研究者データベース

研究者情報

マスター

アカウント(マスター)

  • 氏名

    長浜 太郎(ナガハマ タロウ), ナガハマ タロウ

所属(マスター)

  • 工学研究院 応用化学部門 無機材料化学分野

所属(マスター)

  • 工学研究院 応用化学部門 無機材料化学分野

独自項目

syllabus

  • 2020, 応用物質化学(機能物性化学), Applied Materials Chemistry (Physical Chemistry and Functional Materials), 修士課程, 総合化学院, ナノ構造、半導体デバイス、磁性、薄膜、結晶成長 など
  • 2020, 物質変換工学, Applied Chemistry and Biochemistry, 学士課程, 工学部, 有機合成,有機材料,化学プロセス,反応器設計,生体材料,高分子材料,分子機能,無機材料,複合材料,電子材料,光機能材料
  • 2020, 化学結合論, Theory of Chemical Bonding, 学士課程, 工学部, 元素、原子軌道、周期律表、シュレディンガー方程式、波動関数
  • 2020, 物理化学Ⅱ, Physical Chemistry Ⅱ, 学士課程, 工学部, 平衡論、化学計測,化学量論,溶液内化学平衡
  • 2020, 応用化学学生実験Ⅱ, Applied Chemistry Laboratory Ⅱ, 学士課程, 工学部, 固相反応、焼結、固溶体、X線回折、電子顕微鏡、熱分析、セラミックス

timetable

  • 修士課程, 総合化学院, 2020, 応用物質化学A(機能固体化学)

researchmap

プロフィール情報

学位

  • 博士(理学)(京都大学)

プロフィール情報

  • 長浜, ナガハマ
  • 太郎, タロウ
  • ID各種

    200901000870246534

対象リソース

業績リスト

研究キーワード

  • MgO   トンネル磁気抵抗効果   スピントロニクス   TMR   強磁性トンネルトランジスタ   ホットエレクトロン   コヒーレントトンネリング   スピントロニクス 磁気抵抗   

研究分野

  • ものづくり技術(機械・電気電子・化学工学) / 電子デバイス、電子機器
  • ものづくり技術(機械・電気電子・化学工学) / 電気電子材料工学
  • 自然科学一般 / 磁性、超伝導、強相関系 / 磁性
  • ナノテク・材料 / ナノ材料科学 / 薄膜成長
  • ナノテク・材料 / 結晶工学 / スピントロニクス
  • ナノテク・材料 / 応用物性 / スピントロニクス

経歴

  • 2011年 - 現在 北海道大学 工学(系)研究科(研究院) 准教授
  • 2008年 - 2012年 科学技術振興機構 さきがけ研究員
  • 2008年 - 2010年 (独)産業技術総合研究所 エレクトロニクス研究部門 主任研究員
  • 2002年 - 2007年 (独)産業技術総合研究所 エレクトロニクス研究部門 研究員
  • 2005年10月 - 2006年09月 マサチューセッツ工科大学 フランシスビッター研究所 客員研究員

学歴

  • 1996年 - 2000年   京都大学大学院理学研究院博士後期課程
  • 1994年 - 1996年   京都大学大学院理学研究院博士前期課程
  • 1990年 - 1994年   京都大学理学部

委員歴

  • 2019年 - 現在   応用物理学会 的秩序とその操作に関わる研究グループ   幹事(企画)
  • 2016年 - 2021年   日本磁気学会   編集委員
  • 2019年 - 2020年   Intermag 2020   Program committee
  • 2007年 - 2020年   日本磁気学会   スピントロニクス専門研究会世話人
  • 2015年 - 2016年   応用物理学会北海道支部   庶務幹事
  • 2015年 - 2015年   日本磁気学会   論文委員
  • 2014年 - 2015年   日本学術振興会   科学研究費委員会専門委員
  • 2013年 - 2015年   SSDM   SSDM論文委員
  • 2013年 - 2013年   MML2013   プログラム委員
  • 2013年 - 2013年   科学技術振興機構   JST国際科学技術協力推進委員
  • 2012年 - 2012年   日本磁気学会   第39回学術講演会実行委員会事務局長
  • 2009年 - 2010年   日本磁気学会   企画委員会幹事
  • 2009年 - 2010年   日本磁気学会   論文賞・若手奨励賞審査委員
  • 2008年 - 2009年   日本物理学会   領域3世話人
  • 2007年 - 2009年   日本磁気学会   企画委員

受賞

  • 2020年 Applied Physics Express: 2020 reviewer awards
     
    受賞者: Taro Nagahama
  • 2014年 日本磁気学会 学会活動貢献賞
     
    受賞者: 長浜 太郎
  • 2005年 応用物理学会 論文賞
     High Tunnel Magnetoresistance at Room Temperatue of Fully Epitaxial Fe/MgO/Fe Tunnel Junctions due to Coherent Spin-Polarized Tunneling 
    受賞者: 湯浅新治;福島章男;長浜太郎;安藤功兒;鈴木義茂
  • 2002年 日本応用磁気学会 論文賞
     Fe(001)強磁性トンネル接合の量子井戸効果 
    受賞者: 長浜太郎;湯浅新治;鈴木義茂;田村英一

論文

  • Shoma Yasui, Syuta Honda, Jun Okabayashi, Takashi Yanase, Toshihiro Shimada, Taro Nagahama
    PHYSICAL REVIEW APPLIED 15 3 2021年03月 
    Spinel-type magnetite Fe3O4 is predicted to be a half-metal material with negative spin polarization. However, magnetic tunnel junctions (MTJs) using an Fe3O4 electrode exhibit a small tunnel magnetoresistance (TMR) effect, the sign of which has not been established experimentally. The development of Fe3O4 as an excellent TMR material requires a better understanding of the characteristics of the interface and the phase transition of Fe3O4 called the Verwey transition. We fabricate MTJs using epitaxial Fe3O4-based stacks on MgO(001) substrates and find a large inverse TMR ratio of -55.8% at 80 K, which corresponds to 126% by the optimistic definition of the TMR ratio. The temperature dependence of the TMR ratio is significantly affected by the Verwey transition. Moreover, we investigate the dependence of TMR on oxygen partial pressure during Fe3O4 deposition. It is found that the magneto-transport properties of the MTJs show different behaviors depending on the oxygen partial pressure because the Verwey transition is sensitive to the oxygen concentration. Furthermore, the electronic and magnetic properties at the interfacial regions are investigated by x-ray magnetic spectroscopy and first-principles calculation. These findings greatly support the use of Fe3O4 in spintronic devices and should lead to further developments in oxide spintronics.
  • Tsujie, Asaka, Hara, Yoshinori, Yanase, Takashi, Shimada, Toshihiro, Nagahama, Taro
    Applied Physics Letters 2020年 [査読有り][通常論文]
  • Nodo, Shoto, Ono, Shimpei, Yanase, Takashi, Shimada, Toshihiro, Nagahama, Taro
    Applied Physics Express 2020年 [査読有り][通常論文]
  • Liu, Wei, Yanase, Takashi, Iwasa, Nobuhiro, Koizumi, Hitoshi, Mukai, Shin, Iwamura, Shinichiro, Nagahama, Taro, Shimada, Toshihiro
    International Journal of Hydrogen Energy 2020年 [査読有り][通常論文]
  • Nodo, S., Nodo, S., Yamamoto, T., Yamamoto, T., Yanase, T., Yanase, T., Shimada, T., Shimada, T., Nagahama, T., Nagahama, T.
    Solid State Communications in press 2020年 [査読有り][通常論文]
  • Liu, Wei, Yanase, Takashi, Iwasa, Nobuhiro, Mukai, Shin, Iwamura, Shinichiro, Nagahama, Taro, Shimada, Toshihiro
    Journal of Colloid and Interface Science 2020年 [査読有り][通常論文]
  • Takashi Yanase, Uika Ogihara, Yoshihiro Awashima, Takeshi Yanagida, Kazuki Nagashima, Taro Nagahama, Toshihiro Shimada
    CRYSTAL GROWTH & DESIGN 19 12 7257 - 7263 2019年12月 [査読有り][通常論文]
     
    Single-crystal Fe nanowires (NWs) were synthesized on SiO2/Si substrates by chemical vapor deposition (CVD) using a two-flow system in the presence of FeO nanoparticles as catalysts. Transmission electron microscopy observations and an electron diffraction analysis revealed that the NW was defect-free alpha-Fe (bcc structure) oriented in the < 100 > direction. A plausible mechanism of the Fe NW growth is suggested based on the observations of the morphology and characteristic diameter of the Fe NW. We found that the NWs became polycrystalline when their diameter was approximately 100 nm. The excellent crystal quality of the single-crystal NW was confirmed by electron diffraction and the residual resistance ratio. Magnetic uniformity and a strong shape-anisotropic behavior of magnetization were observed by measuring the hysteresis loop and the angle dependence of the magneto-optical Kerr effect, respectively.
  • 柳瀬 隆, Weng Mengting, Zhang Meiqi, 後藤真菜美, 長浜太郎, 島田 敏宏
    表面と真空 62 10 2019年10月 [査読有り][通常論文]
  • Takashi Yanase, Takuya Miura, Tatsuya Shiratori, Mengting Weng, Taro Nagahama, Toshihiro Shimada
    C-JOURNAL OF CARBON RESEARCH 5 3 2019年09月 [査読有り][通常論文]
     
    The choice of a catalyst for carbon nanotube (CNT) growth is critical to controlling the morphology and chirality of the final product. Plasma-enhanced chemical vapor deposition (PECVD) can alleviate the requirements of the catalyst, i.e., they must be active for both the decomposition of the source gas and graphitization in the conventional thermal CVD. However, it is still not well understood how the catalytic activity of the graphitization affects the yield and quality of CNTs. In this paper, we systematically investigated the influence of the catalytic activity of graphitization by tuning the composition of Fe1-xMnxO (x = 0-1) nanoparticles as catalysts. As the Mn component increased, the number of CNTs decreased because Mn has no catalytic function of the graphitization. The quality of CNTs also affected by the inclusion of the Mn component. Our study may provide useful information to develop a new catalyst for CNT growth in PECVD.
  • Study on the requirements of catalytic function for the growth of carbon nanotubes by plasma-enhanced chemical vapor deposition
    Takashi Yanase, Takuya Miura, Tatsuya Shiratori, Mengting Weng, Taro Nagahama, Toshihiro Shimada
    C: Journal of Carbon Research 5 3 46 - 46 2019年08月 [査読有り][通常論文]
  • Y. Goto, T. Yanase, T. Shimada, M. Shirai, T. Nagahama
    AIP Advances 9 8 2019年08月01日 [査読有り][通常論文]
     
    © 2019 Author(s). In magnetic tunnel junctions (MTJs), the tunnel resistance varies as a function of the relative magnetic configuration of the electrode, in an effect called tunnel magnetoresistance (TMR). The material of which the electrodes are composed is of great importance, because TMR is very sensitive to the electronic states of the electrodes. Additionally, structural defects at the interface also have a significant influence on TMR. In this study, we employ B2-Fe3Sn as the magnetic electrode of MTJs. The use of Fe3Sn could solve the problem of lattice mismatch between Fe and MgO. However, the presence of dissimilar atoms in the electrodes or interface oxidation could be a source of defects at the interface. We find that MTJs with Fe3Sn exhibit a TMR of 50% and an asymmetric bias dependence.
  • Synthesis and Enhanced Room-temperature Ferromagnetic Properties of Carbon-doped Boron Nitride Nanosheets
    Wei Liu, Takashi Yanase, Taro Nagahama, Toshihiro Shimada
    J. Alloys and Compounds 792 1206 - 1212 2019年06月 [査読有り][通常論文]
  • Search of new Nitrogen-doped carbon materials by compressing molecular crystals
    Ichiro Yamane, Takashi Yanase, Taro Nagahama, Toshihiro Shimada
    Japanese Journal of Applied Physics 2019年04月 [査読有り][通常論文]
  • Single crystal growth, structural analysis and electronic band structure of a nitrogen-containing polyacene BBQPP
    Nobuhiko Sakai, Takahiro Tamura, Takashi Yanase, Taro Nagahama, Toshihiro Shimada
    Japanese Journal of Applied Physics 58 in press  2019年04月 [査読有り][通常論文]
  • Room-temperature large magnetocapacitance effect in magnetic tunnel junctions
    H. Kaiju, T. Nagahama, O. Kitakami, J. Nishii, G. Xiao
    Bulletin of Topical Symposium of the Magnetics Society of Japan 224 7 - 13 2019年 [査読有り][招待有り]
  • Wei Xie, Takahiro Tamura, Takashi Yanase, Taro Nagahama, Toshihiro Shimada
    Japanese Journal of Applied Physics 57 4 2018年04月01日 [査読有り][通常論文]
     
    The effect of C doping to hexagonal boron nitride (h-BN) to its electronic structure is examined by first principles calculations using the association from π-electron systems of organic molecules embedded in a two-dimensional insulator. In a monolayered carbon-doped structure, odd-number doping with carbon atoms confers metallic properties with different work functions. Various electronic interactions occur between two layers with odd-number carbon substitution. A direct sp3 covalent chemical bond is formed when C replaces adjacent B and N in different layers. A charge transfer complex between layers is found when C replaces B and N in the next-neighboring region, which results in narrower band gaps (e.g., 0.37 eV). Direct bonding between C and B atoms is found when two C atoms in different layers are at a certain distance.
  • Takashi Yanase, Sho Watanabe, Fumiya Uehara, Mengting Weng, Taro Nagahama, Toshihiro Shimada
    THIN SOLID FILMS 649 171 - 176 2018年03月 [査読有り][通常論文]
     
    The doping of element (Nb, Ta, etc.) into MoS2, one of the layered transition metal dichalcogenides, is a key technology for electronic devices because the lack of the p-type MoS2 has limited the range of applications. We report that the Mo1-xNbxS2 thin films were synthesized on SiO2/Si substrates by chemical vapor deposition (CVD). It was critical to use chloride sources ( MoCl5 and NbCl5) for the synthesis of Mo1-xNbxS2. The Nb concentration can be increased to 10% by controlling the supplied amount of Nb using a separate-flow CVD apparatus. The Raman spectra changed as the Nb concentration increased, appearing E-2(Nb-S) vibrational mode. The photoluminescence (PL) at 655 nm, attributed to emission from excitons, disappeared, when Nb was incorporated into the MoS2. PL due to trions at 680 nm was observed for the Mo1-xNbxS2 thin films.
  • Takashi Yanase, Hiroaki Uwabe, Koki Hasegawa, Taro Nagahama, Makoto Yamaguchi, Toshihiro Shimada
    JAPANESE JOURNAL OF APPLIED PHYSICS 57 3 2018年03月 [査読有り][通常論文]
     
    We synthesized semitransparent conducting thin films of amorphous carbon from sp(3)-rich network polymer. The films showed a reasonable optical transparency (58-73% transmission in the wavelength range of 380-2200 nm), a low electric resistivity (6.7 x 10(-3) Omega cm), and durability against corrosive chemical reagents. The sintering of the amorphous films results in the formation of a carbon honeycomb lattice in the films. (C) 2018 The Japan Society of Applied Physics
  • Mengting Weng, Meiqi Zhang, Takashi Yanase, Fumiya Uehara, Taro Nagahama, Toshihiro Shimada
    Japanese Journal of Applied Physics 57 3 2018年03月01日 [査読有り][通常論文]
     
    MoS2 nanotubes were synthesized by chemical vapor deposition (CVD) using six-horned octahedral FeO nanoparticles and gasified MoO3 and sulfur. The morphology and structure of the nanotubes were characterized by scanning electron microscopy (SEM), Raman spectroscopy, and transmission electron microscopy (TEM), showing that the nanotubes had a hollow structure. The hexagonal lattice of MoS2 was aligned parallel to the direction of the nanotubes. The catalytic activities of the Fe films and nanoparticles of Fe2O3, NiO, and Cu2O for the growth of MoS2 nanotubes were investigated. FeO was found to be the most effective catalyst tested.
  • Yuki Goto, Masato Araki, Nozomi Takahashi, Takashi Yanase, Toshihiro Shimada, Masahito Tsujikawa, Masafumi Shirai, Akira Kamimaki, Satoshi Iihama, Shigemi Mizukami
    Japanese Journal of Applied Physics 2018年 [査読有り][通常論文]
  • Kaiju Hideo, Misawa Takahiro, Nagahama Taro, Komine Takashi, Kitakami Osamu, Fujioka Masaya, Nishii Junji, Xiao Gang
    Scientific Reports 8 2018年 [査読有り][通常論文]
  • Nozomi Takahashi, Teodor Huminiuc, Yuta Yamamoto, Takashi Yanase, Toshihiro Shimada, Atsufumi Hirohata, Taro Nagahama
    SCIENTIFIC REPORTS 7 2017年08月 [査読有り][通常論文]
     
    The application of magnetic oxides in spintronics has recently attracted much attention. The epitaxial growth of magnetic oxide on Si could be the first step of new functional spintronics devices with semiconductors. However, epitaxial spinel ferrite films are generally grown on oxide substrates, not on semiconductors. To combine oxide spintronics and semiconductor technology, we fabricated Fe3O4 films through epitaxial growth on a Si(111) substrate by inserting a gamma-Al2O3 buffer layer. Both of gamma-Al2O3 and Fe3O4 layer grew epitaxially on Si and the films exhibited the magnetic and electronic properties as same as bulk. Furthermore, we also found the buffer layer dependence of crystal structure of Fe3O4 by X-ray diffraction and high-resolution transmission electron microscope. The Fe3O4 films on an amorphous-Al2O3 buffer layer grown at room temperature grew uniaxially in the (111) orientation and had a textured structure in the plane. When Fe3O4 was deposited on Si(111) directly, the poly-crystal Fe3O4 films were obtained due to SiOx on Si substrate. The epitaxial Fe3O4 layer on Si substrates enable us the integration of highly functional spintoronic devices with Si technology.
  • Weng Mengting, Takashi Yanase, Fumiya Uehara, Sho Watanabe, Takuya Miura, Taro Nagahama, Toshihiro Shimada
    CRYSTENGCOMM 19 28 3915 - 3920 2017年07月 [査読有り][通常論文]
     
    Nanowires of layered materials are important because they exhibit the highest sensitivity as electricallydetecting chemical sensors. MoS2 nanowires have been synthesized by a catalytic chemical vapor deposition method on silicon substrates drop-coated with FeO nanoparticles of different shapes. Switching of the products (MoS2 nanowires to SiO2 nanowires) has been observed when the shapes and sizes of the FeO nanoparticles changed. MoS2 nanowires were grown in the presence of six-horned octahedral nanoparticles, whereas SiO2 nanowires were formed in the presence of spherical nanoparticles. Their morphology, crystal structure and elemental composition have been fully investigated to elucidate the growth mechanism of the nanowires. The kinetics of the grown SiO2 and MoS2 nanowires are competing, giving rise to the observed switching.
  • Takahiro Tamura, Takuya Takami, Takashi Yanase, Taro Nagahama, Toshihiro Shimada
    JAPANESE JOURNAL OF APPLIED PHYSICS 56 6 2017年06月 [査読有り][通常論文]
     
    We studied the N-2 plasma etching of cubic boron nitride (cBN). We have developed experimental techniques for handling 200-mu m-size single crystals for the preparation of surfaces with arbitrary crystal indexes, plasma processes, and surface analyses. We successfully prepared smooth surfaces of cBN with roughness smaller than 10 nm and found that the etching behavior was strongly influenced by the surface indexes. The morphology of the etched surfaces can be explained by the chemical stability of (111)B surfaces. (C) 2017 The Japan Society of Applied Physics
  • Hideo Kaiju, Taro Nagahama, Shun Sasaki, Toshihiro Shimada, Osamu Kitakami, Takahiro Misawa, Masaya Fujioka, Junji Nishii, Gang Xiao
    SCIENTIFIC REPORTS 7 2017年06月 [査読有り][通常論文]
     
    Magnetocapacitance (MC) effect, observed in a wide range of materials and devices, such as multiferroic materials and spintronic devices, has received considerable attention due to its interesting physical properties and practical applications. A normal MC effect exhibits a higher capacitance when spins in the electrodes are parallel to each other and a lower capacitance when spins are antiparallel. Here we report an inverse tunnel magnetocapacitance (TMC) effect for the first time in Fe/AlOx/Fe3O4 magnetic tunnel junctions (MTJs). The inverse TMC reaches up to 11.4% at room temperature and the robustness of spin polarization is revealed in the bias dependence of the inverse TMC. Excellent agreement between theory and experiment is achieved for the entire applied frequency range and the wide bipolar bias regions using Debye-Frohlich model (combined with the Zhang formula and parabolic barrier approximation) and spin-dependent drift-diffusion model. Furthermore, our theoretical calculations predict that the inverse TMC effect could potentially reach 150% in MTJs with a positive and negative spin polarization of 65% and -42%, respectively. These theoretical and experimental findings provide a new insight into both static and dynamic spin-dependent transports. They will open up broader opportunities for device applications, such as magnetic logic circuits and multi-valued memory devices.
  • K. Omori, T. Kawai, N. Takahashi, T. Yanase, T. Shimada, T. Nagahama
    APPLIED PHYSICS LETTERS 110 21 2017年05月 [査読有り][通常論文]
     
    Fe3O4 possesses unique properties such as high Curie temperature and half-metallic nature and therefore can function as a high-spin-polarized electrode in spintronic devices. In order to use this compound in such devices, the magnetic characteristics of multilayers used in these devices, which include Fe3O4, should be highly controllable. In this study, we fabricated Fe3O4/Pt or Cr (0-10 nm)/Fe systems through epitaxial growth on MgO(110) substrates in order to investigate the magnetic coupling in Fe3O4 and metallic hybrid systems. We found that these two systems show a marked difference in the dependence of the coercive field (H-c) of the Fe layer on the nonmagneticmetal- layer thickness. Hc for the system with Pt showed variation characteristics of interlayer exchange coupling in metal systems, while H-c increased monotonically with the thickness of the Cr layer. Published by AIP Publishing.
  • Toshihiro Shimada, Takuya Miura, Wei Xie, Takashi Yanase, Taro Nagahama
    MEASUREMENT 102 244 - 248 2017年05月 [査読有り][通常論文]
     
    We report an accurate and easy-to operate instrument for the temperature control of an electrically floated sample using a thermocouple in direct contact with it. The signal was transmitted via 2.45 GHz WiFi. We measured and analyzed the discrepancy between the thermocouple and a radiation thermometer for the plasma CVD of carbon nanomaterials under sample bias. A successful protection method from abnormal discharge in the plasma is also provided. (C) 2017 Elsevier Ltd. All rights reserved.
  • Takahiro Tamura, Takuya Takami, Sachio Kobayashi, Taro Nagahama, Takashi Yanase, Toshihiro Shimada
    DIAMOND AND RELATED MATERIALS 69 127 - 132 2016年10月 [査読有り][通常論文]
     
    This paper describes an attempt to synthesize bismuth-doped diamond by plasma CVD. Solid bismuth was inserted into the reaction plasma of CH4 and H-2. Examination by TEM showed that most of the bismuth was included as Bi nanoparticles in the carbon nanospheres, which segregated at the grain boundary of the diamond polycrystals. Diffraction peaks corresponding to the carbon allotrope Chaoite were observed at the grain boundaries. The Raman spectra showed very complex features between 100-1600 cm(-1), suggesting the existance of molecule-like species. (C) 2016 Elsevier B.V. All rights reserved.
  • Yoichiro Nakano, Takashi Yanase, Taro Nagahama, Hajime Yoshida, Toshihiro Shimada
    SCIENTIFIC REPORTS 6 2016年10月 [査読有り][通常論文]
     
    The water vapor transmission rate (WVTR) of a gas barrier coating is a critically important parameter for flexible organic device packaging, but its accurate measurement without mechanical stress to ultrathin films has been a significant challenge in instrumental analysis. At the current stage, no reliable results have been reported in the range of 10(-6) g m(-2) day(-1) that is required for organic light emitting diodes (OLEDs). In this article, we describe a solution for this difficult, but important measurement, involving enhanced sensitivity by a cold trap, stabilized temperature system, pumped sealing and calibration by a standard conductance element.
  • Takashi Yanase, Sho Watanabe, Mengting Weng, Makoto Wakeshima, Yukio Hinatsu, Taro Nagahama, Toshihiro Shimada
    CRYSTAL GROWTH & DESIGN 16 8 4467 - 4472 2016年08月 [査読有り][通常論文]
     
    This article reports the growth and characterization of z-axis-oriented NbS2 thin films on SiO2/Si substrate by ambient pressure chemical vapor deposition (CVD) using a generic metal chloride source. We found that NbS2 nanosheets can be grown directly on the SiO2/Si substrate with the aid of hydrogen gas mixed in the carrier gas. Detailed examination Of the growth parameters was made possible using a separate-flow CVD apparatus. It appears that the major cause of the misorientation is the off-stoichiometry with surplus Nb. The quality of the films was evaluated by X-ray diffraction and Raman spectroscopy as well as resistivity measurements at low temperatures. They showed a resistivity minimum at the same temperature of the charge density wave (CDW) transition for a bulk single crystal of 3R-NbS2.
  • Takashi Yanase, Sho Watanabe, Mengting Weng, Taro Nagahama, Toshihiro Shimada
    JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY 16 4 3223 - 3227 2016年04月 [査読有り][通常論文]
     
    We report detailed experiments on chemical vapor deposition of an atomic layer semiconductor MoS2. We developed a new type of CVD system in which MoO3 and S sources are separately supplied to the substrates. It has become possible to precisely control the supply of the materials separately in the order of seconds. Raman and XPS analysis of the films grown under various conditions revealed that the initially obtained films are S-deficient and complete stoichiometry is reached after several minutes under S vapor flow.
  • Nozomi Takahashi, Tomohiro Kawai, Takashi Yanase, Toshihiro Shimada, Taro Nagahama
    JAPANESE JOURNAL OF APPLIED PHYSICS 54 11 2015年11月 [査読有り][通常論文]
     
    The combination of magnetic tunnel junctions (MTJs) and magnetic insulating (MI) layers has attracted much attention because of its potential as novel spintronic devices. To realize such devices, the epitaxial growth and magnetoresistance of MTJs with spinel ferrite were investigated. Nonmagnetic (NM) layers were inserted between the MTJs and MI layers as magnetic decoupling layers, the epitaxial growth of which was important to obtain high-quality epitaxial multilayers. Multilayers of MTJ/NM/MI and MI/NM/MTJ were fabricated and tunnel magnetoresistance (TMR) values of 70 and 50% at room temperature, respectively, were observed. The shape of the magnetoresistance curve depended on the sample structure. (C) 2015 The Japan Society of Applied Physics
  • Hideo Kaiju, Masashi Takei, Takahiro Misawa, Taro Nagahama, Junji Nishii, Gang Xiao
    APPLIED PHYSICS LETTERS 107 13 2015年09月 [査読有り][通常論文]
     
    The frequency dependence of tunneling magnetocapacitance (TMC) in magnetic tunnel junctions (MTJs) is investigated theoretically and experimentally. According to the calculation based on Debye-Frohlich model combined with Julliere formula, the TMC ratio strongly depends on the frequency and it has the maximum peak at a specific frequency. The calculated frequency dependence of TMC is in good agreement with the experimental results obtained in MgO-based MTJs with a tunneling magnetoresistance (TMR) ratio of 108%, which exhibit a large TMC ratio of 155% at room temperature. This calculation also predicts that the TMC ratio can be as large as about 1000% for a spin polarization of 87%, while the TMR ratio is 623% for the same spin polarization. These theoretical and experimental findings provide a deeper understanding on AC spin-dependent transport in the MTJs and will open up wider opportunities for device applications, such as highly sensitive magnetic sensors and impedance-tunable devices. (C) 2015 AIP Publishing LLC.
  • Wei Xie, Aiko Kawahito, Takuya Miura, Takashi Endo, Yongming Wang, Takashi Yanase, Taro Nagahama, Yukitoshi Otani, Toshihiro Shimada
    CHEMISTRY LETTERS 44 9 1205 - 1207 2015年09月 [査読有り][通常論文]
     
    We report the synthesis of a new carbon material diamondlike carbon film codeposited with 1% C-60 molecules by plasma-assisted chemical vapor deposition. The synthesized films exhibited strong iridescent colors after being exposed to an atmosphere containing water vapor. An electron microscopy examination revealed that the film expanded by a factor of more than two, while forming 10-100-nm-sized grain-like structures after its exposure to water vapor. This work demonstrates a new concept of incorporating reaction centers in carbon solids to make carbon-based nanostructures.
  • A. Kawahito, T. Yanase, T. Endo, T. Nagahama, T. Shimada
    JOURNAL OF APPLIED PHYSICS 117 17 2015年05月 [査読有り][通常論文]
     
    Magnetic nanowires (NWs) are promising as material for use in spintronics and as the precursor of permanent magnets because they have unique properties due to their high aspect ratio. The growth of magnetic Fe whiskers was reported in the 1960s, but the diameter was not on a nanoscale level and the growth mechanism was not fully elucidated. In the present paper, we report the almost vertical growth of Fe NWs on a single crystal yttrium-stabilized zirconia (Y0.15Zr0.85O2) by a thermal CVD method. The NWs show a characteristic taper part on the bottom growing from a trigonal pyramidal nucleus. The taper angle and length can be controlled by changing the growth condition in two steps, which will lead to obtaining uniformly distributed thin Fe NWs for applications. (C) 2015 AIP Publishing LLC.
  • 島田敏宏, 柳瀬隆, 長浜太郎, 土田裕也, 村谷直紀
    電気学会論文C部門誌 135 2 160 - 163 2015年02月01日 [査読無し][通常論文]
     
    Frontier Chemistry Center, Hokkaido University Kita 13, Nishi 8, Kita-ku, Sapporo 060-8628, Japan Graduate School of Chemical Sciences and Engineering, Hokkaido University Kita 13, Nishi 8, Kita-ku, Sapporo 060-8628, Japan Flexible solar cells made of organic semiconductors are promising for the application to the photovoltaic energy sources for flexible devices and those can be attached to any curved surfaces. Mixing and phase separation of different molecules are important for the formation of bulk-hetero junctions. We found that DNTT (dinaphtotienothiophene) and C60 spontaneously mixes when they are sequentially deposited. The driving force does not seem to be the charge transfer between p-type and n-type molecules but the maximization of van der Waals attraction coming from the molecular shapes.
  • Il Jeon, Dai Kutsuzawa, Yu Hashimoto, Takashi Yanase, Taro Nagahama, Toshihiro Shimada, Yutaka Matsuo
    ORGANIC ELECTRONICS 17 275 - 280 2015年02月 [査読有り][通常論文]
     
    We demonstrate the deposition of multilayered MoS2 on a low-cost metallic-semiconducting carbon nanotube via chemical vapor deposition, and the use of this material as electron acceptor species forming a bulk heterojunction with P3HT in inverted-type organic photovoltaics (OPVs). This is an uplifting discovery, in which MoS2 has been used as an electron acceptor in spite of its innate immiscibility with organic compounds. This is possible because we utilize carbon nanotube's nature to intercalate with P3HT through pi-pi interaction. The successful binding of MoS2 onto carbon nanotube bundles and its optoelectronic effect as a photovoltaic device has been lucidly analyzed through various techniques in this paper. The effect has been ultimately evidenced by a power conversion efficiency of 0.46%, which proves MoS2 with many advantages can also be used as a photoactive layer. (C) 2014 Elsevier B.V. All rights reserved.
  • 島田敏宏, 柳瀬隆, 長浜太郎
    電気学会論文C部門誌 135 2 2015年 [査読無し][通常論文]
  • Takahiro Tamura, Takashi Yanase, Taro Nagahama, Makoto Wakeshima, Yukio Hinatsu, Toshihiro Shimada
    CHEMISTRY LETTERS 43 10 1569 - 1571 2014年10月 [査読有り][通常論文]
     
    We demonstrate a new doping technique for chemical vapor deposition (CVD) growth of diamond. The method involves immersing a solid-state dopant source into the plasma during microwave plasma-assisted CVD. We applied this simple and versatile technique to the growth of boron-doped diamond. The grown films were characterized by X-ray diffraction (XRD), Raman microscopy, glow discharge optical emission spectroscopy (GDOES), and electrical conductivity measurements. The average concentration of boron was 0.5 atom % and the conductivity was 1.5 x 10(-2) Omega cm, which showed irregular behavior at low temperature.
  • Taro Nagahama, Yuya Matsuda, Kazuya Tate, Tomohiro Kawai, Nozomi Takahashi, Shungo Hiratani, Yusuke Watanabe, Takashi Yanase, Toshihiro Shimada
    APPLIED PHYSICS LETTERS 105 10 2014年09月 [査読有り][通常論文]
     
    Fe3O4 is a ferrimagnetic spinel ferrite that exhibits electric conductivity at room temperature (RT). Although the material has been predicted to be a half metal according to ab-initio calculations, magnetic tunnel junctions (MTJs) with Fe3O4 electrodes have demonstrated a small tunnel magnetoresistance (TMR) effect. Not even the sign of the tunnel magnetoresistance ratio has been experimentally established. Here, we report on the magnetic properties of epitaxial Fe3O4 films with various crystal orientations. The films exhibited apparent crystal orientation dependence on hysteresis curves. In particular, Fe3O4(110) films exhibited in-plane uniaxial magnetic anisotropy. With respect to the squareness of hysteresis, Fe3O4 (111) demonstrated the largest squareness. Furthermore, we fabricated MTJs with Fe3O4(110) electrodes and obtained a TMR effect of -12% at RT. The negative TMR ratio corresponded to the negative spin polarization of Fe3O4 predicted from band calculations. (C) 2014 AIP Publishing LLC.
  • Toshihiro Shimada, Taro Nagahama, Takashi Yanase
    JAPANESE JOURNAL OF APPLIED PHYSICS 53 8 2014年08月 [査読有り][通常論文]
     
    We present the analysis of a simple electrostatic model of a solid-state capacitor with ionizable charge traps. It shows a charge voltage characteristic resembling a chemical battery. With a given withstanding voltage and the same geometry, the energy density of the capacitor with ionizable traps will be lower than that of an ordinary capacitor with metal electrodes. (C) 2014 The Japan Society of Applied Physics
  • Toshihiro Shimada, Shosei Kubota, Takashi Yanase, Taro Nagahama
    CARBON 67 300 - 303 2014年02月 [査読有り][通常論文]
     
    We examined the edge structure of graphite formed by a cathodic electrochemical reaction in acid solutions associated with water electrolysis. Pole figures of the X-ray diffraction show that the edge orientation is parallel to the zigzag edge direction on a macroscopic scale. The polarization dependence of the G-band and D-band of the Raman spectra is consistent with the zigzag edge formation on a microscopic scale. It was found that the etched carbon atoms are converted to CH4 in the evolving gas and molecular species in the solution. (C) 2013 Elsevier Ltd. All rights reserved.
  • T. Yanase, A. Kawahito, Y. Hashimoto, T. Endo, Y. Wang, T. Nagahama, T. Shimada
    RSC ADVANCES 4 52 27620 - 27624 2014年 [査読有り][通常論文]
     
    We re-examined the growth of single crystalline Fe whiskers by chemical vapour deposition with regard to modern nanowire (NW) growth techniques. Single Fe NWs (100-300 nm diameter) with a high aspect ratio and unique [021] orientation were grown by optimizing the flow of raw materials (FeCl2 and H-2) and using a Au nanoparticle catalyst and single crystalline substrate. The growth mechanism was investigated by transmission electron microscopy and electron diffraction analysis of the NW-catalyst interface.
  • Wei Xie, Naoki Muraya, Takashi Yanase, Taro Nagahama, Toshihiro Shimada
    JAPANESE JOURNAL OF APPLIED PHYSICS 53 1 2014年01月 [査読有り][通常論文]
     
    We demonstrate the synthesis of diamond-like carbon (DLC) doped with highly pi-conjugated molecules by codepositing organic semiconductor molecules during the plasma-assisted chemical vapor deposition of DLC. From the results of optical absorption spectroscopy, it was revealed that organic semiconductor molecules reacted with plasma but were not completely decomposed. The results of Raman spectroscopy strongly suggested that a certain chemical moiety remained in the film. (C) 2014 The Japan Society of Applied Physics
  • Takashi Yanase, Taro Nagahama, Toshihiro Shimada
    Applied Physics Letters 103 13 2013年09月23日 [査読有り][通常論文]
     
    In order to combine the photovoltaic and piezoelectric activities in a single film, we fabricated tricolor superlattices consisting of an organic p-n junction and a piezoelectric polymer with sub-nm thickness control. The photovoltaic response of each repeating unit was saturated at 10 nm each as a function of the layer thicknesses. The capacitance across the 50 periods of superlattice changed by 0.009 pF upon photo-irradiation, which corresponds to a 2 nm thickness change of a 1-μm thick film. The characteristic time dependent behaviors of the capacitance of the superlattice consist of photocarrier generation, electrostriction, and poling of the piezoelectric polymer. © 2013 AIP Publishing LLC.
  • Manabu Ohtomo, Yuya Tsuchida, Naoki Muraya, Takashi Yanase, Seiji Sakai, Tetsu Yonezawa, Taro Nagahama, Tetsuya Hasegawa, Toshihiro Shimada
    JOURNAL OF PHYSICAL CHEMISTRY C 117 22 11555 - 11561 2013年06月 [査読有り][通常論文]
     
    We report the in-plane orientation control of a high-mobility organic semiconductor: 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT). As previously reported for the monolayer pentacene, it was revealed that bunched steps on the vicinal Si(111) with a bismuth termination break the surface 3-fold symmetry and reduce the multiple orientation of the DPh-BTBT grains into a quasi-single orientation. Interestingly, the critical step height necessary for the orientation control was higher than that of pentacene. We examined several mechanisms of orientation control and concluded that the facet nanostructure fabricated by step bunching works as an anisotropic template for the nucleation. We also show the wettability optimization of the bismuth-terminated silicon surface and show that the growth mode of DPh-BTBT is dependent on the surface nanostructure of Bi-Si.
  • Toshihiro Shimada, Takashi Yanase, Taro Nagahama, Toshiyuki Kanno
    JAPANESE JOURNAL OF APPLIED PHYSICS 52 5 2013年05月 [査読有り][通常論文]
     
    Materials and structures for water vapor barrier sealing are now actively studied, as the commercialization of organic electronic devices has become a reality. In this paper, we focus on the edge sealing barriers, in which diffusion plays an essential role. In the past, the diffusion-limited gas barrier properties were analyzed in the steady-state approximation, which is never reached within the device lifetime in the application for organic electronics. We analyze them using a simple analytical model. The diffusion before reaching the steady state is a strongly non-linear process, as is well known, and the length scale of approximately 1-10mm is very important when a practical polymer resin is used for the edge seal. (C) 2013 The Japan Society of Applied Physics
  • Toshihiro Shimada, Naoki Muraya, Jun Tomita, Takashi Yanase, Taro Nagahama
    Journal of Physics: Conference Series 441 1 12041  2013年 [査読無し][通常論文]
     
    The plasma treatment of organic semiconductor molecules includes issues related with nanoscale plasma interfaces. The process was examined by optical emission spectroscopy. It was found that molecules are sublimed by the heat and decomposed by the interaction with the plasma. The product carbon solids were examined by Raman spectroscopy. The raman spectra are strongly dependent on the molecular structures. The carbon solids which does not show graphitic Raman spectra have nanostructures originating from initial molecules.
  • Sawako Miyamoto, Tetsuya Hasegawa, Hiroyuki Takahashi, Tetsu Yonezawa, Hajime Kiyono, Takashi Yanase, Taro Nagahama, Toshihiro Shimada
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 85 12 1287 - 1292 2012年12月 [査読有り][通常論文]
     
    We report a method to produce vapor species from ZnO powder for the fabrication of ZnO nanorods. Mixing ethanol vapor with the inert carrier gas substantially lowered the gasification temperature of ZnO. c Axis-aligned ZnO nanorods were obtained by dispersing gold nanoparticles on Si substrates. The growth was identified as a vapor-solid (VS) mechanism. Choice of the substrate materials and their surface morphology were both critically important to control the shapes and orientation of grown ZnO.
  • Takashi Yanase, Tetsuya Hasegawa, Taro Nagahama, Toshihiro Shimada
    JAPANESE JOURNAL OF APPLIED PHYSICS 51 11 111803.1-111803.4  2012年11月 [査読無し][通常論文]
     
    The transient characteristics of electric double layer (EDL) gated field-effect transistors with Si as an active semiconductor were studied using various electrolyte solutions of LiBF4 by applying a step-function voltage to determine the optimum electrolyte for semiconductor circuits using EDLs. The t(R), determined by EDL dynamics in the present experiment, was minimum as a function of the kind of solvent used owing to the competing effects of the EDL thickness and viscosity. The responses of the electrolyte solutions with various solvents at the same concentration were classified into three categories on the basis of t(R): slow response of a complex-forming solvent, intermediate response of protic solvents, and fast response of nonprotic solvents. The best response time was 55 mu s when a 1.0M acetonitrile solution was used as the liquid-gate insulator. (C) 2012 The Japan Society of Applied Physics
  • Takashi Yanase, Tetsuya Hasegawa, Taro Nagahama, Toshihiro Shimada
    JAPANESE JOURNAL OF APPLIED PHYSICS 51 4 2012年04月 [査読有り][通常論文]
     
    We demonstrate that polyurea films can be prepared by alternating deposition with automated deposition control involving quartz crystal microbalance monitoring and optical source heating. The thickness of the films was linearly controlled by changing the repetition time of deposition, and the stoichiometry obtained was much higher than +/- 5%. The surface roughness of a 600-nm-thick film was 0.5 nm, which ensures the nm thickness control of the deposited polymers. The piezoelectricity of the films was confirmed by directly measuring the current transient induced by mechanical stress and by measuring the capacitance change induced by electric field. (C) 2012 The Japan Society of Applied Physics
  • Shosei Kubota, Tetsu Yonezawa, Taro Nagahama, Toshihiro Shimada
    CHEMISTRY LETTERS 41 2 187 - 188 2012年02月 [査読有り][通常論文]
     
    We found that graphite surfaces can be etched by electrochemical reduction in sulfuric acid. Terraces with straight edges crossing with 60 degrees-multiple angles were found on surfaces by AFM observation. It suggests that crystallographic edges can be formed at room temperature in a controlled manner. Layer-by-layer etching of graphene sheets was possible by tuning the etching condition.
  • Taro Nagahama, Hitoshi Kubota, Shinji Yuasa
    THIN SOLID FILMS 519 23 8239 - 8242 2011年09月 [査読有り][通常論文]
     
    We investigated the spin filter effect in tunnel junctions with a Ni1 + xFe2 - xO4 ferrimagnetic tunnel barrier. For higher T-c above room temperature, Ni1 + xFe2 - xO4 film should be thicker than 4.0 nm and grown above 400 degrees C. The spin filter junctions (SFJs) of Fe3O4(001)/Ni1 + xFe2 - xO4(001)/Al2O3/Fe/Au grown on MgO(001) substrates exhibited an inverse magnetoresistance effect at room temperature, which is consistent with the band calculation of NiFe2O4. (C) 2011 Elsevier B.V. All rights reserved.
  • Rie Matsumoto, Akio Fukushima, Taro Nagahama, Eiiti Tamura, Yoshishige Suzuki, Koji Ando, Shinji Yuasa
    JAPANESE JOURNAL OF APPLIED PHYSICS 50 6 2011年06月 [査読有り][通常論文]
     
    We studied spin-dependent transport in fully epitaxial Fe/MgO/Cr/Fe(001) magnetic tunnel junctions (MTJs) with a Cr(001) layer grown above or below the MgO barrier layer to clarify the scattering process of tunneling electrons at the barrier/electrode interfaces. From the Cr-thickness dependence of tunneling resistance, it was clearly shown that the Cr layer having no Delta(1) Bloch states at Fermi energy acts as a tunnel barrier for coherent-tunneling current (I(c)oh) carried by electrons with Delta(1) symmetry and acts as a metallic layer for incoherent-tunneling current (I-inc) mediated by nonspecular scattering. We evaluated Icoh and Iinc in Fe/MgO/Fe MTJs from the Cr-thickness dependence. The I-coh/I-inc ratio for the upper MgO/Fe interface was estimated to be 16, which corresponded to tunneling spin polarization above 0.9. The lower Fe/MgO interface with higher-density dislocations had a smaller I-coh/I-inc ratio and spin polarization. Reducing structural defects at the lower interface is the key to achieving higher MR ratios. (C) 2011 The Japan Society of Applied Physics
  • K. Yakushiji, T. Saruya, H. Kubota, A. Fukushima, T. Nagahama, S. Yuasa, K. Ando
    APPLIED PHYSICS LETTERS 97 23 2010年12月 [査読有り][通常論文]
     
    Ultrathin [Co/Pt](n) and [Co/Pd](n) superlattice films consisting of 0.14-0.20-nm-thick Co and Pt(Pd) layers were deposited by sputtering. A large perpendicular magnetic anisotropy [(3-9) x 10(6) ergs/cm(3)] and an ideal square out-of-plane hysteresis loop were attained even for ultrathin superlattice films with a total thickness of 1.2-2.4 nm. The films were stable against annealing up to 370 degrees C. MgO-based perpendicular magnetic tunnel junctions with this superlattice layer as the free layer showed a relatively high magnetoresistance ratio (62%) and an ultralow resistance-area product (3.9 Omega mu m(2)) at room temperature. The use of these films will enable the development of gigabit-scale nonvolatile memory. (C) 2010 American Institute of Physics. [doi:10.1063/1.3524230]
  • W. Y. Zhang, H. Shima, F. Takano, M. Takenaka, M. Yamazaki, K. Masuda, H. Akinaga, T. Nagahama, S. Nimori
    Journal of Magnetism and Magnetic Materials 322 8 900 - 908 2010年04月 [査読有り][通常論文]
     
    In this study, the deposition pressure dependence of the compositional ratio, magnetic domain structure, and perpendicular magnetic anisotropy (PMA) of B-containing PrFe- and PrCo-based films, which are rare-earth-transition-metal (RE-TM) films, was investigated. PrFe- and PrCo-based films were fabricated by magnetron sputtering. The film compositions were controlled in a wide range by varying the deposition pressure. On the basis of experimental results, the residual stress of the films was considered to be the possible origin of their PMA. The films showed strong magneto-optical effects over the entire wavelength range of 300-750 nm. Because of the excellent magnetic and magneto-optical (MO) properties of the films, they have high potential for MO applications at wavelengths of red and blue lasers. © 2009 Elsevier B.V. All rights reserved.
  • Taro Nagahama, Hidekazu Saito, Shinji Yuasa
    APPLIED PHYSICS LETTERS 96 11 112509  2010年03月 [査読有り][通常論文]
     
    We fabricated fully epitaxial magnetic tunnel transistors (MTTs) consisting of a Fe(001)/MgO(001) emitter and a Fe(001) based on a n-GaAs(001) collector. We observed much higher transfer ratio (alpha) and magnetocurrent (MC) ratio than those of the previously reported MTTs that had an amorphous AlO barrier. These improvements are due to the coherent tunneling and single-crystalline base layer. In addition, a pronounced peak structure appeared in the emitter bias dependence of alpha, which might be attributed to the interface resonance states at the Fe(001)/GaAs(001) interface.
  • Kay Yakushiji, Kenji Noma, Takeshi Saruya, Hitoshi Kubota, Akio Fukushima, Taro Nagahama, Shinji Yuasa, Koji Ando
    APPLIED PHYSICS EXPRESS 3 5 2010年 [査読有り][通常論文]
     
    We fabricated perpendicularly magnetized MgO-based magnetic tunnel junctions (p-MgO-MTJs) with a [Co/Pt](n)/CoFeB/CoFe bottom electrode layer (free layer) and a CoFe/CoFeB/TbFeCo top electrode layer (reference layer). The insertion of thin CoFeB/CoFe layers at the barrier/electrode interfaces and post-annealing at a relatively low temperature of 225 degrees C simultaneously yielded high magnetoresistance (MR) ratios of up to 85% at room temperature and a low resistance-area (RA) product of 4.4 Omega mu m(2). Such a high MR ratio in low-RA p-MgO-MTJs is the key to developing ultrahigh-density spin-transfer-torque magnetoresistive random access memories (MRAMs). (C) 2010 The Japan Society of Applied Physics
  • Rie Matsumoto, Akio Fukushima, Kay Yakushiji, Satoshi Yakata, Taro Nagahama, Hitoshi Kubota, Toshikazu Katayama, Yoshishige Suzuki, Koji Ando, Shinji Yuasa, Benoit Georges, Vincent Cros, Julie Grollier, Albert Fert
    Physical Review B - Condensed Matter and Materials Physics 80 17 174405  2009年11月12日 [査読有り][通常論文]
     
    We experimentally investigated current-driven oscillation in fully epitaxial Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions (MTJs) to pave the way for a better understanding of why the linewidth (a few hundred MHz) of microwave oscillation in spin-torque nano-oscillators (STNOs) based on textured MTJs is much larger than that (smaller than 10 MHz) in STNOs based on current-perpendicular-to-plane giant-magnetoresistance junctions. The epitaxial Fe/MgO/Fe STNO is a model system for studying the physics of spin-transfer torque because it has a well-defined single-crystal barrier and electrode layers with atomically flat interfaces. In the Fe/MgO/Fe STNOs, clear spin-torque-induced switching and spin-torque-induced precession were observed in epitaxial MTJs. When the initial magnetic alignment was antiparallel and the bias current exceeded the threshold current, a state in which the spin-torque compensates for the damping, the STNOs showed a rapid increase in the peak intensity, a redshift of the peak frequency, and a minimum linewidth, all clear evidence of spin-torque-induced precession above the threshold current. The minimum linewidth of the STNOs was 200 MHz, which is comparable to that of textured CoFeB/MgO/CoFeB MTJs. This indicates that the origin of the large linewidth cannot be attributed to structural inhomogeneity in textured MTJs. When the initial magnetic alignment was parallel, the microwave spectrum showed a single peak, which has rarely been observed in textured MTJs without application of a perpendicular magnetic field. The mechanism of the single-peak oscillation can be explained by taking account of the induced perpendicular magnetic anisotropy in the 3-nm-thick Fe(001) free layer grown on the MgO(001) barrier layer. © 2009 The American Physical Society.
  • Rie Matsumoto, Akio Fukushima, Kay Yakushiji, Satoshi Yakata, Taro Nagahama, Hitoshi Kubota, Toshikazu Katayama, Yoshishige Suzuki, Koji Ando, Shinji Yuasa, Benoit Georges, Vincent Cros, Julie Grollier, Albert Fert
    PHYSICAL REVIEW B 80 17 2009年11月 [査読有り][通常論文]
     
    We experimentally investigated current-driven oscillation in fully epitaxial Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions (MTJs) to pave the way for a better understanding of why the linewidth (a few hundred MHz) of microwave oscillation in spin-torque nano-oscillators (STNOs) based on textured MTJs is much larger than that (smaller than 10 MHz) in STNOs based on current-perpendicular-to-plane giant-magnetoresistance junctions. The epitaxial Fe/MgO/Fe STNO is a model system for studying the physics of spin-transfer torque because it has a well-defined single-crystal barrier and electrode layers with atomically flat interfaces. In the Fe/MgO/Fe STNOs, clear spin-torque-induced switching and spin-torque-induced precession were observed in epitaxial MTJs. When the initial magnetic alignment was antiparallel and the bias current exceeded the threshold current, a state in which the spin-torque compensates for the damping, the STNOs showed a rapid increase in the peak intensity, a redshift of the peak frequency, and a minimum linewidth, all clear evidence of spin-torque-induced precession above the threshold current. The minimum linewidth of the STNOs was 200 MHz, which is comparable to that of textured CoFeB/MgO/CoFeB MTJs. This indicates that the origin of the large linewidth cannot be attributed to structural inhomogeneity in textured MTJs. When the initial magnetic alignment was parallel, the microwave spectrum showed a single peak, which has rarely been observed in textured MTJs without application of a perpendicular magnetic field. The mechanism of the single-peak oscillation can be explained by taking account of the induced perpendicular magnetic anisotropy in the 3-nm-thick Fe(001) free layer grown on the MgO(001) barrier layer.
  • Rie Matsumoto, Akio Fukushima, Kay Yakushiji, Shingo Nishioka, Taro Nagahama, Toshikazu Katayama, Yoshishige Suzuki, Koji Ando, Shinji Yuasa
    Physical Review B - Condensed Matter and Materials Physics 79 17 174436  2009年05月01日 [査読有り][通常論文]
     
    We fabricated fully epitaxial Fe/Cr/MgO/Fe(001) magnetic tunnel junctions (MTJs) with an atomically flat ultrathin Cr(001) layer grown below the MgO barrier layer and studied the spin-dependent transport to clarify scattering process of tunneling electrons. Because Cr does not have Bloch states with Δ1 symmetry at the Fermi energy (EF), Δ1 evanescent states in MgO, which dominantly mediate the tunneling current, cannot couple with Cr Bloch states without a scattering process. The Fe/Cr/MgO/Fe(001) MTJs are therefore a model system for studying nonspecular scattering processes where the orbital symmetry of tunneling states is not conserved. The resistance-area (RA) product of the MTJs was found to not increase exponentially as a function of the Cr thickness (tCr), indicating that the Cr layer does not act as a perfect tunnel barrier despite of the absence of Δ1 states at EF. Moreover, the magnetoresistance ratio of the MTJs was seen to oscillate as a function of tCr with a period of 2 monolayers, reflecting the layered antiferromagnetic structure of Cr(001). Surprisingly, the MR ratio showed local maxima at the odd numbers of Cr monolayers and local minima at the even numbers of Cr monolayers, indicating that the tunneling current is oppositely spin polarized with respect to the interface magnetization. These results suggest that nonspecular scatterings mediate the coupling between evanescent states in MgO and certain non- Δ1 Bloch states in Cr that have negative spin polarization, thereby inducing nonspecular tunneling current even at a low temperature and a small bias voltage. We also investigated, as a reference sample, Fe/MgO/Cr/Fe MTJs with a less-oxidized Cr/MgO interface by growing the Cr(001) layer on the MgO barrier layer and found that their RA product increased much more rapidly with increasing tCr. This indicates that partial oxidation of interface Cr atoms in the Fe/Cr/MgO/Fe MTJs is one of the major origins of nonspecular scatterings. Both an increase in temperature and the application of bias voltage were found to greatly enhance the electron scattering that gives rise to tunneling conductance. While the temperature dependence of the antiparallel conductance due to magnon scatterings follows the Bloch T3/2 law, the conductance due to nonspecular scatterings deviates from the Bloch T3/2 law. The present experimental results give us important clues to a clearer understanding of the tunneling process in MgO-based MTJs. © 2009 The American Physical Society.
  • Rie Matsumoto, Akio Fukushima, Kay Yakushiji, Shingo Nishioka, Taro Nagahama, Toshikazu Katayama, Yoshishige Suzuki, Koji Ando, Shinji Yuasa
    PHYSICAL REVIEW B 79 17 2009年05月 [査読有り][通常論文]
     
    We fabricated fully epitaxial Fe/Cr/MgO/Fe(001) magnetic tunnel junctions (MTJs) with an atomically flat ultrathin Cr(001) layer grown below the MgO barrier layer and studied the spin-dependent transport to clarify scattering process of tunneling electrons. Because Cr does not have Bloch states with Delta(1) symmetry at the Fermi energy (E-F), Delta(1) evanescent states in MgO, which dominantly mediate the tunneling current, cannot couple with Cr Bloch states without a scattering process. The Fe/Cr/MgO/Fe(001) MTJs are therefore a model system for studying nonspecular scattering processes where the orbital symmetry of tunneling states is not conserved. The resistance-area (RA) product of the MTJs was found to not increase exponentially as a function of the Cr thickness (t(Cr)), indicating that the Cr layer does not act as a perfect tunnel barrier despite of the absence of Delta(1) states at E-F. Moreover, the magnetoresistance ratio of the MTJs was seen to oscillate as a function of t(Cr) with a period of 2 monolayers, reflecting the layered antiferromagnetic structure of Cr(001). Surprisingly, the MR ratio showed local maxima at the odd numbers of Cr monolayers and local minima at the even numbers of Cr monolayers, indicating that the tunneling current is oppositely spin polarized with respect to the interface magnetization. These results suggest that nonspecular scatterings mediate the coupling between evanescent states in MgO and certain non-Delta(1) Bloch states in Cr that have negative spin polarization, thereby inducing nonspecular tunneling current even at a low temperature and a small bias voltage. We also investigated, as a reference sample, Fe/MgO/Cr/Fe MTJs with a less-oxidized Cr/MgO interface by growing the Cr(001) layer on the MgO barrier layer and found that their RA product increased much more rapidly with increasing t(Cr). This indicates that partial oxidation of interface Cr atoms in the Fe/Cr/MgO/Fe MTJs is one of the major origins of nonspecular scatterings. Both an increase in temperature and the application of bias voltage were found to greatly enhance the electron scattering that gives rise to tunneling conductance. While the temperature dependence of the antiparallel conductance due to magnon scatterings follows the Bloch T-3/2 law, the conductance due to nonspecular scatterings deviates from the Bloch T-3/2 law. The present experimental results give us important clues to a clearer understanding of the tunneling process in MgO-based MTJs.
  • Do Bang, T. Nozaki, D. D. Djayaprawira, M. Shiraishi, Y. Suzuki, A. Fukushima, H. Kubota, T. Nagahama, S. Yuasa, H. Maehara, K. Tsunekawa, Y. Nagamine, N. Watanabe, H. Itoh
    JOURNAL OF APPLIED PHYSICS 105 7 2009年04月 [査読有り][通常論文]
     
    We investigated bias-voltage and temperature dependence of conductivity arising from the magnon contribution in MgO-based magnetic tunneling junctions (MTJs) with different ferromagnetic electrodes. Second derivative conductance curves showed broad peak structure, which extends from 5 to 200 mV, accompanied by additional peaks at around 23, 54, and 85 mV. The peak intensities were larger for antiparallel configuration than for parallel configuration except that at 85 mV. This difference in the peak intensity was observed to be larger for the MTJs having higher tunneling magnetoresistance ratio, indicating a magnetic origin of these peaks. Abrupt increase in the second derivative conductance at very low biasing voltage in the antiparallel configuration suggests the important role of the surface magnon excitation. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3063674]
  • Shinji Yuasa, Hitoshi Kubota, Akio Fukushima, Kay Yakushiji, Taro Nagahama, Yoshishige Suzuki, Koji Ando
    Synthesiology 2 3 194 - 205 2009年 [査読有り][通常論文]
     
    We have been promoting Full Research to develop ultimate green IT devices based on non-volatile electronics. A core technology of non-volatile electronics is a non-volatile memory possessing features such as large capacity, high-speed operation, and high endurance. To develop such an ultimate non-volatile memory, we developed a novel high-performance magnetic tunnel junction device based on magnesium oxide (MgO) and its mass-manufacturing technology. These technologies have already been commercialized for the production of the magnetic heads of ultra-high density hard disk drives (HDD). Now we are also performing intensive R & D activities for developing the ultimate non-volatile memory called Spin-RAM.
  • I. C. Lekshmi, G. Berera, Y. Afsar, G. X. Miao, T. Nagahama, T. Santos, J. S. Moodera
    JOURNAL OF APPLIED PHYSICS 103 9 2008年05月 [査読有り][通常論文]
     
    In this study, we examine the possibility of electrode-barrier interactions in modifying the electrical characteristics and current switching behavior of a trilayer junction with silver sulfide as the barrier layer. A series of Al-Ag(2)S-Ag crossbar junction is fabricated by thermal evaporation technique varying the thickness (30-110 A) of the sulfide layer. Current-voltage characteristics of the junctions are studied as a function of barrier layer thickness, which can suggest any role that electrode-barrier interaction may have in tuning their electrical behavior. To fully understand the performance of the barrier layer, structure and physical properties of Ag(2)S films are independently investigated. The microstructure of Ag(2)S films strongly depends on the deposition conditions that, in turn, affect their electrical and optical characteristics. The polarization of the lattice prevalent in Ag(2)S is shown to affect the charge carrier conduction in their films and dominates their electrical behavior and that of the junctions. (c) 2008 American Institute of Physics.
  • Kay Yakushiji, Shinji Yuasa, Taro Nagahama, Akio Fukushima, Hitoshi Kubota, Toshikazu Katayama, Koji Ando
    APPLIED PHYSICS EXPRESS 1 4 2008年04月 [査読有り][通常論文]
     
    We fabricated a current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) nanopillar with a 1-nm-thick FePt free layer having perpendicular anisotropy using the alternate monatomic layer deposition method. Nanopillars consisting of [Fe (1 monolayer (ML))/Pt (1 ML)]n (n: the number of the alternation period) ferromagnetic layers and an Au spacer layer showed spin-transfer induced switching at room temperature. An average critical switching current density (J(c0)) of 1.1 x 1(0)7 A/cm(2) with a large thermal stability parameter (Delta) of 60 was obtained in a nanopillar with a free-layer thickness of 1.02 nm (n = 3) and a pillar diameter of 110 nm. The ultrathin free-layer with high perpendicular anisotropy is effective to obtain both large Delta and small J(c). (C) 2008 The Japan Society of Applied Physics.
  • Hitoshi Kubota, Akio Fukushima, Kay Yakushiji, Taro Nagahama, Shinji Yuasa, Koji Ando, Hiroki Maehara, Yoshinori Nagamine, Koji Tsunekawa, David D. Djayaprawira, Naoki Watanabe, Yoshishige Suzuki
    NATURE PHYSICS 4 1 37 - 41 2008年01月 [査読有り][通常論文]
     
    When an electric current passes from one ferromagnetic layer via a non-magnetic layer into another ferromagnetic layer, the spin polarization and subsequent rotation of this current can induce a transfer of angular momentum that exerts a torque on the second ferromagnetic layer(1-4). This provides a potentially useful method to reverse(3,5-7) and oscillate(8) the magnetic momenta in nanoscale magnetic structures. Owing to the large current densities required to observe spin-torque-induced magnetization switching and microwave emission (similar to 10(7) A cm(-2)), accurately measuring the strength, or even the direction, of the associated spin torque has proved difficult. Yet, such measurements are crucial to refining our understanding of the mechanisms responsible and the theories that describe them(9,10). To address this, we present quantitative experimental measurements of the spin torque in MgO-based magnetic tunnel junctions(11-14) for a wide range of bias currents covering the switching currents. The results verify the occurrence of two different spin-torque regimes with different bias dependences that agree well with theoretical predictions(10).
  • Rie Matsumoto, Shingo Nishiokaa, Masaki Mizuguchi, Masashi Shiraishi, Hiroki Maehara, Koji Tsunekawa, David D. Djayaprawira, Naoki Watanabe, Yuichi Otani, Taro Nagahama, Akio Fukushima, Hitoshi Kubota, Shinji Yuasa, Yoshishige Suzuki
    SOLID STATE COMMUNICATIONS 143 11-12 574 - 578 2007年09月 [査読有り][通常論文]
     
    We investigated the annealing temperature dependence of differential tunneling conductance spectra (dI/dV as a function of V) in CoFeB/teXtUred MgO(001)/CoFeB magnetic tunnel junctions (MTJs) that exhibit the giant tunneling magnetoresistance (TMR) effect at room temperature. The spectra were strongly affected by annealing the MTJs. A reduction in dI/dV at around 300 mV was observed only in annealed MTJs in which the CoFeB electrodes were crystallized in a bcc(001) structure. Because the reduction in conductance was observed in both MTJs that have a 1.8-nm-thick MgO barrier and MTJs that have a 3.2-nm-thick MgO barrier, we concluded that Delta(5) and Delta(2') evanescent states, which rapidly decay in the MgO tunneling barrier, are not the cause of the reduction in conductance. We believe the cause of the reduction is either the electronic structure of the interfaces between MgO(001)/bcc CoFeB(001) after annealing or a particular feature of Delta(1) states in MgO(001) or bee CoFeB(001). () 2007 Published by Elsevier Ltd
  • M. Mizuguchi, Y. Suzuki, T. Nagahama, S. Yuasa
    APPLIED PHYSICS LETTERS 91 1 2007年07月 [査読有り][通常論文]
     
    Topological surface analysis using in situ scanning tunneling microscopy was performed for highly oriented polycrystalline (textured) MgO(001) tunneling barrier layers grown on amorphous CoFeB electrode layers. The microscopy revealed a MgO surface structure in which nanosized grains were dispersed on clusters that originated from the CoFeB underlayer. In situ annealing reduced this surface roughness. Local tunneling spectroscopy measurements revealed the formation of a nearly perfect and uniform tunneling barrier in spite of grain boundaries in the textured MgO(001) layer, which is consistent with the fact that textured CoFeB/MgO/CoFeB and fully epitaxial MgO-based magnetic tunnel junctions exhibit comparable spin-dependent tunneling properties. (c) 2007 American Institute of Physics.
  • T. Nagahama, T. S. Santos, J. S. Moodera
    PHYSICAL REVIEW LETTERS 99 1 2007年07月 [査読有り][通常論文]
     
    In quasimagnetic tunnel junctions with a EuS spin-filter tunnel barrier between Al and Co electrodes, we observed large magnetoresistance (MR). The bias dependence shows an abrupt increase of MR ratio in high bias voltage, which is contrary to conventional magnetic tunnel junctions. This behavior can be understood as due to Fowler-Nordheim tunneling through the fully spin-polarized EuS conduction band. The I-V characteristics and bias dependence of MR calculated using tunneling theory show excellent agreement with experiment.
  • Rie Matsumoto, Akio Fukushima, Taro Nagahama, Yoshishige Suzuki, Koji Ando, Shinji Yuasa
    APPLIED PHYSICS LETTERS 90 25 252506  2007年06月 [査読有り][通常論文]
     
    The authors fabricated fully epitaxial Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions (MTJs) with various MgO thicknesses (t(MgO)) and investigated spin-dependent transport properties. Both the tunneling resistance in the parallel magnetic state (R-P) and that in the antiparallel magnetic state (R-AP) exhibited short-period oscillations as functions of t(MgO) with the same period of 3.2 angstrom and different phases. R-AP also showed a long-period oscillation with a period of 9.9 angstrom. As a result, t(MgO) dependence of magnetoresistance is expressed as a superposition of the short- and long-period oscillations. These results provide important clues for understanding the oscillatory t(MgO) dependence of the tunneling magnetoresistance effect. (c) 2007 American Institute of Physics.
  • Jagadeesh S. Moodera, Tiffany S. Santos, Taro Nagahama
    JOURNAL OF PHYSICS-CONDENSED MATTER 19 16 2007年04月 [査読有り][通常論文]
     
    The spin filtering phenomenon allows one to obtain highly spin-polarized charge carriers generated from nonmagnetic electrodes using magnetic tunnel barriers. The exponential dependence of tunnel current on the tunnel barrier height is operative here. The magnetic, semiconducting europium chalcogenide compounds have strikingly demonstrated this effect. The possibility of employing ferrites and other methods opens the potential for display of this phenomenon at room temperature, which can be expected to lead to huge progress in spin injection and detection in semiconductors. But first, extremely challenging material-related issues have to be addressed. This review covers the field.
  • S. Nishioka, Y. V. Hamada, R. Matsumoto, M. Mizuguchi, M. Shiraishi, A. Fukushima, H. Kubota, T. Nagahama, S. Yuasa, H. Maehara, Y. Nagamine, K. Tsunekawa, D. D. Djayaprawira, N. Watanabe, Y. Suzuki
    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 310 2 E649 - E651 2007年03月 [査読有り][通常論文]
     
    We measured differential conductance spectra of magnetic tunnel junctions (MTJs) with thin MgO barrier and low-resistance area product. The spectra of MTJs with MgO barrier thicker than 1.05 nm were essentially the same except for slight decrease of contributions from low-energy excitations, such as magnons. The spectra of MTJ with 1.01 nm MgO barrier were thoroughly different from the MTJs with thicker barrier. The result reveals that an MTJ with very thin MgO barrier thickness has different conduction characteristics from those with thicker MgO barriers. (C) 2006 Elsevier B.V. All rights reserved.
  • M. Mizuguchi, Y. Suzuki, T. Nagahama, S. Yuasa
    Journal of Nanoscience and Nanotechnology 7 1 255 - 258 2007年01月 [査読有り][通常論文]
     
    The surface morphology of epitaxial Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions, which show the giant tunneling magnetoresistance effect, was investigated by in situ scanning tunneling microscopy. It was observed that an epitaxial MgO barrier layer forms flat surface structures. The surface was flatter with distinct steps and terraces after annealing, which would lead to an increase of the tunneling magnetoresistance ratio. Examination of the local electronic structures of 1.05-nm-thick MgO barrier layers by scanning tunneling spectroscopy revealed no pinholes in the layers, so they would be perfect barriers in magnetic tunnel junctions. Copyright © 2007 American Scientific Publishers All rights reserved.
  • M. Mizuguchi, Y. Suzuki, T. Nagahama, S. Yuasa
    APPLIED PHYSICS LETTERS 88 25 251901-1-251901-3  2006年06月 [査読無し][通常論文]
     
    The microscopic structures of MgO(001) barrier layers in magnetic tunnel junctions showing giant tunneling magnetoresistance were characterized by in situ scanning tunneling microscopy. The MgO thin films formed exceedingly flat surfaces, and their terraces were made even flatter by annealing after deposition. This flattening of MgO surfaces apparently promotes coherent transport of electrons, which should enhance the tunneling magnetoresistance ratio. Local tunneling spectroscopy revealed that an annealed MgO layer has a critical thickness between 3 and 5 ML (monolayer), and a continuous film without pinholes can be formed over the thickness. (c) 2006 American Institute of Physics.
  • M Mizuguchi, Y Hamada, R Matsumoto, S Nishioka, H Maehara, K Tsunekawa, DD Djayaprawira, N Watanabe, T Nagahama, A Fukushima, H Kubota, S Yuasa, M Shiraishi, Y Suzuki
    JOURNAL OF APPLIED PHYSICS 99 8 08T309-1-08T309-3  2006年04月 [査読無し][通常論文]
     
    Tunneling spectroscopy measurements of magnetic tunneling junctions including two different barrier layers were performed. Intense dips at bias voltages of +/- 0.3 V were observed in second derivative conductance spectra only for a magnetic tunneling junction with a MgO barrier. It was concluded that the electronic structure of the MgO barrier has significant influence on the tunneling process of electrons through magnetic tunnel junctions. (C) 2006 American Institute of Physics.
  • M Mizuguchi, Y Suzuki, T Nagahama, S Yuasa
    JOURNAL OF APPLIED PHYSICS 99 8 2006年04月 [査読有り][通常論文]
     
    Scanning tunneling microscopy observations of a single-crystal Fe(001)/MgO(001)/Fe(001) magnetic tunnel junction which shows a large tunnel magnetoresistance effect were performed. Step-and-terrace structures of an epitaxial MgO barrier layer, which are very similar to surfaces of a bottom Fe underlayer, were observed. This indicates that the MgO terraces were grown flatly on each terrace of the Fe electrode. It is supposed that these atomically flat terraces enable a coherent tunneling of electrons through magnetic tunnel junctions. (C) 2006 American Institute of Physics.
  • Shinji Yuasa, Rie Matsumoto, Akio Fukushima, Hitoshi Kubota, Taro Nagahama, David D. Djayaprawira, Koji Tsunekawa, Hiroki Maehara, Yoshinori Nagamine, Motonobu Nagai, Shinji Yamagata, Yoshishige Suzuki, Masaki Mizuguchi, Alina M. Deac, Koji Ando
    IEEE NMDC 2006: IEEE NANOTECHNOLOGY MATERIALS AND DEVICES CONFERENCE 2006, PROCEEDINGS 186 - + 2006年 [査読有り][通常論文]
     
    First-principle theories predicted an extremely high magnetoresistance (MR) ratio over 1000% in epitaxial Fe(001)/MgO(001)/Fe(001) MTJs. We have fabricated fully epitaxial Fe-Co(001)/MgO(001)/Fe-Co(001) MTJs and textured CoFeB/MgO(001)/CoFeB MTJs and achieved giant MR ratios above 400% at room temperature. An ultra-low resistance-area (RA) product indispensable for magnetic sensor application has also been achieved in CoFeB/MgO(001)/CoFeB MTJs. The giant TMR effect in MgO-based MTJs is the key for next-generation spintronic devices.
  • Moodera Jagadeesh S, Nagahama Taro
    2006年 [査読有り][通常論文]
  • R Matsumoto, Y Hamada, M Mizuguchi, M Shiraishi, H Maehara, K Tsunekawa, DD Djayaprawira, N Watanabe, Y Kurosaki, T Nagahama, A Fukushima, H Kubota, S Yuasa, Y Suzuki
    SOLID STATE COMMUNICATIONS 136 11-12 611 - 615 2005年12月 [査読無し][通常論文]
     
    We measured differential tunneling conductance (dI/dV, d(2)I/dV(2)) spectra of spin-valve-type magnetic tunnel junctions (MTJs) with a MgO(001) tunnel barrier layer and amorphous CoFeB ferromagnetic electrodes that show 315% magnetoresistance (MR) ratio at 4.3 K. The dI/dV spectra showed clear reduction in the conductance at around +/- 400 mV for a parallel magnetic configuration. Such anomalous spectra have never been observed for MTJs with an amorphous Al-O barrier. The d(2)I/dV(2) spectra showed several distinct peaks between 5 and 100 mv. Magnon excitations are assigned to an origin of those peaks and thought to be a dominant process to reduce MR at finite bias voltage. (c) 2005 Elsevier Ltd. All rights reserved.
  • S Yuasa, T Katayama, T Nagahama, A Fukushima, H Kubota, Y Suzuki, K Ando
    APPLIED PHYSICS LETTERS 87 22 222508-222508-3  2005年11月 [査読無し][通常論文]
     
    Fully epitaxial bcc Fe1-xCox (001)/MgO(001)/Fe(001) magnetic tunnel junctions (x= 0, 0.5, 1) were fabricated with molecular-beam epitaxy and microfabrication techniques. While the bcc Fe(001) and Fe0.5Co0.5(001) electrodes had similar magnetoresistance (MR) ratios of about 180% at room temperature, the bcc Co(001) electrode exhibited a higher MR ratio up to 271% at room temperature (353% at 20 K). The fact that the MR ratio for a bcc Co electrode is much higher than that for a bcc Fe electrode is consistent with first-principle calculations, indicating the importance of electrode band structure in the k(parallel to) = 0 direction. (c) 2005 American Institute of Physics.
  • M Mizuguchi, Y Suzuki, T Nagahama, S Yuasa
    APPLIED PHYSICS LETTERS 87 17 2005年10月 [査読有り][通常論文]
     
    Observation using in situ scanning tunneling microscopy of the layers constituting a magnetic tunnel junction with a naturally oxidized aluminum barrier layer revealed an extremely flat aluminum-oxide surface. It was clarified from line-scan images that the aluminum-oxide barrier layer has atomic steps. This flatness, which is surprising given that the aluminum-oxide film is amorphous, reduced electron scattering within the barrier, leading to momentum-dependent tunneling, which should enable the fabrication of advanced devices, such as spin-polarized resonant tunneling transistors. (C) 2005 American Institute of Physics.
  • T Nagahama, S Yuasa, E Tamura, Y Suzuki
    PHYSICAL REVIEW LETTERS 95 8 086602  2005年08月 [査読有り][通常論文]
     
    The tunnel magnetoresistance effect (TMR), which is intrinsically determined by the interface monolayer of an electrode, was realized by using magnetic tunnel junctions (MTJs) with a single-crystal Cr(001) layer inserted between a tunnel barrier and an electrode. The MTJs showed an oscillation of the TMR ratio as a function of the thickness of the Cr(001) layer with a period of 2 monatomic layers, which corresponds to the layered antiferromagnetic structure of Cr(001). These oscillations originate from electron scattering at the interface, due to the mismatching of the symmetry of the wave functions and band structure in Cr(001).
  • 黒崎 義成, 長浜 太郎, 湯浅 新治, 鈴木 義茂, 片山 利一
    日本応用磁気学会誌 29 4 446 - 449 社団法人日本磁気学会 2005年04月01日 [査読無し][通常論文]
     
    We fabricated Fe(001)/Al-O/NiFe magnetic tunnel junctions (MTJs) with an ultrathin single-crystal Fe(001) electrode and studied the Fethickness (t_<Fe>) dependence of the TMR effect. The upper NiFe electrode (polycrystal) was pinned by a IrMn layer in order to make an antiparallel alignment between upper and lower electrode magnetizations. As a result, we succeeded in observing the TMR effect with an ultrathin Fe electrode down to t_<Fe>=0.5nm. It was found that a magnetoresistance (MR) ratio of 7% was achieved even at t_<Fe>=0.5nm, although the MR ratio decreased slightly with decreasing t...
  • S Yuasa, T Nagahama, A Fukushima, Y Suzuki, K Ando
    NATURE MATERIALS 3 12 868 - 871 2004年12月 [査読有り][通常論文]
  • T. Saito, T. Katayama, Y. Kurosaki, M. Endo, S. Saito, T. Kamino, K. Kobayashi, Y. Suzuki, T. Nagahama, S. Yuasa, T. Koide, T. Shidara, H. Manaka, H. Tokano
    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 272 E1489 - E1490 2004年05月 [査読有り][通常論文]
     
    We have studied the electronic state of a Co(0 0 1) monatomic layer facing an amorphous-Al2O3 tunneling barrier using X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements at the Co L-2,L-3 edges. XAS and XMCD measurements evidence that the 1-ML Co facing Al2O3 layer is not oxidized. (C) 2003 Elsevier B. V. All rights reserved.
  • S Yuasa, A Fukushima, T Nagahama, K Ando, Y Suzuki
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 43 4B L588 - L590 2004年04月 [査読有り][通常論文]
     
    We fabricated fully epitaxial Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions (MTJs) and observed a magnetoresistance (MR) ratio of 88% at T = 293 K (146% at T = 20 K), the highest value yet reported. The origin of the high MR ratio is not the diffusive tunneling' of Julliere's model but the coherent spin-polarized tunneling in epitaxial MTJs, in which only the electrons with totally symmetric wave functions with respect to the barrier-normal axis can tunnel. The bias-voltage dependence of the MR was very small, resulting in a high output voltage of 380 mV. This high voltage will help overcome problems in the development of high-density magnetoresistive random-access-memory (MRAM).
  • 金属スピントロニクスの展開
    鈴木義茂, 湯浅新治, 長浜太郎, 安藤功兒
    アネルバ技報 第10巻 5 - 12 2004年03月 [査読有り][通常論文]
  • 小林明子, 前原大樹, 長田智明, 原市聡, 長浜太郎, 福島章雄, 鈴木義茂
    日本応用磁気学会誌 28 2 176 - 179 2004年 [査読無し][通常論文]
  • 小林 明子, 前原 大樹, 長田 智明, 原市 聡, 長浜 太郎, 福島 章雄, 鈴木 義茂
    日本応用磁気学会誌 28 2 176 - 179 社団法人日本磁気学会 2004年 [査読無し][通常論文]
     
    Reactive Ion Etching (RIE) is the most promising technique to realize sub-micron patterns of tunneling magneto-resistance (TMR) junctions for Magnetic Random Access Memory (MRAM) devices. However, it is difficult to be applied into practical use due to re-deposition of the etched material and corrosion by etching gases. In order to eliminate these drawbacks, an etching process utilizing CO + NH_3 gas chemistry and Ta metal mask has been applied. Electron microscopy observations confirmed that there is no re-deposition or corrosion occurs. Using the same process we have succeeded in fabricat...
  • 長浜太郎, 湯浅新治, 鈴木義茂, 田村英一
    日本応用磁気学会誌 27 1 14 - 20 2003年01月01日 [査読無し][通常論文]
  • T Nagahama, S Yuasa, Y Suzuki
    INTERNATIONAL CONFERENCE ON MEMS, NANO AND SMART SYSTEMS, PROCEEDINGS 316 - 320 2003年 [査読有り][通常論文]
     
    Control of coherent electron transports in spintronic devices is an essential issue to realize highly functional spin-devices such as spin-dependent resonant-tunneling transistors. The spin dependent resonant tunneling via the quantum-well states in the electrode gives its many clues to understand and to control transport property in magnetic tunnel junctions (MTJs). To observe the quantum-well oscillations in TMR effect, we prepared the magnetic tunnel junctions with single-crystal ultrathin electrode or insertion layers. The MTJs with an ultrathin Fe (001) electrode shows the oscillation of TMR ratio as a Junction of the biasing voltage. In the case of an ultrathin Cu (001) layer inserted between a Co electrode and Al2O3 barrier layer, we found large oscillations of TMR with respect to the thickness of the Cu electrode. These results clearly show a feasibility, of the coherent spintronic devices using metallic systems.
  • S Yuasa, T Nagahama, T Kawakami, K Ando, Y Suzuki
    JOURNAL OF PHYSICS D-APPLIED PHYSICS 35 19 2427 - 2431 2002年10月 [査読無し][通常論文]
     
    Magnetic tunnel junctions (MTJs) with ultrathin single-crystal electrodes were prepared, and the influence of the quantum-well (QW) states formed in the electrodes on the tunnel magnetoresistance (TMR) effect was studied. MTJs with an ultrathin Fe(001) electrode showed oscillations in the conductivity and the TMR ratio in tunnel spectra. MTJs with an ultrathin non-magnetic Cu(001) layer inserted between an Al-O tunnel barrier and a ferromagnetic Co(001) electrode showed an oscillation in the TMR as a function of the Cu-layer thickness. The period of the TMR oscillation coincides with the oscillation period of interlayer exchange coupling in a Co(001)/Cu(001)/Co(001) film, indicating that the TMR oscillation originates from the spin-polarized QW states in Cu(001). The amplitude of the oscillation was so large that even the sign of the TMR ratio alternated.
  • LT Baczewski, K Mibu, T Nagahama, S Hamada, T Shinjo
    JOURNAL OF PHYSICS D-APPLIED PHYSICS 35 20 2479 - 2483 2002年10月 [査読有り][通常論文]
     
    Magnetic properties of the Co/Nd multilayers with a 2 Angstrom probe layer of Fe-57 evaporated in UHV conditions were studied in the temperature range 4.2 K < T < 300 K by means of VSM magnetometry and Mossbauer spectroscopy. The isomer shift, hyperfine field distribution and its direction were obtained from a Mossbauer data computer fitting. The easy magnetization direction was found close to the sample normal for all samples at low temperatures and there was a stronger tendency to the perpendicular direction for the sample when the probe layer was placed in a close vicinity of the Co/Nd interface. For this sample the out-of-plane easy direction was preserved at 300 K. For all other studied samples, in-plane anisotropy was observed at room temperature.
  • S Yuasa, T Nagahama, Y Suzuki
    SCIENCE 297 5579 234 - 237 2002年07月 [査読有り][通常論文]
     
    Insertion of a thin nonmagnetic copper Cu(001) layer between the tunnel barrier and the ferromagnetic electrode of a magnetic tunnel junction is shown to result in the oscillation of the tunnel magnetoresistance as a function of the Cu layer thickness. The effect is interpreted in terms of the formation of spin-polarized resonant tunneling. The amplitude of the oscillation is so large that even the sign of the tunnel magnetoresistance alternates. The oscillation period depends on the applied bias voltage, reflecting the energy band structure of Cu. The results are encouraging for the development of spin-dependent resonant tunneling devices.
  • T Nagahama, S Yuasa, Y Suzuki, E Tamura
    JOURNAL OF APPLIED PHYSICS 91 10 7035 - 7037 2002年05月 [査読無し][通常論文]
     
    The transport properties of magnetic tunnel junctions with single-crystalline ultrathin Fe(001) electrodes are studied. The tunnel spectra and the bias dependence of the differential magnetoresistance show quantum-well oscillations. This effect provides evidence of ballistic transport through the Al-O barrier and shows that magnetic tunnel junctions with quantum-well states can be a tool for studying spin-dependent transport mechanisms in magnetic tunnel junctions. (C) 2002 American Institute of Physics.
  • 長浜 太郎, 湯浅 新治, 鈴木 義茂, 田村 英一
    日本応用磁気学会誌 26 4 410 - 413 社団法人日本磁気学会 2002年04月01日 [査読無し][通常論文]
     
    We studied the tunnel spectra of MTJs with a single-crystal ultrathin Fe (100) electrode. As a result, an enhancement of the MR ratio compared to that for the MTJ with thick Fe (100) electrode was observed. In addition, the tunnel spectra of MTJs with ultrathin Fe electrodes were found to have oscillatory components in the positive bias region, and the differential magnetoresistance also showed oscillations. This is the first observation of the quantum-well effect in the bias dependence of TMR. This new effect provides us with the possibility of creations new voltage-controlled spin-functio...
  • T Nagahama, S Yuasa, Y Suzuki, E Tamura
    APPLIED PHYSICS LETTERS 79 26 4381 - 4383 2001年12月 [査読無し][通常論文]
     
    We studied the tunnel spectra of magnetic tunnel junctions with a single-crystal ultrathin Fe(100) electrode. The tunnel spectra show oscillations of the differential conductivity and the differential tunnel magnetoresistance. The positions of the maxima of the oscillations move systematically with the change in the Fe(100) electrode's thickness, indicating that the oscillations originate from the quantum-well states in the ultrathin Fe(100) electrode. This effect provides us with an opportunity to create voltage-controlled spin functional devices. (C) 2001 American Institute of Physics.
  • 強磁性トンネル接合〜スピントロニクスへの展望
    鈴木義茂, 湯浅新治, 長浜太郎, 田村英一
    応用電子物性分科会誌 Vol.7 No.5 251 - 256 2001年10月 [査読有り][通常論文]
  • T Shinjo, K Shigeto, T Nagahama, K Mibu, T Ono
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 69 91 - 98 2000年05月 [査読有り][通常論文]
     
    Several recent experimental studies on microstructured samples of ferromagnetic materials are described. Magnetization reversal phenomena were investigated on submicron wire samples consisting of two magnetic layers with different coercivities utilizing the giant magnetoresistance (GMR) effect. From resistivity measurements, the domain wall movements are sensitively monitored and the velocity of propagation is determined. Experiments to estimate the contribution of domain wall to the resistivity are also introduced.
  • T Nagahama, K Mibu, T Shinjo
    JOURNAL OF APPLIED PHYSICS 87 9 5648 - 5650 2000年05月 [査読有り][通常論文]
     
    The contribution of a magnetic domain wall to electric resistivity was measured using NiFe wires (width: 1 mu m) partially covered with hard magnetic pads (CoSm). When the wire is covered with N pinning pads, 2N domain walls can be produced in the wire by reversing the magnetization only at the uncovered parts. The resistance for the magnetically saturated state (no domain wall structure) and that for the magnetic structure with 2N domain walls were compared at zero applied field. It was found that the resistance is smaller when magnetic domain walls exist, and that the domain wall resistance is almost temperature independent. (C) 2000 American Institute of Physics. [S0021-8979(00)38608-X].
  • LT Baczewski, K Mibu, T Nagahama, T Shinjo
    ACTA PHYSICA POLONICA A 97 3 439 - 442 2000年03月 [査読有り][通常論文]
     
    Multilayers of Go/Nd with probe layer of 2 Angstrom of Fe-57 evaporated in UHV conditions were studied by means of vibrating sample magnetometry and Mossbauer spectroscopy at T = 4.2 K. Isomer shift, hyperfine field distribution and direction was obtained from the Mossbauer data computer fitting. Magnetic anisotropy was studied and discussed. The easy magnetization direction was dose to the perpendicular direction for both samples but the tendency to perpendicular anisotropy was stronger when the Fe probe layer was placed in the vicinity of Co/Nd interface.
  • K Mibu, T Nagahama, T Shinjo, T Ono
    PHYSICAL REVIEW B 58 10 6442 - 6446 1998年09月 [査読有り][通常論文]
     
    The magnetoresistance originating from magnetic structures with gradually rotating magnetic moments, like a Bloch wall, was investigated using soft-magnetic (NiFe)/hard-magnetic (CoSm) bilayers, whose magnetic structures were well characterized. The magnetoresistance was measured with an electric current in the film plane; the magnetoresistance in this geometry corresponds to that due to a current parallel to a Bloch wall. The main feature of the magnetoresistance curves was ruled by the anisotropic magnetoresistance. It was found that a giant magnetoresistance-type effect coexisted; the effect was very small in comparison with the anisotropic magnetoresistance effect.
  • T Nagahama, K Mibu, T Shinjo
    JOURNAL OF PHYSICS D-APPLIED PHYSICS 31 1 43 - 49 1998年01月 [査読有り][通常論文]
     
    A perfectly reversible magnetization process was observed in NiFe/CoSm bilayers. During this process, the magnetic moments in the soft magnetic layer (NiFe) are pinned at the interface with the hard magnetic layer (CoSm), so that the direction of the magnetic moment distributes successively like a Bloch wall. The characteristic reversible magnetization process is explained by an atomic layer model. The magnetoresistance also exhibits a reversible change reflecting the magnetization process. The basic feature of the reversible magnetoresistance curve is understood to be anisotropic magnetoresistance.
  • K Mibu, T Nagahama, T Ono, T Shinjo
    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 177 1267 - 1268 1998年01月 [査読有り][通常論文]
     
    The magnetoresistance (MR) originating from a magnetic structure with continuous rotation of magnetic moments was studied using soft-magnetic/hard-magnetic bilayers. The feature of the MR curves was explained with anisotropic magnetoresistance (AMR) applying to twisted magnetic structures. The giant magnetoresistance (GMR)-type effect was found to be very small compared with the AMR effect. (C) 1998 Elsevier Science B.V. All rights reserved.
  • K Mibu, T Nagahama, T Shinjo
    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 163 1-2 75 - 79 1996年10月 [査読有り][通常論文]
     
    The magnetization process and magnetoresistance were studied for soft-magnetic (NiFe)/hard-magnetic (CoSm) bilayers. In the course of the magnetization reversal, the magnetic moments in the soft magnetic layer rotate reversibly, while they are pinned by the hard magnetic layer at the interface; consequently, the direction of the magnetic moment distributes successively as in a Bloch wall. The magnetoresistance also shows a reversible change, reflecting the magnetization process. The change is explained by means of the anisotropic magnetoresistance due to angle distributed magnetic moments.
  • K Mibu, T Nagahama, T Shinjo
    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 156 1-3 299 - 300 1996年04月 [査読有り][通常論文]
     
    Cobalt-samarium alloy films were prepared by alternate deposition of Co and Sm in the presence of an external magnetic field to obtain thin magnetic layers with large in-plane coercive fields. These anisotropic CoSm layers were used as hard magnetic layers of noncoupled-type magnetoresistance multilayers or as pinning layers of spin-valve structures. Large magnetoresistance changes up to 4.6% were attained in the CoSm/Co/Cu/NiFe system.

MISC

書籍等出版物

  • 執筆者:67名, 編集:技術情報協会 (担当:共著範囲:第8章3節 スピネルフェライト層を用いたトンネル磁気抵抗素子の開発)
    技術情報協会 2018年03月 (ISBN: 4861047056) 572p
  • 強磁性トンネル接合がIT技術の革命を引き起こす スピン偏極トンネリングのフロンティア
    J.S.moodera, T.S.Santos, 長浜太郎 (担当:単訳)
    パリティ 2011年04月
  • スピントロニクスの基礎と材料・応用技術の最前線
    長浜太郎, 柳原英人 (担当:共著範囲:第18章 磁性絶縁体とスピンフィルター接合)
    シーエムエー出版 2009年

講演・口頭発表等

  • Tunnel magnetoresistance effect in magnetic tunnel junctions with spinel ferrite electrodes  [招待講演]
    T. Nagahama
    10th NTTH Joint Symposium 2019年07月
  • スピネル型フェライト薄膜を含むスピントロニクスデバイスの開発  [招待講演]
    長浜太郎,高橋望,安井彰馬,島田敏宏
    平成30年度日本表面真空学会東北北海道支部講演会 2019年03月 口頭発表(招待・特別)
  • スピネルフェライト層を用いたトンネル磁気抵抗素子の開発  [招待講演]
    長浜太郎
    日本磁気学会第59回スピンエレクトロニクス専門研究会 2016年 茨城県つくば市
  • Magnetic tunnel junctions using Fe3O4 electrodes  [招待講演]
    T. Nagahama
    2016 Japan & France joint workshop _ Oxide electronics and spintronics seminar _ 2016年 Paris, France
  • Progress of Magnetic Tunnel Junctions —Connection between Charge and Spin—  [招待講演]
    Taro Nagahama
    THE 14th RIES-HOKUDAI INTERNATIONAL SYMPOSIUM 2013年12月
  • スピントロニクス素子を目指した磁性材料薄膜開発  [招待講演]
    長浜太郎
    電気化学会北海道支部 第18回若手研究者交流会 2012年 北海道小樽市
  • Fabrication of NiFe2O4 ultrathin films for oxide spin-filter junctions  [招待講演]
    Taro Nagahama
    International Joint Symposium between HU-CNU Symposium 2012年 Sapporo, Japan
  • Fabrication of epitaxial thin films of magnetic spinel materials by molecular beam epitaxy method  [招待講演]
    T. Nagahama, Y. Matsuda, K. Tate, S. Hiratani, Y. Watanabe, T. Shimada
    ICAUMS2012 2012年 Nara, Japan
  • スピントロニクス素子材料としての高機能トンネル磁気抵抗素子の開発  [招待講演]
    長浜太郎
    日本粉末冶金協会 平成23年度春季講演大会 2011年 東京都新宿区
  • 磁性薄膜とスピントロニクス  [招待講演]
    長浜太郎
    日本表面科学会第11回薄膜基礎講座 2008年 東京都新宿区
  • 結晶性MgOトンネルバリアの精密構造制御とコヒーレントスピントンネル  [招待講演]
    長浜太郎
    日本物理学会第62回年次大会 2007年 北海道札幌市
  • Giant tunneling magnetoresistance in MgO-MTJs and its industrial applications  [招待講演]
    T.Nagahama, S.Yuasa, R.Matsumoto, A.Fukushima, H.Kubota, K.Yakushiji, Y.Suzuki, K.Ando
    Workshop NANOSPINTRONICS Flagship 2007年 Netherland, Enshede
  • Giant tunneling magnetoresistance in magnetic tunnel junctions with MgO(001)  [招待講演]
    T.Nagahama, S.Yuasa, R.Matsumoto, A.Fukushima, H.Kubota, K.Yakushiji, Y.Suzuki, K.Ando
    International conference on Nanospintronic Design and Realization 2007 2007年 Germany, Dresden
  • Spin polarized resonant tunneling for various magnetic electrode  [招待講演]
    T.Nagahama, S.Yuasa, Y.Suzuki
    5th International Symposium on Metallic Multilayers (MML04) 2004年 USA, Boulder
  • 強磁性トンネル接合のTMR効果と量子サイズ効果  [招待講演]
    長浜太郎, 湯浅新治, 鈴木義茂, 田村英一
    将来加工技術第136委員会第2部会研究会 2003年 東京四谷
  • Quantum Size Effect in Magnetic Tunnel Junctions with Single-Crystal Ultrathin Electrodes  [招待講演]
    T.Nagahama, S.Yuasa, E.Tamura, Y.Suzuki
    The 2003 International Conference on MEMS, NANO, and Smart Systems 2003年 Banff, Canada
  • Magnetic Tunnel Junctions with Single-Crystal Electrodes  [招待講演]
    長浜太郎, 湯浅新治, 鈴木義茂, 安藤功兒
    第26回日本応用磁気学会学術講演会アジアンフォーラム 2002年 東京都小金井市
  • TMRの量子サイズ効果とその応用(共鳴トンネルトランジスタに向けて)  [招待講演]
    長浜太郎, 湯浅新治, 鈴木義茂, 安藤功兒
    日本応用磁気学会 第10回磁性人工構造膜の物性と機能専門委員会 2001年 東京

担当経験のある科目(授業)

  • 応用物質化学北海道大学大学院総合化学院
  • 物理化学2北海道大学工学部
  • 化学結合論北海道大学工学部
  • 物質変換工学(オムニバス)北海道大学工学部
  • 応用化学実験2(無機化学)北海道工学部
  • 薄膜材料の化学と機能北海道大学

所属学協会

  • 日本セラミックス協会   日本磁気学会   応用物理学会   日本物理学会   

共同研究・競争的資金等の研究課題

  • 強磁性ナノ構造を用いた光照射巨大磁気誘電デバイスの創製
    日本学術振興会:科学研究費助成事業 挑戦的研究(萌芽)
    研究期間 : 2019年06月 -2021年03月 
    代表者 : 海住 英生, 西井 準治, 長浜 太郎
  • 磁気トンネル接合における電圧誘起型巨大磁気キャパシタンス効果の発現
    日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2018年04月 -2021年03月 
    代表者 : 海住 英生, 西井 準治, 長浜 太郎
     
    近年、磁気キャパシタンス(MC)効果は、静的なスピン蓄積や動的なスピンダイナミクスに関する新たな学術的知見を与える一方で、高感度磁気センサ、省エネメモリ、大容量蓄電材料への応用も期待されていることから、国内外で大きな注目を集めている。これまでに、MC効果はマルチフェロイック材料、スピントロニクスデバイス、磁気スーパーキャパシタ、有機ヘテロ接合など、様々な物質・材料・デバイスにおいて発見されてきた。中でも、スピントロニクスデバイスにおける磁気トンネル接合(MTJ)は室温にて巨大なMC効果を示すことから、世界中で精力的に研究が進められている。本研究課題では、MTJにおける電圧誘起型MC効果に着目し、従来値を凌駕する巨大なMC比の達成とそのメカニズム解明を目指す。 本研究目標を達成するため、当該年度では、超高真空マグネトロンスパッタ装置を用いて、熱酸化Si基板上にCoFeB/MgO/CoFeBベースのMTJを作製し、磁場中交流4端子法によりMC効果を調べた。CoFeB磁性層の膜厚は3nm、MgO絶縁層の膜厚は2nmとした。微細加工にはフォトリソグラフィーとイオンミリング法を用いた。接合面積は1800μm2とした。磁気インピーダンス測定を行った結果、電圧を印加するとトンネル磁気抵抗(TMR)比は減少する一方で、TMC比は増大することを初めて見出した。また、この電圧誘起TMC効果は4次関数バリア近似とスピン依存ドリフト拡散モデルを取り入れたDebye-Frohlich(DF)モデルにより定量的に説明できることがわかった。
  • Fe3O4/絶縁体の界面精密制御によるトンネル磁気抵抗素子の高機能化
    日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2018年04月 -2021年03月 
    代表者 : 長浜 太郎, 岡林 潤, 本多 周太
     
    本研究はFe3O4を電極とするトンネル磁気抵抗(TMR)素子の現象解明と高機能化を目的とする。Fe3O4は第一原理計算ではハーフメタルになると期待されており、大きなMR比が期待されているが、これまでの研究では20%程度のMR比しか得られておらず、その理由も明らかではない。TMR素子では電極とバリア界面の電子状態磁気状態が重要である。本研究ではFe3O4/酸化物界面の電子状態磁気状態を詳細に調べ、これまでのFe3O4-TMR素子で大きなMRが観測されていない理由を明らかにし、Fe3O4本来のハーフメタル性を発揮した素子を作成することを目的とした。 今年はMgO(001)/Fe3O4(001)/MgO(001)/Feという構造のTMR素子を作製し、その磁気抵抗効果を調べた。その結果、80Kという低温ではあるが、約60%という大きな磁気抵抗効果を得ることができた。またこのMR比は-200mVのバイアス電圧化で得られた。バイアス電圧依存性を調べたところ、通常のMR素子と同様に電圧の増加とともにMR比が減少した。また、素子抵抗が大きすぎるため低電圧領域ではMRを測定することができなかった。 電子状態について調べるためにKEK-PFでのXASおよびXMCD実験を行った。用いた試料は、Fe3O4薄膜であり(100)(110)(111)の3つの結晶方位について調べた。またキャップ層をMgOとAl2O3とし、MgO-TMR素子とAl2O3-TMR素子についてそれぞれ情報が得られるように測定を行った。結果は結晶方位についてはほぼ同じスペクトルであったが若干の結晶方位依存性が見られた。また、MgOキャップ層とAl2O3キャップ層についてはほとんど差はなかった。
  • 日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2015年04月 -2018年03月 
    代表者 : 海住 英生, 西井 準治, 長浜 太郎, 小峰 啓史
     
    電子の電荷とスピンの2つの自由度を利用する「スピントロニクス」は、現代のエレクトロニクスを凌駕する次世代技術として期待され、近年大きな注目を集めている。本研究課題では、強磁性ナノ薄膜間に極薄絶縁層を挟んだスピントロニクス素子を作製し、磁気キャパシタンス効果を調べることを目的とした。その結果、これまでで最大の155%の磁気キャパシタンス比を観測することに成功した。これは従来値(=50%)の約3倍を示す。また、本実験結果はデバイ・フレーリッヒ模型を用いた理論計算により定量的に説明できることがわかった。本理論によると磁気キャパシタンス比は1000%を超えることから、今後更なる発展が期待できる。
  • 日本学術振興会:科学研究費助成事業 挑戦的萌芽研究
    研究期間 : 2015年04月 -2017年03月 
    代表者 : 島田 敏宏, 長浜 太郎, 柳瀬 隆
     
    本研究では中~遠赤外線に対応した波長程度の微細構造を持つメタマテリアルを耐火性材料を用いて作製するプロセスを開発し、作製した構造の物性を評価することを目的とした。大気中高温に耐える耐火性材料の利用により高温における熱の制御、赤外線光学部品などへの応用が可能となる。さまざまな高融点材料の薄膜を作製し微細加工と透明薄膜でのコーティングによる高温大気安定性を検討した。その結果、MoSi2を主成分とする薄膜の作製とμmスケールの微細加工プロセスを開発することができ、赤外発光特性を評価してほぼ所定の結果を得た。
  • 日本学術振興会:科学研究費助成事業 若手研究(A)
    研究期間 : 2011年11月 -2014年03月 
    代表者 : 長浜 太郎
     
    スピントロニクス分野ではMRAMやスピントルクRAMが注目を浴びている。とくにスピントルクRAMでは省電力な磁化反転技術として、スピン注入磁化反転を用いている。Slonczewskiは磁性絶縁体中に励起されるスピン波によって、高効率なスピン流生成が可能であることを示した。本研究ではエピタキシャルなフェライト層を含むMTJ素子の作製技術を確立するとともに、その磁気特性、および磁気抵抗効果の評価を行った。その結果、非磁性挿入層としてPtが適していることや、非磁性挿入により保磁力の増大が観測されるなどの新たな知見を得た。また電子線リソグラフィーを用いた微小素子を作製し、70%の磁気抵抗を観測した。
  • 日本学術振興会:科学研究費助成事業 挑戦的萌芽研究
    研究期間 : 2011年 -2012年 
    代表者 : 柳原 英人, 長浜 太郎
     
    本研究では、電流を流した回折格子状のPt試料に対して磁気カー効果(MOKE)を用いてPt端面に蓄積するスピン情報を検出することを試みた。2軸X線回折計を改造し、簡便かつ高感度である偏光変調法をもちいて可視光用のMOKE回折計を作製した。偏光変調に加えて回折格子に印加する電流を交流電流とすることで二重変調法によるロックイン検波を試みた。1次の回折線に関する磁気カー信号は、電流による変化を示さず、スピン蓄積に起因した応答は得られなかった。今後、S/Nを改善するためには、交流電流の増大および、レーザーダイオード強度のゆらぎに対する対策を講じる必要があることがわかった。
  • 日本学術振興会:科学研究費助成事業 挑戦的萌芽研究
    研究期間 : 2011年 -2012年 
    代表者 : 島田 敏宏, 長浜 太郎
     
    ドープした有機半導体の結晶の育成法とその熱電効果を実験的に研究した。常温で固体であるアントラキノン(AQ)と、融点を持たない銅フタロシアニン(CuPc)を混合して加熱することにより溶解させ、同系の融点-組成曲線を得た。また、溶媒AQを昇華させることにより結晶成長を試み、CuPcのmmサイズの単結晶を得た。さらにFeCl3をドーパントとして加えて多結晶試料を作製した。XPSと近赤外分光により電荷移動度が組成に依存することを明らかにした。ドープした試料は純粋なCuPcに比べて電気伝導度とゼーベック係数の両方が向上した。
  • 強磁性絶縁体超薄膜を用いた新規スピントロニクスデバイスの創製
    JST戦略的創造研究推進制度(個人研究型) (個人研究推進事業:さきがけ研究21‐PRESTO)
    研究期間 : 2008年 -2011年
  • 日本学術振興会:科学研究費助成事業 特定領域研究
    研究期間 : 2008年 -2008年 
    代表者 : 長浜 太郎
     
    優れたコヒーレントトンネリング特性と示すMgO バリアをエミッタに用いたMgO-強磁性トンネルトランジスタ(MgO-MTT)を作製し、その伝導特性を調べた。ベース層の薄膜化、コレクタ材料の最適化により伝導特性は大きく向上することが分かった。また、その特性は界面共鳴状態の影響を受けており、界面の電子状態を調べる新しいツールとしても活用可能であることが分かった。
  • 交換スプリング多層膜の磁性と伝導性
    日本学術振興会:科学研究費助成事業 特別研究員奨励費
    研究期間 : 1998年 -1999年 
    代表者 : 長濱 太郎

産業財産権

  • 大 坊 忠 臣, 伊 藤 順 一, 甲 斐 正, 天 野 実, 與 田 博 明, 宮▲崎▼照 宣, 水 上 成 美, 窪 田 崇 秀, 薬師寺 啓, 湯 浅 新 治, 久保田 均, 福 島 章 雄, 長 浜 太 郎, 安 藤 功 兒  株式会社東芝, 国立大学法人 東北大学 原子分子材料科学高等研究機構, 独立行政法人産業技術総合研究所  201203085075659950
  • 北川 英二, 大坊 忠臣, 橋本 豊, 都甲 大, 甲斐 正, 長嶺 真, 永瀬 俊彦, 西山 勝哉, 上田 公二, 與田 博明, 薬師寺 啓, 湯浅 新治, 久保田 均, 長浜 太郎, 福島 章雄, 安藤 功兒  株式会社東芝, 独立行政法人産業技術総合研究所  201203044120433324
  • 永 瀬 俊 彦, 甲 斐 正, 長 嶺 真, 西 山 勝 哉, 北 川 英 二, 大 坊 忠 臣, 上 田 公 二, 與 田 博 明, 薬師寺 啓, 湯 浅 新 治, 久保田 均, 長 浜 太 郎, 福 島 章 雄, 安 藤 功 兒  株式会社東芝, 独立行政法人産業技術総合研究所  201203068209490131
  • 甲斐 正, 西山 勝哉, 永瀬 俊彦, 吉川 将寿, 北川 英二, 大坊 忠臣, 長嶺 真, 中山 昌彦, 下村 尚治, 與田 博明, 薬師寺 啓, 湯浅 新治, 久保田 均, 長浜 太郎, 福島 章雄, 安藤 功兒  株式会社東芝, 独立行政法人産業技術総合研究所  201103048480364771
  • 久保田 均, 福島 章雄, 薬師寺 啓, 長浜 太郎, 湯浅 新治, 安藤 功兒, 大兼 幹彦, 渡邉 美穂, 安藤 康夫, 宮崎 照宣  独立行政法人産業技術総合研究所, 国立大学法人東北大学  200903076122654976

メディア報道

  • 酸化鉄で巨大な負のトンネル磁気抵抗効果を実現~酸化物スピントロニクスの実現に道筋~
    報道 : 2021年03月
    執筆者 : 本人
    発行元・放送局 : 北海道大学プレスリリース
    https://www.hokudai.ac.jp/news/2021/03/post-815.html
  • 新たな物理現象,逆磁気キャパシタンス(iTMC)効果を発見 —磁石の向きにより電気の溜まり方を自在にコントロール—
    報道 : 2017年06月
    執筆者 : 本人以外
    発行元・放送局 : 北海道大学電子科学研究所プレスリリース
    https://www.es.hokudai.ac.jp/result/2017-06-02-nfm/


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