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論文

Vortex rectenna powered by environmental fluctuations

Lustikova, J.*; 塩見 雄毅*; 横井 直人*; 壁谷 典幸*; 木村 憲彰*; 家永 紘一郎*; 金子 真一*; 大熊 哲*; 高橋 三郎*; 齊藤 英治

Nature Communications (Internet), 9, p.4922_1 - 4922_6, 2018/11

 被引用回数:1 パーセンタイル:49.95(Multidisciplinary Sciences)

A rectenna, standing for a rectifying antenna, is an apparatus which generates d.c. electricity from electric fluctuations. It is expected to realize wireless power transmission as well as energy harvesting from environmental radio waves. To realize such rectification, devices that are made up of internal atomic asymmetry such as an asymmetric junction have been necessary so far. Here we report a material that spontaneously generates electricity by rectifying environmental fluctuations without using atomic asymmetry. The sample is a common superconductor without lowered crystalline symmetry, but, just by putting it in an asymmetric magnetic environment, it turns into a rectifier and starts generating electricity. Superconducting vortex strings only annihilate and nucleate at surfaces, and this allows the bulk electrons to feel surface fluctuations in an asymmetric environment: a vortex rectenna. The rectification and generation can be switched on and off with only a slight change in temperature or external magnetic fields.

論文

Enhanced magneto-optical Kerr effect at Fe/insulator interfaces

Gu, B.; 高橋 三郎*; 前川 禎通

Physical Review B, 96(21), p.214423_1 - 214423_6, 2017/12

 被引用回数:1 パーセンタイル:81.49(Materials Science, Multidisciplinary)

Using density functional theory calculations, we have found an enhanced magneto-optical Kerr effect in Fe/insulator interfaces. The results of our study indicate that interfacial Fe atoms in the Fe films have a lowdimensional nature, which causes the following two effects: (1) The diagonal component $$sigma_{xx}$$ of the optical conductivity decreases dramatically because the hopping integral for electrons between Fe atoms is suppressed by the low dimensionality. (2) The off-diagonal component $$sigma_{xy}$$ of the optical conductivity does not change at low photon energies, but it is enhanced at photon energies around 2 eV, where we obtain enhanced orbital magnetic moments and spin-orbit correlations for the interfacial Fe atoms. A large Kerr angle develops in proportion to the ratio $$sigma_{xy}$$/$$sigma_{xx}$$. Our findings indicate an efficient way to enhance the effect of spin-orbit coupling at metal/insulator interfaces without using heavy elements.

論文

Generation of megahertz-band spin currents using nonlinear spin pumping

渡辺 真悟*; 廣部 大地*; 塩見 雄毅*; 井口 亮*; 大門 俊介*; 亀田 麻衣*; 高橋 三郎*; 齊藤 英治

Scientific Reports (Internet), 7, p.4576_1 - 4576_6, 2017/07

 被引用回数:2 パーセンタイル:66.33(Multidisciplinary Sciences)

Spin pumping enables the generation of d.c. and gigahertz-band (GHz-band) voltages from an applied microwave via magnetization dynamics when combined with inverse spin Hall effects. However, generating such voltages in the in-between frequency region, or the megahertz (MHz) band, has been difficult since ferromagnetic resonance usually occurs in the GHz band. Here we show that in spite of GHz-band microwaves applied, MHz-band voltages can be generated by spin pumping with use of nonlinear magnetization dynamics in Y$$_3$$Fe$$_5$$O$$_{12}$$. The mechanism is ascribed to the MHz-band oscillation of the amplitude of the magnetization precession, which is projected onto a rectified voltage component via spin pumping. The present finding could be useful for frequency down-conversion thanks to the simple and durable structure, continuous-wave operation, and the tunability of an output frequency with low magnetic fields.

論文

Optically transparent ferromagnetic nanogranular films with tunable transmittance

小林 伸聖*; 増本 博*; 高橋 三郎*; 前川 禎通

Scientific Reports (Internet), 6, p.34227_1 - 34227_7, 2016/09

 被引用回数:7 パーセンタイル:40.56(Multidisciplinary Sciences)

Developing optically transparent magnets at room temperature is an important challenge. They would bring many innovations to various industries, not only for electronic and magnetic devices but also for optical applications. Here we introduce FeCo-(Al-fluoride) nanogranular films exhibiting ferromagnetic properties with high optical transparency in the visible light region. These films have a nanocomposite structure, in which nanometer-sized FeCo ferromagnetic granules are dispersed in an Al-fluoride crystallized matrix. The optical transmittance of these films is controlled by changing the magnetization. This is a new type of magneto-optical effect and is explained by spin-dependent charge oscillation between ferromagnetic granules due to quantum-mechanical tunneling.

論文

Theory of spin Hall magnetoresistance (SMR) and related phenomena

Chen, Y.-T.*; 高橋 三郎*; 中山 裕康*; Althammer, M.*; Goennenwein, S. T. B.*; 齊藤 英治; Bauer, G. E. W.*

Journal of Physics; Condensed Matter, 28(10), p.103004_1 - 103004_15, 2016/03

 被引用回数:34 パーセンタイル:30.51(Physics, Condensed Matter)

スピンホール効果によってスピン流が発生する金属と磁性絶縁体との二重層におけるいわゆるスピンホール磁気抵抗(SMR)を概観する。関連する角運動量が強磁性層に伝達され、それにより電気抵抗が印加電流と磁化方向との間の角度によって変調される。SMRは、絶縁体の磁化方向とスピン伝達トルクを非侵入的に測定するための便利なツールを提供する。SMR、すなわちスピン拡散理論および量子力学的境界条件を計算するための最小の理論的手段を紹介する。これは、実験に適合させることができる少数のパラメータを導出する。理論の限界と、強磁性近接効果やRashbaスピン軌道トルクなどの代替メカニズムについて議論し、新しい展開を指摘する。

論文

Spin hydrodynamic generation

高橋 遼*; 松尾 衛; 小野 正雄; 針井 一哉; 中堂 博之; 岡安 悟; 家田 淳一; 高橋 三郎*; 前川 禎通; 齊藤 英治

Nature Physics, 12, p.52 - 56, 2016/01

 被引用回数:24 パーセンタイル:7.93(Physics, Multidisciplinary)

Magnetohydrodynamic generation is the conversion of fluid kinetic energy into electricity. Such conversion, which has been applied to various types of electric power generation, is driven by the Lorentz force acting on charged particles and thus a magnetic field is necessary. On the other hand, recent studies of spintronics have revealed the similarity between the function of a magnetic field and that of spin-orbit interactions in condensed matter. This suggests the existence of an undiscovered route to realize the conversion of fluid dynamics into electricity without using magnetic fields. Here we show electric voltage generation from fluid dynamics free from magnetic fields; we excited liquid-metal flows in a narrow channel and observed longitudinal voltage generation in the liquid. This voltage has nothing to do with electrification or thermoelectric effects, but turned out to follow a universal scaling rule based on a spin-mediated scenario. The result shows that the observed voltage is caused by spin-current generation from a fluid motion: spin hydrodynamic generation. The observed phenomenon allows us to make mechanical spin-current and electric generators, opening a door to fluid spintronics.

論文

Giant dielectric and magnetoelectric responses in insulating nanogranular films at room temperature

小林 伸聖*; 増本 博*; 高橋 三郎*; 前川 禎通

Nature Communications (Internet), 5, p.4417_1 - 4417_6, 2014/07

 被引用回数:25 パーセンタイル:15.35(Multidisciplinary Sciences)

これまでの誘電体材料の研究は主として結晶性セラミックスだったが、最近はマルチフェロイックスと言われる多機能性の材料が注目され、磁界による誘電率の応答特性の研究が盛んに行われている。しかしながら、その応答特性はマイナス170$$^{circ}$$C程度の極低温でなければ発現せず、実用デバイスに使用することは不可能だった。今回開発された材料は、誘電体中にナノメーターサイズの磁性粒子を均一分散させることで、室温で大きな磁化を有すると共にナノ量子効果による新しい誘電特性を期待できる。さらに、この現象が発現する機構を明らかにするため、理論的考察を行った結果、新しいナノ量子効果であるスピン依存電荷分極に基づく現象であることを解明した。

論文

Anomalous temperature dependence of current-induced torques in CoFeB/MgO heterostructures with Ta-based underlayers

Kim, J.*; Sinha, J.*; 三谷 誠司*; 林 将光*; 高橋 三郎*; 前川 禎通; 山ノ内 路彦*; 大野 英男*

Physical Review B, 89(17), p.174424_1 - 174424_8, 2014/05

 被引用回数:55 パーセンタイル:4.96(Materials Science, Multidisciplinary)

磁気トンネル接合素子に用いられるCoFeB/Mgoヘテロ構造膜の下地に用いられるTa膜の影響を調べた。Ta下地膜は電流による磁化反転に大きな効果を持っていることがわかった。この結果はスピン拡散モデルで解析された。

論文

Effect of anisotropic spin absorption on the Hanle effect in lateral spin valves

井土 宏*; 福間 康裕*; 高橋 三郎*; 前川 禎通; 大谷 義近*

Physical Review B, 89(8), p.081308_1 - 081308_5, 2014/02

 被引用回数:21 パーセンタイル:19.16(Materials Science, Multidisciplinary)

We have succeeded in fully describing dynamic properties of spin current including the different spin absorption mechanisms for longitudinal and transverse spins in lateral spin valves, which enable one to elucidate intrinsic spin transport and relaxation mechanisms in the nonmagnet. The deduced spin lifetimes are found independent of the contact type. From the transit-time distribution of spin current extracted from the Fourier transform in Hanle measurement data, the velocity of the spin current in Ag with Py/Ag Ohmic contact turns out much faster than that expected from the widely used model.

論文

Theory of spin Hall magnetoresistance

Chen, Y.-T.*; 高橋 三郎*; 中山 裕康*; Althammer, M.*; Goennenwein, S. T. B.*; 齊藤 英治; Bauer, G. E. W.*

Physical Review B, 87(14), p.144411_1 - 144411_9, 2013/04

 被引用回数:300 パーセンタイル:0.4(Materials Science, Multidisciplinary)

Ptを代表とするスピン軌道相互作用を持つ常磁性層と、イットリウム鉄ガーネットのような強磁性絶縁体からなる複合膜におけるスピンHall磁気抵抗効果についての理論を提案した。スピンHall磁気抵抗効果はスピンHall効果と逆スピンHall効果によって起きる非平衡な界面磁化に起因している。強磁性/常磁性2層膜および強磁性/常磁性/強磁性3層膜におけるスピンHall磁気抵抗効果を計算し、2層膜の計算がスピンHall磁気抵抗効果の実験結果を説明できること、3層膜では磁化が平行配置になることでスピンHall磁気抵抗効果の強度が増強されることが分かった。特に、3層膜におけるスピンHall磁気抵抗効果とスピン移行トルクが磁化配置によって制御可能であることが示された。

論文

Spin Hall effect

前川 禎通; 高橋 三郎

Spin Current (2nd Edition), p.194 - 208, 2012/10

We consider the effect of spin-orbit scattering on spin and charge transport in nonmagnetic metals (N) such as Cu, Al, and Ag, and discuss SHE by taking into account the side-jump (SJ) and skew-scattering (SS) mechanisms, and derive formulas for the SHE induced by spin-orbit scattering in nonmagnetic metals.

論文

Spin Hall effect in superconductors

高橋 三郎*; 前川 禎通

Japanese Journal of Applied Physics, 51(1), p.010110_1 - 010110_7, 2012/01

 被引用回数:11 パーセンタイル:44.64(Physics, Applied)

Spin Hall effect in a superconductor is theoretically studied. The spin injection from a ferromagnet into a superconductor creates quasiparticle spin and charge currents in SC, which generate charge and spin currents in the transverse direction to accumulate spin and charge imbalance near the side edges of a superconductor. A giant enhancement of spin and charge accumulation signals in the side jump and skew scattering mechanisms of the spin Hall effect is predicted to occur at low temperatures in the superconducting state.

論文

Electrically tunable spin injector free from the impedance mismatch problem

安藤 和也*; 高橋 三郎; 家田 淳一; 紅林 秀和*; Trypiniotis, T.*; Barnes, C. H. W.*; 前川 禎通; 齊藤 英治

Nature Materials, 10(9), p.655 - 659, 2011/09

 被引用回数:176 パーセンタイル:1.17(Chemistry, Physical)

Injection of spin currents into solids is crucial for exploring spin physics and spintronics. There has been significant progress in recent years in spin injection into high-resistivity materials, for example, semiconductors and organic materials, which uses tunnel barriers to circumvent the impedance mismatch; the impedance mismatch between ferromagnetic metals and high-resistivity materials drastically limits the spin-injection efficiency. However, because of this problem, there is no route for spin injection into these materials through low-resistivity interfaces, that is, Ohmic contacts, even though this promises an easy and versatile pathway for spin injection without the need for growing high-quality tunnel barriers. Here we show experimental evidence that spin pumping enables spin injection free from this condition; room-temperature spin injection into GaAs from Ni$$_{81}$$Fe$$_{19}$$ through an Ohmic contact is demonstrated through dynamical spin exchange. Furthermore, we demonstrate that this exchange can be controlled electrically by applying a bias voltage across a Ni$$_{81}$$Fe$$_{19}$$/GaAs interface, enabling electric tuning of the spin-pumping efficiency.

論文

Composite excitation of Josephson phase and spin waves in Josephson junctions with ferromagnetic insulator

挽野 真一*; 森 道康; 高橋 三郎; 前川 禎通

Journal of the Physical Society of Japan, 80(7), p.074707_1 - 074707_8, 2011/07

 被引用回数:5 パーセンタイル:54.24(Physics, Multidisciplinary)

Coupling of Josephson-phase and spin-waves is theoretically studied in a superconductor/ferromagnetic insulator/superconductor (S/FI/S) junction. Electromagnetic (EM) field inside the junction and the Josephson current coupled with spin-waves in FI are calculated by combining Maxwell and Landau-Lifshitz-Gilbert equations. In the S/FI/S junction, it is found that the current-voltage (I-V) characteristic shows two resonant peaks for each mode of the EM field. Voltages at the resonant peaks are obtained as a function of the normal modes of EM field, which indicates a composite excitation of the EM field and spin-waves in the S/FI/S junction. We also examine another type of junction, in which a nonmagnetic insulator (I) is located at one of interfaces between S and FI. In such an S/I/FFI/S junction, three resonant peaks appear in the I-V curve, since the Josephson-phase couples to the EM field in the I layer.

論文

Giant enhancement of spin accumulation and long-distance spin precession in metallic lateral spin valves

福間 康裕*; Le, W.*; 井土 宏*; 高橋 三郎; 前川 禎通; 大谷 義近*

Nature Materials, 10(7), p.527 - 531, 2011/07

 被引用回数:128 パーセンタイル:2.03(Chemistry, Physical)

The non-local spin injection in lateral spin valves is strongly expected to be an effective method to generate a pure spin current for potential spintronic application. However, the spin-valve voltage, which determines the magnitude of the spin current flowing into an additional ferromagnetic wire, is typically of the order of 1$$mu$$V. Here we show that lateral spin valves with low-resistivity NiFe/MgO/Ag junctions enable efficient spin injection with high applied current density, which leads to the spin-valve voltage increasing 100-fold. Hanle effect measurements demonstrate a long-distance collective 2$$pi$$ spin precession along a 6-$$mu$$m-long Ag wire. These results suggest a route to faster and manipulable spin transport for the development of pure spin-current-based memory, logic and sensing devices.

論文

Detection of spin-wave spin current in a magnetic insulator

梶原 瑛祐*; 高橋 三郎*; 前川 禎通; 齊藤 英治*

IEEE Transactions on Magnetics, 47(6), p.1591 - 1594, 2011/06

 被引用回数:6 パーセンタイル:54.54(Engineering, Electrical & Electronic)

Conduction electron can carry a flow spin angular momentum, a spin current. This paper describes another type of spin current: a spin current carried by spin waves. Some types of spin waves can carry a spin current, which can propagate both in insulators and metals. These two types of spin currents were found to exchange each other at a Pt/Y$$_{3}$$Fe$$_{5}$$O$$_{12}$$ interface.

論文

Inverse spin-Hall effect induced by spin pumping in metallic systems

安藤 和也*; 高橋 三郎; 家田 淳一; 梶原 瑛祐*; 中山 裕康*; 吉野 達郎*; 針井 一哉*; 藤川 安仁*; 松尾 衛*; 前川 禎通; et al.

Journal of Applied Physics, 109(10), p.103913_1 - 103913_11, 2011/05

 被引用回数:225 パーセンタイル:0.75(Physics, Applied)

強磁性/常磁性複層系において、スピンポンピングによって誘起された逆スピンホール効果(ISHE)の系統的な研究を行う。強磁性共鳴により駆動されたスピンポンピングが常磁性層にスピン流を注入し、常磁性層におけるISHEの作用によりスピン流に対して横向きに起電力が生じる。Ni$$_{81}$$Fe$$_{19}$$/Pt薄膜系では、強磁性共鳴条件において印加磁場の向きと直交する方向に起電力が発生することがわかった。起電力のスペクトル形状はローレンツ関数を用いてよく再現され、観測された起電力は完全にスピンポンピング誘起のISHEによるものであることを示している。すなわち、本実験ではその他の外的な磁気電圧効果は排除されている。起電力はマイクロ波強度,印加磁場の方向,薄膜の大きさに対し系統的に変化する。この振る舞いはLandau-Lifshitz-Gilbert方程式に基づく理論計算によりよく再現される。また、この起電力は、Ni$$_{81}$$Fe$$_{19}$$を強磁性絶縁体Y$$_3$$Fe$$_4$$GaO$$_{12}$$で置き換えたPt/Y$$_3$$Fe$$_4$$GaO$$_{12}$$複層膜系においても観測され、スピンポンピング誘起ISHEが起電力の起源となっていることを示唆している。

論文

Spin transfer torque in MTJs with synthetic ferrimagnetic layers by the Keldysh approach

市村 雅彦*; 濱田 智之*; 今村 裕志*; 高橋 三郎*; 前川 禎通

Journal of Applied Physics, 109(7), p.07C906_1 - 07C906_3, 2011/03

 被引用回数:4 パーセンタイル:80.83(Physics, Applied)

Based on the Keldysh Green's function method within the tight-binding model, the spin transfer torque is analyzed in a magnetic tunnel junction with a synthetic ferrimagnetic (SyF) free layer in the ballistic regime. The spin transfer torque exerted on the magnetizations of ferromagnetic bilayers in the SyF free layer tends to rotate the magnetizations in the same direction as a combined motion of the SyF free layer regardless of the bias direction. This rotation causes a reduction of the critical current for the magnetization switching of the SyF free layer which is consistent with experimental observations.

論文

Numerical study on the spin Seebeck effect

大江 純一郎; 安立 裕人; 高橋 三郎; 前川 禎通

Physical Review B, 83(11), p.115118_1 - 115118_5, 2011/03

 被引用回数:38 パーセンタイル:12.65(Materials Science, Multidisciplinary)

熱勾配によって誘起される強磁性絶縁体中のスピン波について理論解析を行った。特に、非磁性端子によって観測されるスピン波スピン流の電気的シグナルを計算できる数値シミュレーション法を開発した。揺動散逸定理により温度勾配中の磁化の運動を再現し、計算によって得られたシグナルは実験と良い一致を示した。

論文

Linear-response theory of spin Seebeck effect in ferromagnetic insulators

安立 裕人; 大江 純一郎; 高橋 三郎; 前川 禎通

Physical Review B, 83(9), p.094410_1 - 094410_6, 2011/03

 被引用回数:166 パーセンタイル:1.31(Materials Science, Multidisciplinary)

強磁性体に温度勾配を印可したときにスピン流を駆動する、いわゆるスピンゼーベック効果の線形応答理論を構築する。このアプローチでは、局在磁化の低エネルギー励起であるマグノンに着目する。われわれの線形応答に基づくアプローチは、ごく最近イットリウム鉄ガーネットという典型的な磁性絶縁体で観測されたスピンゼーベック効果を、定性的にも定量的にも記述可能であることを示す。

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