研究者詳細 - 谷山　智康

論文 - 谷山　智康

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Kondo effect in a semiconductor quantum dot coupled to ferromagnetic electrodes 査読有り国際誌

K. Hamaya, M. Kitabatake, K. Shibata, M. Jung, M. Kawamura, K. Hirakawa, T. Machida, T. Taniyama, S. Ishida

APPLIED PHYSICS LETTERS 91 巻 ( 23 ) 頁： 232105/1 - 3 2007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：AMER INST PHYSICS

Using a laterally fabricated quantum-dot (QD) spin-valve device, we experimentally study the Kondo effect in the electron transport through a semiconductor QD with an odd number of electrons (N). In a parallel magnetic configuration of the ferromagnetic electrodes, the Kondo resonance at N=3 splits clearly without external magnetic fields. With applying magnetic fields (B), the splitting is gradually reduced, and then the Kondo effect is almost restored at B=1.2 T. This means that, in the Kondo regime, an inverse effective magnetic field of B similar to 1.2 T can be applied to the QD in the parallel magnetic configuration of the ferromagnetic electrodes. (c) 2007 American Institute of Physics.

DOI： 10.1063/1.2820445

Field-induced order-disorder transition in antiferromagnetic BaCo2V2O8 driven by a softening of spinon excitation 査読有り国際誌

S. Kimura, H. Yashiro, K. Okunishi, M. Hagiwara, Z. He, K. Kindo, T. Taniyama, M. Itoh

PHYSICAL REVIEW LETTERS 99 巻 ( 8 ) 頁： 087602/1 - 4 2007年8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

High field magnetization and ESR measurements on the quasi-one-dimensional (1D) antiferromagnet BaCo2V2O8 have been performed in magnetic fields up to 50 T along the chain. The experimental results are explained well in terms of a 1D S=1/2 antiferromagnetic XXZ model in longitudinal fields. We show that the quantum phase transition from the Neel ordered phase to the spin liquid one in the model is responsible for a peculiar order to disorder transition in BaCo2V2O8.

DOI： 10.1103/PhysRevLett.99.087602

その他リンク： http://orcid.org/0000-0003-3683-5416

Electric-field control of tunneling magnetoresistance effect in a Ni/InAs/Ni quantum-dot spin valve 査読有り国際誌

K. Hamaya, M. Kitabatake, K. Shibata, M. Jung, M. Kawamura, K. Hirakawa, T. Machida, T. Taniyama, S. Ishida, Y. Arakawa

APPLIED PHYSICS LETTERS 91 巻 ( 2 ) 頁： 022107/1 - 3 2007年7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 INST PHYSICS

The authors demonstrate an electric-field control of tunneling magnetoresistance (TMR) effect in a semiconductor quantum-dot spin-valve device. By using ferromagnetic Ni nanogap electrodes, they observe the Coulomb blockade oscillations at a small bias voltage. In the vicinity of the Coulomb blockade peak, the TMR effect is significantly modulated and even its sign is switched by changing the gate voltage, where the sign of the TMR value changes at the resonant condition. (C) 2007 American Institute of Physics.

DOI： 10.1063/1.2759264

Flux growth and magnetic anomalies of Co3V2O8 crystals 査読有り国際誌

Zhangzhen He, Tomoyasu Taniyama, Mitsuru Itoh, Yutaka Ueda

CRYSTAL GROWTH & DESIGN 7 巻 ( 6 ) 頁： 1055 - 1057 2007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 CHEMICAL SOC

Large single crystals of the novel vanadate Co3V2O8 are grown at a slow cooling rate using the flux method. X-ray diffraction and chemical analyses show that the grown crystals have high quality. Magnetic susceptibility and heat capacity measurements confirm that Co3V2O8 exhibits unusual magnetic behaviors, where five magnetic phase transitions occur below 15 K along the magnetic easy a-axis.

DOI： 10.1021/cg060539u

その他リンク： http://orcid.org/0000-0003-3683-5416

AgNbO3: A lead-free material with large polarization and electromechanical response 査読有り国際誌

Desheng Fu, Makoto Endo, Hiroki Taniguchi, Tomoyasu Taniyama, Mitsuru Itoh

APPLIED PHYSICS LETTERS 90 巻 ( 25 ) 頁： 252907/1 - 3 2007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 INST PHYSICS

Polarization measurements reveal that AgNbO3 has an extremely large polarization, which can reach a value of 52 mu C/cm(2) in polycrystals. Experiments also show that the large internal atom distortion in AgNbO3 is also strongly coupled to the electric field, indicating that high piezoelectric performance can be realized in AgNbO3 system. This finding opens the way to designing a new class of lead-free, high-performance piezoelectric materials based on AgNbO3. (c) 2007 American Institute of Physics.

DOI： 10.1063/1.2751136

Electrical voltage manipulation of ferromagnetic microdomain structures in a ferromagnetic/ferroelectric hybrid structure 査読有り国際誌

T. Taniyama, K. Akasaka, D. Fu, M. Itoh, H. Takashima, B. Prijamboedi

JOURNAL OF APPLIED PHYSICS 101 巻 ( 9 ) 頁： 09F512/1 - 3 2007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：AMER INST PHYSICS

We report on electrical bias voltage manipulation of the magnetic domain structures of patterned ferromagnetic Fe dots on a ferroelectric BaTiO(3) underlayer using a scanning probe microscope technique. Piezoresponse force microscopy is used to apply local electric field on the Fe dots/BaTiO(3) hybrid structure and to map the ferroelectric domain structures simultaneously. Magnetic force microscope observation also demonstrates that the magnetic domain structures of the Fe dots significantly change with the application of voltage at room temperature through the coupling between piezoelectric and magnetostrictive effects, indicating that the Fe dots/BaTiO(3) hybrid structure provides a promising basis for controlling micromagnetic domain structures using electric voltage.

DOI： 10.1063/1.2711280

その他リンク： http://orcid.org/0000-0003-3683-5416

Ferromagnetism at the surface of a LaCoO3 single crystal observed using scanning SQUID microscopy 査読有り国際誌

A. Harada, T. Taniyama, Y. Takeuchi, T. Sato, T. Kyomen, M. Itoh

PHYSICAL REVIEW B 75 巻 ( 18 ) 頁： 184426/1 - 5 2007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：AMERICAN PHYSICAL SOC

Evidence for ferromagnetism at the surface of a LaCoO3 single crystal is reported using a scanning superconducting quantum interference device (SQUID) microscope. Stray magnetic flux detected with the scanning SQUID shows typical ferromagnetic behavior in LaCoO3 below T-c similar to 85 K, in agreement with previous work on LaCoO3 particles. Analysis of the magnetization of LaCoO3 particle samples clearly shows that the magnetization is inversely proportional to the particle radius, giving the information that the ferromagnetism is restricted within a few unit cell layers from the surface. X-ray photoemission spectroscopy also indicates that the ferromagnetism likely originates from the metallic surface due to hole doping with oxygen chemisorption.

DOI： 10.1103/PhysRevB.75.184426

その他リンク： http://orcid.org/0000-0003-3683-5416

Size and field effect on mesoscopic spin glass 査読有り国際誌

K. Komatsu, H. Maki, T. Taniyama, T. Sato

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 310 巻 ( 2 ) 頁： 1500 - 1502 2007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：ELSEVIER SCIENCE BV

Spin glass particles were prepared as the mesoscopic system in order to examine the space scale of spin glass domain (droplet). The peak temperature T-peak in the temperature-dependent magnetization is systematically reduced with decreasing average particle size. This is due to the imitation of droplet growth to the particle diameter. The magnetic field H also decreases T-peak, which is caused by the reduction of the barrier height by Zeeman energy. However, there appears different tendency in the relation between H and T-peak below 100 Oe. This indicates the existence of crossover between the two regimes in which the free energy and Zeeman energy govern the droplet excitation. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.640

その他リンク： http://orcid.org/0000-0003-3683-5416

Surface ferromagnetism of LaCoO3 crystals 査読有り国際誌

A. Harada, T. Taniyama, M. Itoh

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 310 巻 ( 2 ) 頁： 2172 - 2173 2007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：ELSEVIER SCIENCE BV

Linear and nonlinear magnetic susceptibilities of a LaCoO3 particle sample were measured to shed light on the surface ferromagnetism of LaCoO3. The ferromagnetic transition temperature T-C estimated from the nonlinear magnetic susceptibility agrees well with that inferred from Kouvel-Fisher's analysis of the linear susceptibility, while the critical exponent gamma deduced simultaneously from the same analysis is incompatible with any values reported for 2d and 3d ferromagnetism. The results are discussed in view of surface and/or size effects in detail. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.11.117

その他リンク： http://orcid.org/0000-0003-3683-5416

Spin-glass behavior in zero magnetic field using tunnel resistance 査読有り国際誌

Y. Takeuchi, K. Komatsu, H. Maki, T. Taniyama, T. Sato

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 310 巻 ( 2 ) 頁： 1503 - 1505 2007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：ELSEVIER SCIENCE BV

The measurement of spin glass in zero magnetic field is essential to investigate the intrinsic nature of spin glass. We pay attention to tunnel resistance between spin-glass layers. The temperature dependence of tunnel resistance between spin-glass layers with the structure of AgMn/Al2O3/AgMn was measured, and was compared with that of Al/Al2O3/AgMn. For the junction of AgMn/Al2O3/AgMn, tunnel resistance shows a peak around the spin-glass transition temperature, but no peak was observed for the junction of Al/Al2O3/AgMn. The results indicate that the magnetic correlation between spin-glass layers is reflected to tunnel resistance. The singularity of the tunnel resistance between spin-glass layers is discussed by the droplet theory. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.1099

その他リンク： http://orcid.org/0000-0003-3683-5416

Two magnetic phase transitions in quasi-one-dimensional system SrCo2V2O8 査読有り国際誌

Zhangzhen He, Tomoyasu Taniyama, Mitsuru Itoh, Jun-Ichi Yamaura, Yutaka Ueda

SOLID STATE COMMUNICATIONS 141 巻 ( 12 ) 頁： 667 - 670 2007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：PERGAMON-ELSEVIER SCIENCE LTD

Magnetic properties of SrCo2V2O8 single crystal are investigated by means of ac magnetic susceptibility, dc magnetization and heat capacity measurements. The results show that SrCo2V2O8 possesses two canted antiferromagnetic transitions at similar to 5 and similar to 3 K, which is different from isostructural BaCo2V2O8 with only one anti ferromagnetic transition at similar to 5 K. We suggest that such different magnetic properties are mainly due to their different structural symmetry. (C) 2007 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ssc.2007.01.010

その他リンク： http://orcid.org/0000-0003-3683-5416

Spin transport through a single self-assembled InAs quantum dot with ferromagnetic leads 査読有り国際誌

K. Hamaya, S. Masubuchi, M. Kawamura, T. Machida, M. Jung, K. Shibata, K. Hirakawa, T. Taniyama, S. Ishida, Y. Arakawa

APPLIED PHYSICS LETTERS 90 巻 ( 5 ) 頁： 053108 2007年1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 INST PHYSICS

The authors have fabricated a lateral double barrier magnetic tunnel junction (MTJ) which consists of a single self-assembled InAs quantum dot (QD) with ferromagnetic Co leads. The MTJ shows clear hysteretic tunnel magnetoresistance (TMR) effect, which is evidence for spin transport through a single semiconductor QD. The TMR ratio and the curve shapes are varied by changing the gate voltage.

DOI： 10.1063/1.2435957

Conductive Boundary Layer in CaCu3Ti4O12 with Giant-Dielectric-Response 査読有り国際誌

Desheng Fu, Yosuke Arima, Hiroki Taniguchi, Tomoyasu Taniyama, Mitsuru Itoh, Jianding Yu, Shin-Ya Koshihara

FERROELECTRICS 347 巻 ( 1 ) 頁： 140 - 144 2007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

Dense polycrystalline samples of CaCu3Ti4O12 (CCTO) were prepared by solid-state reaction method under a high hydraulic pressure of 700 MPa, and characterized by X-ray diffraction, scanning electron microscopy and atomic force microscopy (AFM). Probing of the conduction path in nanoscale by AFM reveals that the grain boundary layer is conductive in CCTO. Therefore, reported giant-dielectric response in CCTOcan not be understood by the model of conducting grains surrounded by insulating grain boundaries, which is usually used for the explanation of conventional internal barrier layer capacitors.

DOI： 10.1080/00150190601187500

Long-range antiferromagnetic ordering in Cu2NiB2O6 査読有り国際誌

Zhangzhen He, Toru Kyomen, Tomoyasu Taniyama, Mitsuru Itoh

JOURNAL OF SOLID STATE CHEMISTRY 179 巻 ( 12 ) 頁： 3937 - 3941 2006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：ACADEMIC PRESS INC ELSEVIER SCIENCE

Magnetic properties of peculiar structural-type borate Cu2NiB2O6 are investigated by means of ac susceptibility, dc magnetization, and heat capacity measurements. This material is isostructural to Cu2CoB2O6, of which the structural configuration is composed of quasi-one-dimensional six-columns ribbons. Our experimental results show that this material displays an antiferromagnetic phase transition at similar to 15 K. which differs from Cu2CoB2O6 showing spin-glass behavior below 5 K. The different magnetic ground states in these compounds may be attributed mainly to the subtle balance of the exchange energy in the scalene triangles. (c) 2006 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.jssc.2006.08.034

その他リンク： http://orcid.org/0000-0003-3683-5416

Magnetic behavior and structural feature of quasi-one-dimensional BaCu(2)V2O(8) crystal 査読有り国際誌

Zhangzhen He, Tomoyasu Taniyama, Mitsuru Itoh

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 306 巻 ( 2 ) 頁： 277 - 280 2006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 properties of BaCu2V2O8 single crystals obtained by the spontaneous nucleation method using V2O5 as a self-flux are investigated. Singlet ground states with spin gaps of 215 and 223 K are observed along the directions parallel and perpendicular to the c-axis of the crystal, respectively, being in good agreement with powder sample measurements. Reinvestigation of the crystal structure reveals that the whole structural framework of BaCu2V2O8 crystal can be described as an assembly of alternating chains, which are built by edge-sharing pairs of CuO4 square-plaquettes and VO4 tetrahedra along the c-axis. The magnetic nature of BaCu2V2O8 is well understood based on this structural interpretation. (C) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.03.041

その他リンク： http://orcid.org/0000-0003-3683-5416

Crystal growth and magnetic properties of SrCO2V2O8 査読有り国際誌

Zhangzhen He, Tomoyasu Taniyama, Mitsuru Itoh

JOURNAL OF CRYSTAL GROWTH 293 巻 ( 2 ) 頁： 458 - 461 2006年8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

We report on the growth condition and magnetic properties of SrCo2V2O8 crystal. High-quality single crystals with dimensions 3x3x6 mm(3) have been successfully grown from its stoichiometrical melt in air by spontaneous nucleation method. Preliminary magnetic measurements show large canted ferromagnetism and unusual paramagnetic anisotropy of the grown crystals. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2006.05.039

その他リンク： http://orcid.org/0000-0003-3683-5416

Magnetic anisotropy switching in (Ga,Mn)As with increasing hole concentration 査読有り国際誌

K. Hamaya, T. Watanabe, T. Taniyama, A. Oiwa, Y. Kitamoto, Y. Yamazaki

PHYSICAL REVIEW B 74 巻 ( 4 ) 頁： 045201/1 - 4 2006年7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

We study a possible mechanism of the switching of the magnetic easy axis as a function of hole concentration in (Ga,Mn)As epilayers. In-plane uniaxial magnetic anisotropy along [110] is found to exceed intrinsic cubic magnetocrystalline anisotropy above a hole concentration of p=1.5x10(21) cm(-3) at 4 K. This anisotropy switching can be realized by postgrowth annealing, and the temperature-dependent ac susceptibility is significantly changed with increasing annealing time. On the basis of our recent scenario [Phys. Rev. Lett. 94, 147203 (2005) and Phys. Rev. B 73, 155204 (2006)], we deduce that the growth of highly hole-concentrated cluster regions with [110] uniaxial anisotropy is likely the predominant cause of the enhancement in [110] uniaxial anisotropy at the high hole concentration regime. We can clearly rule out anisotropic lattice strain as a possible origin of the switching of the magnetic anisotropy.

DOI： 10.1103/PhysRevB.74.045201

その他リンク： http://orcid.org/0000-0003-3683-5416

Effect of the shape anisotropy on the magnetic configuration of (Ga,Mn)As and its evolution with temperature 査読有り国際誌

K Hamaya, T Taniyama, T Koike, Y Yamazaki

JOURNAL OF APPLIED PHYSICS 99 巻 ( 12 ) 頁： 123901/1 - 3 2006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 INST PHYSICS

We study the effect of the shape anisotropy on the magnetic domain configurations of a ferromagnetic semiconductor (Ga,Mn)As/GaAs(001) epitaxial wire as a function of temperature. Using magnetoresistance measurements, we deduce the magnetic configurations and estimate the relative strength of the shape anisotropy compared with the intrinsic anisotropies. Since the intrinsic anisotropy is found to show a stronger temperature dependence than the shape anisotropy, the effect of the shape anisotropy on the magnetic domain configuration is relatively enhanced with increasing temperature. This information about the shape anisotropy provides a practical means of designing nanostructured spin electronic devices using (Ga,Mn)As. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2196113

Antiferromagnetic-paramagnetic transitions in longitudinal and transverse magnetic fields in a SrCo2V2O8 crystal 査読有り国際誌

ZZ He, T Taniyama, M Itoh

PHYSICAL REVIEW B 73 巻 ( 21 ) 頁： 212406/1 - 4 2006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：AMERICAN PHYSICAL SOC

Magnetic and heat capacity studies of the 1D anisotropic antiferromagnet SrCo2V2O8 demonstrate field-induced magnetic transitions in longitudinal and transverse magnetic fields. An antiferromagnetic (AF)-paramagnetic (PM) transition is observed instead of spin-flop transition in longitudinal magnetic field parallel to the magnetic easy c-axis. The temperature-field (T-H) phase diagram clearly shows that the AF-PM transition occurs in the same framework for both longitudinal and transverse magnetic fields in SrCo2V2O8. This is an experimental observation of such magnetic behaviors in an anisotropic antiferromagnet, irrespective of the field directions.

DOI： 10.1103/PhysRevB.73.212406

その他リンク： http://orcid.org/0000-0003-3683-5416

Out-of-plane magnetization reversal processes of (Ga,Mn)As with two different hole concentrations 査読有り国際誌

K.Hamaya, T. Taniyama, Y. Yamazaki

J. Appl. Phys. 99 巻 ( 9 ) 頁： 093903/1 - 5 2006年5月

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