; 福田 竜生; 橋爪 大輔*; 賀川 史敬*; 高橋 陽太郎*; 島野 亮*; et al.

Physica B; Condensed Matter, 407(11), p.1685 - 1688, 2012/06

 被引用回数:3 パーセンタイル:79.94(Physics, Condensed Matter)

Perovskite-type manganites exhibit various interesting phenomena arising from complex interplay among spin, charge, orbital, and lattice degrees of freedom. As the respective examples, perovskite-type YMnO$$_3$$ and Sr$$_{1-x}$$Ba$$_x$$MnO$$_3$$ are discussed. In the YMnO$$_3$$, the ferroelectric lattice distortion associated with the $$E$$-type spin order is observed for the first time. Displacement-type ferroelectricity with off-center magnetic ions is discovered for Sr$$_{0.5}$$Ba$$_{0.5}$$MnO$$_3$$, which shows both large polarization value and strong coupling between ferroelectricity and magnetism.

論文

Multiferroic $$M$$-type hexaferrites with a room-temperature conical state and magnetically controllable spin helicity

徳永 祐介*; 金子 良夫*; 奥山 大輔*; 石渡 晋太郎*; 有馬 孝尚*; 脇本 秀一; 加倉井 和久; 田口 康二郎*; 十倉 好紀*

Physical Review Letters, 105(25), p.257201_1 - 257201_4, 2010/12

 被引用回数:162 パーセンタイル:1.99(Physics, Multidisciplinary)

Magnetic and magnetoelectric (ME) properties have been studied for single crystals of Sc-doped $$M$$-type barium hexaferrites. Magnetization ($$M$$) and neutron diffraction measurements revealed that by tuning Sc concentration a longitudinal conical state is stabilized up to above room temperatures. ME measurements have shown that a transverse magnetic field ($$H$$) can induce electric polarization ($$P$$) at lower temperatures and that the spin helicity is nonvolatile and endurable up to near the conical magnetic transition temperature.

論文

Neutron diffraction studies on the multiferroic conical magnet Ba$$_{2}$$Mg$$_{2}$$Fe$$_{12}$$O$$_{22}$$

石渡 晋太郎*; 奥山 大輔*; 加倉井 和久; 西 正和*; 田口 康二郎*; 十倉 好紀*

Physical Review B, 81(17), p.174418_1 - 174418_8, 2010/05

 被引用回数:54 パーセンタイル:9.09(Materials Science, Multidisciplinary)

Magnetic structure of a multiferroic Y-type hexaferrite Ba$$_{2}$$Mg$$_{2}$$Fe$$_{12}$$O$$_{22}$$ with helical-spin structures propagating along [001] below 195K has been extensively studied, using polarized and unpolarized neutron diffractions under transverse magnetic fields up to 4.5T. The magnetic phase diagram was investigated as a function of transverse B and coexistence of several phases with different helical-spin periodicity was confirmed at low-B region. By using the polarized neutron diffraction under transverse B, we verified that the ferroelectric phase with the largest electric polarization P with a commensurate k0 vector (0,0,3/2) has a transverse-conical structure. The relation between the transverse-conical spin structure and P as a function of the transverse B is discussed in terms of the inverse Dzyaloshinskii-Moriya model.

論文

Resonant magnetic X-ray diffraction study on the successive metamagnetic transitions of TbB$$_4$$ up to 30T

稲見 俊哉; 大和田 謙二; 松田 康弘*; Ouyang, Z. W.*; 野尻 浩之*; 松村 武*; 奥山 大輔*; 村上 洋一*

Journal of Physics; Conference Series, 211, p.012010_1 - 012010_6, 2010/02

 被引用回数:3 パーセンタイル:17.09

Quite recently, successive metamagnetic transitions between 16T and 28T were reported on the rare-earth tetraboride TbB$$_4$$. Considerable attentions have been attracted on this peculiar bahavior. Recently, X-ray diffraction experiments under pulsed high magnetic field have been developed at several places. In order to examine the magnetic structures at high magnetic fields, we applied this technique to TbB$$_4$$. Small magnetic X-ray scattering cross section was overcome by utilizing resonance enhancement. The obtained field, polarization, and scattering angle dependence of magnetic reflections indicates that magnetic moments perpendicular to the magnetic field exist in the high-field plateau phases. This result is inconsistent with normalfractional magnetization plateau phases that posses only magnetic moments parallel or antiparallel to the field. In order to resolve this discrepancy, we propose magnetic structures that consist of Ising spins and XY spins.

論文

Resonant magnetic X-ray diffraction study on successive metamagnetic transitions in TbB$$_4$$

稲見 俊哉; 大和田 謙二; 松田 康弘*; Ouyang, Z. W.*; 野尻 浩之*; 松村 武*; 奥山 大輔*; 村上 洋一*

Journal of the Physical Society of Japan, 78(3), p.033707_1 - 033707_4, 2009/03

 被引用回数:15 パーセンタイル:30.11(Physics, Multidisciplinary)

Resonant magnetic X-ray diffraction experiments on the Shastry-Sutherland lattice TbB$$_4$$ were carried out under pulsed magnetic fields up to 30 T at 8 K. TbB$$_4$$ exhibits a multi-step magnetization process above 16 T below about 20 K when magnetic fields are applied along the $$c$$-axis. We examined the intensity of the 010 magnetic reflection as a function of magnetic field and found that the magnetization plateau phases are accompanied by large XY components of magnetic moments. We propose novel magnetic structures which consist of XY spins and Ising spins for the plateau phases. We also propose that energy level-crossing and its inherent degeneracy give rise to the successive met magnetic transitions in TbB$$_4$$.

口頭

TbB$$_4$$のパルス磁場下共鳴X線回折実験

稲見 俊哉; 大和田 謙二; 松田 康弘*; 野尻 浩之*; 奥山 大輔*; 松村 武*; 村上 洋一*

no journal, , 

希土類四硼化物RB$$_4$$は希土類イオンがShastry-Sutherland格子と等価な格子を組み、磁気及び四極子のフラストレーションの観点から関心を集めている。このうちTbB$$_4$$では、c軸に磁場を印加すると、30Tまでに多段の相転移をすることが磁化測定において発見され、それぞれの相でどのような磁気構造をとっているのか興味が持たれている。そこで、TbB$$_4$$の高磁場相での磁気構造に関する知見を得るために、共鳴X線回折実験をパルス強磁場下で行った。実験はSPring-8の原子力機構専用ビームラインBL22XUで行った。磁場発生は小型のパルスマグネットを用いて行った。中性子回折実験で磁気反射が観測されている100反射で共鳴回折ピークを探し、ここで、30Tまでのパルス磁場をc軸方向に印加し、50回の積算で強度の磁場依存性の測定に成功した。

口頭

TbB$$_4$$の多段磁化過程に関する考察

稲見 俊哉; 大和田 謙二; Ouyang, Z. W.*; 松田 康弘*; 野尻 浩之*; 松村 武*; 奥山 大輔*; 村上 洋一*

no journal, , 

正方晶希土類四硼化物RB$$_4$$は希土類イオンがShastry-Sutherland格子と等価な格子を組み、磁気及び四極子のフラストレーションの観点から関心を集めている。このうちTbB$$_4$$では、c軸に磁場を印加すると、約30Tで飽和するまでに磁化プラトーを伴う多段の相転移をすることが磁化測定において発見され、それぞれの相でどのような磁気構造をとっているのか、また、どういう機構によるものか、興味が持たれている。われわれは、前回、パルス磁場下共鳴X線回折実験の結果から、多段転移相が$$ab$$面内成分の秩序を伴うという、磁化プラトーでは異例の報告を行った。この結論を元に、強いXY型の基底状態とIsing型の励起状態というモデルを考える。このモデルは、面内異方性が無限大の場合、第1,第2,第3近接相互作用の広い範囲で、1/2プラトーを示す。さらに、フラストレーションの効果により、この磁化ステップが分裂するという計算結果を示す。

口頭

Resonant magnetic X-ray diffraction study under pulsed high magnetic field

稲見 俊哉; 大和田 謙二; 松田 康弘*; Ouyang, Z. W.*; 野尻 浩之*; 松村 武*; 奥山 大輔*; 村上 洋一*

no journal, , 

In the last five years, the active development of X-ray diffraction techniques under pulsed high magnetic field has enabled us to observe structural changes of crystals above 30 T. Since X-rays are also an experimental tool for detecting electronic degrees of freedom through resonant process, the development of resonant X-ray diffraction techniques under pulsed high magnetic field introduces a new microscopic probe to the high magnetic field science. After several improvements of the experimental techniques, we observed field dependence of the 100 and 500 magnetic reflections of the tetragonal rare-earth tetraboride TbB$$_4$$, which exhibits a number of magnetization plateaus between 16 T and 28 T. The obtained field, polarization, and scattering angle dependence indicates that magnetic moments perpendicular to the magnetic field exist in the high-field plateau phases. This result is inconsistent with normal fractional magnetization plateau phases that consist of up and down spins.

口頭

偏極中性子を用いたらせん磁性体Ba$$_{2}$$Mg$$_{2}$$Fe$$_{12}$$O$$_{22}$$における磁場誘起強誘電相の研究

石渡 晋太郎*; 奥山 大輔*; 加倉井 和久; 西 正和*; 田口 康二郎*; 十倉 好紀*

no journal, , 

Y型六方晶フェライトBa$$_{2}$$Mg$$_{2}$$Fe$$_{12}$$O$$_{22}$$は、195K以下で[001]方向に伝搬ベクトルk0を有するproper screw型らせん磁性を示し、伝搬ベクトルに垂直な磁場下で強誘電性を示し、非常に弱い磁場を用いて分極の方向を制御できることが知られている。そこで偏極中性子解析を含む詳細な中性子磁気構造解析研究をJRR-3に設置されたTAS-1で実施し、この強誘電相においてサイクロイド型コニカルらせん磁性構造が実現している事を明らかにした。

口頭

Neutron scattering activities on multiferroic systems; Neutron diffraction experiments on hexaferrites

加倉井 和久; 脇本 秀一; 石渡 晋太郎*; 奥山 大輔*; 西 正和*; 徳永 祐介*; 有馬 孝尚*; 田口 康二郎*; 十倉 好紀*

no journal, , 

The discovery of magnetoelectric (ME) effects caused by a cycloidal spin order has initiated an intense research on new class of ME materials. Among them one interesting class of materials is the hexaferrites such as Y-type (A$$_{2}$$Me$$_{2}$$Fe$$_{12}$$O$$_{22}$$: Me=transition metal), M-type (AFe$$_{12}$$O$$_{19}$$: A=Pb, Ca, Sr, Ba, etc.), where types of elementary blocks and their stacking order are different. These materials show a variety of complex magnetic ordering. In this presentation recent neutron diffraction experiments on multiferroic hexaferrites performed at the JRR-3 Neutron Science Facility of Japan Atomic Energy Agency (JAEA) are reported.

口頭

Study of complex magnetic structures in frustrated magnets by means of polarized neutrons

加倉井 和久; 脇本 秀一; 松田 雅昌*; 石渡 晋太郎*; 奥山 大輔*; 田口 康二郎*; 十倉 好紀*; 西 正和*; 中島 多朗*; 満田 節生*; et al.

no journal, , 

In studying modern functional materials, one is often confronted with complex spin configurations, for example, non-collinear, incommensurate magnetic structure such as helimagnetic structure as a result of frustrated magnetic interactions. Since the giant functional responses in these materials are direct consequences of these complicated magnetic structures, the detailed knowledge of the structure is mandatory to understand the essence of the magnetic functional materials. In this presentation some recent results on complex magnetic materials such as frustrated and multiferroic systems are reported, where the polarized neutron investigations provided important insight into the complex behavior of these functional materials.

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