Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
山崎 匠*; 平井 孝昌*; 八木 貴志*; 山下 雄一郎*; 内田 健一*; 関 剛斎*; 高梨 弘毅
Physical Review Applied (Internet), 21(2), p.024039_1 - 024039_11, 2024/02
被引用回数：0This study presents a measurement method for determining the figure of merit for transverse thermoelectric conversion in thin-film forms. Leveraging the proposed methodology, we comprehensively investigate the transverse thermoelectric coefficient, in-plane electrical conductivity, and out-of-plane thermal conductivity in epitaxial and polycrystalline Fe/Pt metallic multilayers.
増田 啓人*; 山根 結太*; 関 剛斎*; Raab, K.*; 土肥 昂尭*; Modak, R.*; 内田 健一*; 家田 淳一; Kl
ui, M.*; 高梨 弘毅
Applied Physics Letters, 122(16), p.162402_1 - 162402_7, 2023/04
被引用回数：1 パーセンタイル：54.89(Physics, Applied)We report current-induced magnetization switching in Pt/Co/Ir/Co/Pt multilayers with different Ir layer thicknesses (
), where the perpendicularly magnetized Co layers are coupled ferromagnetically or antiferromagnetically through an interlayer exchange coupling and are sandwiched by the Pt spin Hall layers. The domain structures formed during switching vary depending on the magnetization alignment, i.e., ferromagnetically coupled or antiferromagnetically coupled configuration. These results clarify the macroscopic picture of switching process for interlayer exchange-coupled systems. The local picture of the switching process is also examined by a numerical calculation based on a macrospin model, which reveals the switching dynamics triggered by dual spin-orbit torque for both antiferromagnetically and ferromagnetically coupled cases. The numerical calculation shows that the dual spin-orbit torque from the two Pt layers effectively acts on the two Co layers not only for the antiferromagnetically coupled case but also for the ferromagnetically coupled one. Our findings deepen the under- standing of the switching mechanism in a magnetic multilayer and provide an avenue to design spintronic devices with more efficient spin-orbit torque switching.
Zhou, Z.*; Frost, W.*; Lloyd, D. C.*; 関 剛斎*; 窪田 崇秀*; Ramos, R.*; 齊藤 英治; 高梨 弘毅; 廣畑 貴文*
Journal of Magnetism and Magnetic Materials, 571, p.170575_1 - 170575_5, 2023/04
被引用回数：0 パーセンタイル：0(Materials Science, Multidisciplinary)Recent development in neuromorphic computation allows us to achieve low power and highly efficient calculations better than the conventional von Neumann computation. In order to achieve realistic synaptic operation, potentiation to add weighting to strengthen a selected artificial synapse. Such functionality can be achieved by reducing the electrical resistance of the artificial synapse. Recently, a ferromagnetic Heusler alloy used in a magnetoresistive junction has been demonstrated to crystallise via the layer-by-layer mode by introducing an electrical current pulse. In this study, we have extended the current-induced crystallisation to a junction with epitaxially-grown Heusler alloy after post-annealing for crystallisation. By combining this potentiation functionality with the neuromorphic operation, realistic synaptic computation can be developed.
増田 啓人*; 関 剛斎*; 山根 結太*; Modak, R.*; 内田 健一*; 家田 淳一; Lau, Y.-C.*; 深見 俊輔*; 高梨 弘毅
Physical Review Applied (Internet), 17(5), p.054036_1 - 054036_9, 2022/05
被引用回数：6 パーセンタイル：73.14(Physics, Applied)反対称層間交換結合(AIEC)が最近発見され、人工反強磁性体(SAF)の傾角磁化の誘起を通じた磁化スイッチングにおいて極めて重要な役割を果たしている。本研究では、くさび形の層を持つ垂直磁化多層膜Pt/Co/Ir/Co/Ptにおける大きなAIECを報告をする。AIECの有効磁場は、対称的な層間交換結合に関連しており、AIECを強化するための指針を提供する。SAFに対する拡張Stoner-Wohlfarthモデルを開発し、その磁化スイッチングの重要な要素を明らかにする。理論的知識と実験結果を組み合わせることで、面内磁場のみによる垂直磁化スイッチングが達成される。
CoGa
films窪田 崇秀*; 高野 大地*; 小田 洋平*; Mohanty, S.*; 伊藤 啓太*; 松木 充弘*; 林田 誠弘*; Sun, M.*; 竹田 幸治; 斎藤 祐児; et al.
Physical Review Materials (Internet), 6(4), p.044405_1 - 044405_12, 2022/04
被引用回数：5 パーセンタイル：59.75(Materials Science, Multidisciplinary)This study focuses on an inverse-type Heusler alloy, MnCoGa (MCG), with a small off-stoichiometry (
), showing perpendicular magnetic anisotropy (PMA). We observed a relatively large uniaxial magnetocrystalline anisotropy constant (
) on the order of 10
J/m
at room temperature in the MCG films with a small tetragonal distortion of a few percent. X-ray magnetic circular dichroism (XMCD) was employed to get insight into the origin for the PMA. Negligible angular variation of the orbital magnetic moment evaluated using the XMCD spectra suggested a minor role of the so-called Bruno's term to . The origin of the PMA was discussed based on the second-order perturbation theory in terms of the spin-orbit coupling, and first principles calculations, claiming that the mixing of the occupied 
- and the unoccupied 
-spin states is responsible for the PMA of the MCG films.
VAl/Fe layered films showing exchange bias窪田 崇秀*; 嶋田 雄介*; 土屋 朋生*; 吉川 智己*; 伊藤 啓太*; 竹田 幸治; 斎藤 祐児; 今野 豊彦*; 木村 昭夫*; 高梨 弘毅*
Nanomaterials (Internet), 11(7), p.1723_1 - 1723_11, 2021/07
被引用回数：2 パーセンタイル：17.84(Chemistry, Multidisciplinary)Heusler alloys exhibit various magnetic properties. In this study, a layered sample consisting of a Heusler alloy, MnVAl and a ferromagnet, Fe, is selected as a material system exhibiting exchange bias. Although the fully ordered MnVAl is known as a ferrimagnet, the MnVAl/Fe layered structure exhibits exchange bias. The high-angle annular dark field STEM images demonstrated the formation of MnVAl clusters with the L2
phase distributed only around the interface to the Fe layer in the sample. Furthermore, the antiferromagnetic coupling between the Mn- and Fe-moments were observed in element specific hysteresis loops measured using XMCD. The locally ordered L2 phase and antiferromagnetic Mn-moments in the MnVAl layer are important for the exchange bias.
Frost, W.*; 関 剛斎*; 窪田 崇秀*; Ramos, R.*; 齊藤 英治; 高梨 弘毅*; 廣畑 貴文*
Applied Physics Letters, 118(17), p.172405_1 - 172405_5, 2021/04
被引用回数：1 パーセンタイル：7.86(Physics, Applied)We demonstrate that the spin-Seebeck effect can be used to estimate the volume of edge domains formed in a giant magnetoresistive (GMR) device. The thermal gradient induced by Joule heating can be harnessed by the addition of a ferromagnetically insulating channel of Fe
O
on the sides of the GMR pillar. This generates a spin wave in FeO, which couples with the free-layer edge magnetization and controls the reversal of the ferromagnetic layers in one direction only, increasing the current density from 
A/cm
to 
A/cm. By simple assumption, we estimate the effect of the edge domain on magnetization reversal to be 
by spin-transfer torque.
Zhou, W.*; 関 剛斎*; 今村 裕志*; 家田 淳一; 高梨 弘毅*
Physical Review B, 100(9), p.094424_1 - 094424_5, 2019/09
被引用回数：6 パーセンタイル：32.87(Materials Science, Multidisciplinary)The generation of the spinmotive force (SMF) requires the temporal and spatial variations of the magnetic moments. We explore an approach to satisfy this requirement by creating asymmetry in a thin film structure along the out-of-plane direction, through the use of an exchange-coupled 
1
-FePt / Ni
Fe
bilayer element. As the spin wave is excited by rf magnetic field, a continuous dc voltage signal along the out-of-plane direction of the bilayer element appears. The sign of the voltage signal and its microwave power dependency agree with the theoretical framework of the SMF. The corresponding spin wave modes are revealed by carrying out the micromagnetic simulation. Our results demonstrate the generation of the SMF using a vertically structured element in addition to previously reported in-plane structured devices.
VAl Heusler alloy thin films; Appearance of antiferromagnetism and exchange bias in a layered structure with Fe土屋 朋生*; 小林 亮太*; 窪田 崇秀*; 斉藤 耕太郎*; 小野 寛太*; 大原 高志; 中尾 朗子*; 高梨 弘毅*
Journal of Physics D; Applied Physics, 51(6), p.065001_1 - 065001_7, 2018/02
被引用回数：10 パーセンタイル：45.99(Physics, Applied)MnVAl Heusler alloy films were epitaxially grown on MgO(100) single-crystal substrates by ultra-high-vacuum magnetron sputtering. A2- and L21-type MnVAl order was controlled by the deposition temperature. A2-type MnVAl films showed no spontaneous magnetization, while L21-type MnVAl films showed ferrimagnetic behaviour with a maximum saturation magnetization of 220 emu/cm
at room temperature (RT). An antiferromagnetic reflection was observed with neutron diffraction at RT for an A2-type MnVAl film deposited at 400
C. A bilayer sample of the antiferromagnetic A2-type MnVAl and Fe showed an exchange bias of 120 Oe at 10 K.
atoms in Ni
Fe
N (
= 1 and 3) films revealed by X-ray absorption spectroscopy and magnetic circular dichroism高田 郁弥*; 伊藤 啓太*; 竹田 幸治; 斎藤 祐児; 高梨 弘毅*; 木村 昭夫*; 末益 崇*
Physical Review Materials (Internet), 2(2), p.024407_1 - 024407_5, 2018/02
被引用回数：14 パーセンタイル：48.52(Materials Science, Multidisciplinary)X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism measurements were performed at the Ni and Fe 
absorption edges for NiFeN ( = 1 and 3) epitaxial films. Shoulders at approximately 2 eV above the Ni main peaks in the XAS spectrum of Ni
FeN were interpreted to originate from hybridization of orbitals between Ni 3 at face-centered (II) sites and N 2
at body-centered sites, while such features were missing in NiFeN film. Similar shoulders were observed at Fe edges in both films. Hence, Ni atoms preferentially occupied corner (I) sites, where the hybridization was weak because of the relatively long distance between Ni at I sites and N atoms. The relatively large magnetic moment deduced from sum-rule analysis of NiFeN also showed a good agreement with the presence of Ni atoms at I sites.
内田 健一*; 吉川 貴史*; 関 剛斎*; 小宅 教文*; 塩見 淳一郎*; Qiu, Z.*; 高梨 弘毅*; 齊藤 英治
Physical Review B, 92(9), p.094414_1 - 094414_6, 2015/09
被引用回数：86 パーセンタイル：93.07(Materials Science, Multidisciplinary)The anomalous Nernst effect (ANE) has been investigated in alternately stacked multilayer films comprising paramagnetic and ferromagnetic metals. We found that the ANE is enhanced by increasing the number of the paramagnet/ferromagnet interfaces and keeping the total thickness of the films constant, and that the enhancement appears even in the absence of magnetic proximity effects; similar behavior was observed not only in Pt/Fe multilayers but also in Au/Fe and Cu/Fe multilayers free from proximity ferromagnetism. This universal enhancement of the ANE in metallic multilayers suggests the presence of unconventional interface-induced thermoelectric conversion in the Fe films attached to the paramagnets.
関 剛斎*; 内田 健一*; 吉川 貴史*; Qiu, Z.*; 齊藤 英治; 高梨 弘毅*
Applied Physics Letters, 107(9), p.092401_1 - 092401_4, 2015/08
被引用回数：39 パーセンタイル：81.57(Physics, Applied)We experimentally observed the inverse spin Hall effect (ISHE) of ferromagnetic FePt alloys. Spin Seebeck effect due to the temperature gradient generated the spin current (
) in the FePt
YFe
O
(YIG) structure, and was injected from YIG to FePt and converted to the charge current through ISHE of FePt. The significant difference in magnetization switching fields for FePt and YIG led to the clear separation of the voltage of ISHE from that of anomalous Nernst effect in FePt. We also investigated the effect of ordering of FePt crystal structure on the magnitude of ISHE voltage in FePt.
Zhang, H.; 山本 春也; 深谷 有喜; 前川 雅樹; Li, H.; 河裾 厚男; 関 剛斎*; 齊藤 英治*; 高梨 弘毅*
Scientific Reports (Internet), 4, p.4844_1 - 4844_5, 2014/04
被引用回数：39 パーセンタイル：81.84(Multidisciplinary Sciences)Current-induced spin polarization (CISP) on the outermost surfaces of Au, Cu, Pt, Pd, Ta, and W nanoscaled films were studied using a spin-polarized positron beam. The Au and Cu surfaces showed no significant CISP. In contrast, the Pt, Pd, Ta, and W films exhibited large CISP (3 to 15% per input charge current of 10
A/cm
) and the CISP of Ta and W were opposite to those of Pt and Pd. The sign of the CISP obeys the same rule in spin Hall effect suggesting that the spin-orbit coupling is mainly responsible for the CISP. The magnitude of the CISP is explained by the Rashba-Edelstein mechanism rather than the diffusive spin Hall effect. This settles a controversy, that which of these two mechanisms dominates the large CISP on metal surfaces.
河裾 厚男; 深谷 有喜; 前川 雅樹; Zhang, H.; 関 剛斎*; 吉野 達郎*; 齊藤 英治*; 高梨 弘毅*
Journal of Magnetism and Magnetic Materials, 342, p.139 - 143, 2013/09
被引用回数：19 パーセンタイル：62.39(Materials Science, Multidisciplinary)電流通電下にある白金超薄膜表面におけるスピン偏極陽電子消滅を観測した。その結果、表面で形成されるオルソポジトロニウムの強度が通電方向に応じて変化することが明らかになった。この結果は、通電下において白金表面の伝導電子がスピン偏極していることを示唆している。その場合、伝導電子のスピン偏極率は1%以上であると推定される。
-Co films with current-perpendicular-to-plane geometry境 誠司; 三谷 誠司*; 松本 吉弘; 圓谷 志郎; Avramov, P.; 大伴 真名歩; 楢本 洋*; 高梨 弘毅
Journal of Magnetism and Magnetic Materials, 324(12), p.1970 - 1974, 2012/06
被引用回数：3 パーセンタイル：14.91(Materials Science, Multidisciplinary)Voltage-dependence of the tunneling magnetoresistance effect in the granular C-Co films has been investigated under the current-perpendicular-to-plane geometry. The transport measurements demonstrate that the granular C-Co films show unusually an exponential MR-V dependence. Small characteristic energies of less than 10's meV are derived from the temperature dependences of the characteristic voltage in the exponential relationship. Considering the voltage drops between Co nanoparticles and also the effect of cotunneling on the energy values, the characteristic energies for the voltage-induced degradation of the spin polarization are found to show a satisfactory agreement with that for the thermally-induced one. It can reasonably be expected that the onset of magnetic disorder to the localized d-electron spins at the interface region of the C-Co compound with Co nanoparticles will lead to the unusual voltage and temperature dependence of the MR ratio and the spin polarization.
-Co films境 誠司; 三谷 誠司*; 菅井 勇; 高梨 弘毅; 松本 吉弘; 圓谷 志郎; 楢本 洋*; Avramov, P.; 前田 佳均
Physical Review B, 83(17), p.174422_1 - 174422_6, 2011/05
被引用回数：8 パーセンタイル：36.17(Materials Science, Multidisciplinary)Magnetotransport properties of the granular C-Co films are investigated over the broad bias voltage range from the region near zero bias by using the samples with the CPP (current-perpendicular-to-plane) geometry. It is revealed that the granular C-Co films show the I-V characteristics ascribed to cotunneling whose magnitudes are comparable to other molecular-based granular films. Furthermore, by considering the contribution of cotunneling on the magnitude of the TMR effect, it is successfully demonstrated that the tunnelling electrons generated at the interface between Co nanoparticles and a C-based matrix (C-Co compound) in the films have a significantly higher spin polarization (50-80%) than those in Co crystal and at the Al-oxide/Co interface. The present results clearly suggest that the high interfacial spin polarization is the most important cause of the large TMR effect in the granular C-Co films, in spite of the cotunneling-induced enhancement.
MnSi中堂 博之; 安藤 和也*; 斉藤 今朝美*; 岡安 悟; 春木 理恵; 桜庭 裕弥*; 安岡 弘志; 高梨 弘毅; 齊藤 英治
Journal of Applied Physics, 109(7), p.073915_1 - 073915_4, 2011/04
被引用回数：19 パーセンタイル：61.3(Physics, Applied)スピン流の生成及び制御は、スピントロニクス研究分野の基本技術として重要である。近年、強磁性共鳴(FMR)を利用したスピンポンピングがスピン流を生成する手段としてスピントロニクスの基礎,応用研究に広く用いられるようになってきた。本研究では、強磁性材料として高スピン偏極CoMnSi(CMS)に着目し、スピンポンピング実験を行った。測定に用いた試料は単結晶CMS/Ptヘテロ構造薄膜である。CMS層で生成されたスピン流はPt層の逆スピンホール効果によって電流に変換されるので、電極間に生じた起電力を測定しCMSのスピン変換効率を見積もる。測定の結果、FMR共鳴磁場で起電力が最大になっており、磁場方向を反転させると起電力の符号は反転する。このことは逆スピンホール効果の幾何的要因を反映しており、CMSのスピンポンピングで生成されたスピン流がPt層に注入されたことを示唆している。見積もられたスピン変換効率はパーマロイより小さいことがわかった。このことから、スピン変換効率はバルクの分極率ではなく、界面の分極率が重要であると考えられる。
Gu, B.; 管井 勇*; Ziman, T.*; Guo, G. Y.*; 永長 直人; 関 剛斎*; 高梨 弘毅; 前川 禎通
Physical Review Letters, 105(21), p.216401_1 - 216401_4, 2010/11
被引用回数：71 パーセンタイル：90.4(Physics, Multidisciplinary)We show, both experimentally and theoretically, a novel route to obtain giant room temperature spin Hall effect due to surface-assisted skew scattering. In the experiment, we report the spin Hall effect in Pt-doped Au films with different thicknesses 
. The giant spin Hall angle 
= 
is obtained for = 10 nm at room temperature, while it is much smaller for = 20 nm sample. Combined 
and quantum Monte Carlo calculations for the skew scattering due to a Pt impurity show 
0.1 on the Au (111) surface, while it is small in bulk Au. The quantum Monte Carlo results show that the spin-orbit interaction of the Pt impurity on the Au (111) surface is enhanced, because the Pt 5 levels are lifted to the Fermi level due to the valence fluctuation. In addition, there are two spin-orbit interaction channels on the Au (111) surface, while only one in bulk Au.
-Co granular thin films菅井 勇*; 境 誠司; 松本 吉弘; 楢本 洋*; 三谷 誠司*; 高梨 弘毅; 前田 佳均
Journal of Applied Physics, 108(6), p.063920_1 - 063920_7, 2010/09
被引用回数：8 パーセンタイル：34.38(Physics, Applied)Composition dependence of magnetic and magnetotransport properties in CCo thin films exhibiting large magnetoresistance TMR effect was investigated in the Co composition range of x=8-20, where x denotes the number of Co atoms per C molecule. The composition dependence of magnetic property revealed a structural transition from well-defined granular structures in the range of x=8-17 to magnetically and electronically coupled states of Co nanoparticles over x=17. As a result of the structural change, the MR behavior became different between the two composition regions separated at x=17. It is found that the zero-bias MR ratio and also the strength of the voltage dependence are in proportion to the charging energy to Co nanoparticles in the samples with well-defined granular structures. The present results indicate that the charging effect of Co nanoparticles plays an important role in the anomalously large MR effect of C-Co granular films.
松本 吉弘; 境 誠司; 楢本 洋*; 平尾 法恵*; 馬場 祐治; 島田 敏宏*; 菅井 勇; 高梨 弘毅; 前田 佳均
Materials Research Society Symposium Proceedings, Vol.1081 (Internet), 6 Pages, 2008/03
近年、C-Co化合物中にCoナノ粒子が分散したC/Co混合材料において、非常に大きな磁気抵抗効果(MR=80%)が発現することを確認した。このような巨大な磁気抵抗効果の発現は、Coナノ粒子間のトンネル伝導のみでは十分に説明することができず、C-Co化合物やその界面が物性発現に大きく関与しているのではないかと推測される。以上の点から、本研究ではC/Co混合分子の電子状態を得ることを目的とした。結果として、純粋なCとは異なる吸収スペクトルがCCo混合分子において得られた。特に、
(LUMO)
C1s励起では顕著な強度の減衰が観測された。また、光電子スペクトルからはC1sピークがわずかに低結合エネルギー側にシフトしていることも観測された。これらの結果は、Coの3d電子がCの軌道へ遷移することで、C-Co化合物に新たな電子状態が形成されたことを意味する。実際に、フェルミ準位近傍に混成軌道形成に対応する状態が観測されていることも、前述の結果と矛盾しない。
