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論文

Triplon current generation in solids

Chen, Y.*; 佐藤 正寛*; Tang, Y.*; 塩見 雄毅*; 大柳 洸一*; 益田 隆嗣*; 南部 雄亮*; 藤田 全基*; 齊藤 英治

Nature Communications (Internet), 12, p.5199_1 - 5199_7, 2021/08

 被引用回数:2 パーセンタイル:30.58(Multidisciplinary Sciences)

A triplon refers to a fictitious particle that carries angular momentum $$S=1$$ corresponding to the elementary excitation in a broad class of quantum dimerized spin systems. Such systems without magnetic order have long been studied as a testing ground for quantum properties of spins. Although triplons have been found to play a central role in thermal and magnetic properties in dimerized magnets with singlet correlation, a spin angular momentum flow carried by triplons, a triplon current, has not been detected yet. Here we report spin Seebeck effects induced by a triplon current: triplon spin Seebeck effect, using a spin-Peierls system CuGeO$$_3$$. The result shows that the heating-driven triplon transport induces spin current whose sign is positive, opposite to the spin-wave cases in magnets. The triplon spin Seebeck effect persists far below the spin-Peierls transition temperature, being consistent with a theoretical calculation for triplon spin Seebeck effects.

論文

Observation of nuclear-spin Seebeck effect

吉川 貴史*; Reitz, D.*; 伊藤 宏陽*; 巻内 崇彦*; 杉本 宜陽*; 恒川 翔*; 大門 俊介*; 大柳 洸一*; Ramos, R.*; 高橋 三郎*; et al.

Nature Communications (Internet), 12, p.4356_1 - 4356_7, 2021/07

 被引用回数:11 パーセンタイル:87.43(Multidisciplinary Sciences)

Thermoelectric effects have been applied to power generators and temperature sensors that convert waste heat into electricity. The effects, however, have been limited to electrons to occur, and inevitably disappear at low temperatures due to electronic entropy quenching. Here, we report thermoelectric generation caused by nuclear spins in a solid: nuclear-spin Seebeck effect. The sample is a magnetically ordered material MnCO$$_3$$ having a large nuclear spin $$(I=5/2)$$ of $$^{55}$$Mn nuclei and strong hyperfine coupling, with a Pt contact. In the system, we observe low-temperature thermoelectric signals down to 100 mK due to nuclear-spin excitation. Our theoretical calculation in which interfacial Korringa process is taken into consideration quantitatively reproduces the results. The nuclear thermoelectric effect demonstrated here offers a way for exploring thermoelectric science and technologies at ultralow temperatures.

論文

Modulation of Dirac electrons in epitaxial Bi$$_2$$Se$$_3$$ ultrathin films on van der Waals ferromagnet Cr$$_2$$Si$$_2$$Te$$_6$$

加藤 剛臣*; 菅原 克明*; 伊東 直洋*; 山内 邦彦*; 佐藤 匠*; 小口 多美夫*; 高橋 隆*; 塩見 雄毅*; 齊藤 英治; 佐藤 宇史*

Physical Review Materials (Internet), 4(8), p.084202_1 - 084202_6, 2020/08

 被引用回数:3 パーセンタイル:22.66(Materials Science, Multidisciplinary)

We investigated the Dirac-cone state and its modulation when an ultrathin film of topological insulator Bi$$_2$$Se$$_3$$ was epitaxially grown on a van der Waals ferromagnet Cr$$_2$$Si$$_2$$Te$$_6$$ (CST) by angle-resolved photoemission spectroscopy. We observed a gapless Dirac-cone surface state in six quintuple-layer (6QL) Bi$$_2$$Se$$_3$$ on CST, whereas the Dirac cone exhibits a gap of 0.37 eV in its 2QL counterpart. Intriguingly, this gap is much larger than those for Bi$$_2$$Se$$_3$$ films on Si(111). We also revealed no discernible change in the gap magnitude across the ferromagnetic transition of CST, suggesting the very small characteristic length and energy scale of the magnetic proximity effect. The present results suggest a crucial role of interfacial coupling for modulating Dirac electrons in topological-insulator hybrids.

論文

Observation of magnon polarization

南部 雄亮*; Barker, J.*; 沖野 夕貴*; 吉川 貴史*; 塩見 雄毅*; Enderle, M.*; Weber, T.*; Winn, B.*; Graves-Brook, M.*; Tranquada, J. M.*; et al.

Physical Review Letters, 125(2), p.027201_1 - 027201_6, 2020/07

 被引用回数:27 パーセンタイル:91.71(Physics, Multidisciplinary)

We measure the mode-resolved direction of the precessional motion of the magnetic order, i.e., magnon polarization, via the chiral term of inelastic polarized neutron scattering spectra. The magnon polarization is a unique and unambiguous signature of magnets and is important in spintronics, affecting thermodynamic properties such as the magnitude and sign of the spin Seebeck effect. However, it has never been directly measured in any material until this work. The observation of both signs of magnon polarization in Y$$_3$$Fe$$_5$$O$$_{12}$$ also gives direct proof of its ferrimagnetic nature. The experiments agree very well with atomistic simulations of the scattering cross section.

論文

Electric readout of magnetic stripes in insulators

Chen, Y.*; 塩見 雄毅*; Qiu, Z.*; 新関 友彦*; 埋田 真樹*; 齊藤 英治

Scientific Reports (Internet), 9, p.19052_1 - 19052_8, 2019/12

 被引用回数:0 パーセンタイル:0(Multidisciplinary Sciences)

In superconductors, a topological configuration of the superconducting order parameter called a superconducting vortex carries magnetization. Such a magnetic topological object behaves like a minute particle generating a magnetic flux. Since the flux is localized with a nanometer scale, the vortex provides a nano-scale probe for local magnetic fields. Here we show that information of magnetic stripes in insulators can be read out by using vortices in an adjacent superconductor film as a probe. The orientation and width of magnetic micro stripes are both transcribed into resistance change of the superconductor through the modulation of vortex mobility affected by local magnetization. By changing the direction of external magnetic fields, zero-field resistance changes continuously according to the stripe orientation, and its modulation magnitude reaches up to 100%. The width of the stripes can also be estimated from the oscillatory magnetoresistance. Our results demonstrate a new possibility for non-volatile analog memory devices based on topological objects.

論文

Magnon pairs and spin-nematic correlation in the spin Seebeck effect

廣部 大地*; 佐藤 正寛*; 萩原 雅人*; 塩見 雄毅*; 益田 隆嗣*; 齊藤 英治

Physical Review Letters, 123(11), p.117202_1 - 117202_7, 2019/09

 被引用回数:8 パーセンタイル:62.78(Physics, Multidisciplinary)

Investigating exotic magnetic materials with spintronic techniques is effective at advancing magnetism as well as spintronics. In this work, we report unusual field-induced suppression of the spin Seebeck effect (SSE) in a quasi-one-dimensional frustrated spin-$$frac{1}{2}$$ magnet LiCuVO$$_4$$, known to exhibit spin-nematic correlation in a wide range of external magnetic field $$B$$. The suppression takes place above $$|B| gtrsim 2$$ T in spite of the $$B$$-linear isothermal magnetization curves in the same $$B$$ range. The result can be attributed to the growth of the spin-nematic correlation while increasing $$B$$. The correlation stabilizes magnon pairs carrying spin 2, thereby suppressing the interfacial spin injection of SSE by preventing the spin-1 exchange between single magnons and conduction electrons at the interface. This interpretation is supported by integrating thermodynamic measurements and theoretical analysis on the SSE.

論文

Spin Seebeck effect in the layered ferromagnetic insulators CrSiTe$$_3$$ and CrGeTe$$_3$$

伊東 直洋*; 吉川 貴史*; Barker, J.*; 廣部 大地*; 塩見 雄毅*; 齊藤 英治

Physical Review B, 100(6), p.060402_1 - 060402_6, 2019/08

 被引用回数:27 パーセンタイル:85.25(Materials Science, Multidisciplinary)

We have studied the longitudinal spin Seebeck effect (LSSE) in the layered ferromagnetic insulators CrSiTe$$_3$$ and CrGeTe$$_3$$ covered by Pt films in the measurement configuration where spin current traverses the ferromagnetic Cr layers. The LSSE response is clearly observed in the ferromagnetic phase and, in contrast to a standard LSSE magnet Y$$_3$$Fe$$_5$$O$$_{12}$$, persists above the critical temperatures in both CrSiTe$$_3$$/Pt and CrGeTe$$_3$$/Pt samples. With the help of a numerical calculation, we attribute the LSSE signals observed in the paramagnetic regime to exchange-dominated interlayer transport of in-plane paramagnetic moments reinforced by short-range ferromagnetic correlations and strong Zeeman effects.

論文

Spin Seebeck mechanical force

針井 一哉; Seo, Y.-J.*; 堤 康雅*; 中堂 博之; 大柳 洸一*; 松尾 衛; 塩見 雄毅*; 小野 崇人*; 前川 禎通; 齊藤 英治

Nature Communications (Internet), 10, p.2616_1 - 2616_5, 2019/06

 被引用回数:27 パーセンタイル:84.51(Multidisciplinary Sciences)

Electric current has been used to send electricity to far distant places. On the other hand, spin current, a flow of electron spin, can in principle also send angular momentum to distant places. In a magnet, there is a universal spin carrier called a spin wave, a wave-type excitation of magnetization. Since spin waves exhibit a long propagation length, it should be able to send angular momentum that can generate torque and force at a distant place: a new function of magnets. Here we observe mechanical angular momentum transmission and force generation due to spin waves injected into Y$$_3$$Fe$$_5$$O$$_{12}$$ by the spin-Seebeck effect. The spin-wave current, transmitted through a Y$$_3$$Fe$$_5$$O$$_{12}$$ micro cantilever, was found to create a mechanical force on the cantilever as a non-local reaction of the spin-Seebeck effect. Spin-wave current can be generated remotely even in open circuits, and it can be used to drive micro mechanical devices.

論文

Microscopic formulation of nonlinear spin current induced by spin pumping

亀田 麻衣*; 廣部 大地*; 大門 俊介*; 塩見 雄毅*; 高橋 三郎*; 齊藤 英治

Journal of Magnetism and Magnetic Materials, 476, p.459 - 463, 2019/04

 被引用回数:1 パーセンタイル:6.16(Materials Science, Multidisciplinary)

Nonlinear effects of spin pumping have been investigated by numerically calculating the temporal evolution of the magnon number affected by three- and four-magnon interactions. We showed that the three-magnon interaction gives rise to the enhancement of spin-pumping spin current. We also found that the four-magnon interaction in combination with the three-magnon interaction can be responsible for the experimentally observed low-frequency oscillation of spin current.

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:24 パーセンタイル:82.88(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

Spin pumping from nuclear spin waves

塩見 雄毅*; Lustikova, J.*; 渡辺 真悟*; 廣部 大地*; 高橋 三郎*; 齊藤 英治

Nature Physics, 15(1), p.22 - 26, 2018/10

 被引用回数:8 パーセンタイル:59.11(Physics, Multidisciplinary)

Various spintronic phenomena originate from the exchange of angular momentum between the spin of electrons and other degrees of freedom in crystalline materials. Many degrees of freedom, such as magnetization and mechanical motion, have already been united into this exchange framework. However, the nuclear spin -a key angular momentum- has yet to be incorporated. Here we observe spin pumping from nuclear magnetic resonance (NMR), in which nuclear spin dynamics emits a spin current, a flow of spin angular momentum of electrons. By using the canted antiferromagnet MnCO$$_3$$, in which typical nuclear spin-wave formation is established due to the reinforced hyperfine coupling, we find that a spin current is generated from an NMR. Nuclear spins are indispensable for quantum information technology and are also frequently used in various sensors, such as in magnetic resonance imaging. The observed NMR spin pumping allows spin-current generation from nuclei and will enable spintronic detection of nuclear spin states.

論文

Experimental evidence consistent with a magnon Nernst effect in the antiferromagnetic insulator MnPS$$_3$$

塩見 雄毅*; 高嶋 梨菜*; 齊藤 英治

Physical Review B, 96(13), p.134425_1 - 134425_9, 2017/10

 被引用回数:67 パーセンタイル:93.69(Materials Science, Multidisciplinary)

A magnon Nernst effect, an antiferromagnetic analog of the magnon Hall effect in ferromagnetic insulators, has been studied experimentally for the layered antiferromagnetic insulator MnPS$$_3$$ in contact with two Pt strips. Thermoelectric voltage in the Pt strips grown on MnPS$$_3$$ single crystals exhibits nonmonotonic temperature dependence at low temperatures, which is unlikely to be explained by electronic origins in Pt but can be ascribed to the inverse spin Hall voltage induced by a magnon Nernst effect. Control of antiferromagnetic domains in the MnPS$$_3$$ crystal by magnetoelectric cooling is found to modulate the low-temperature thermoelectric voltage in Pt, which is evidence consistent with the emergence of the magnon Nernst effect in Pt-MnPS$$_3$$ hybrid structures.

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:5 パーセンタイル:28.01(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

Oscillatory Nernst effect in Pt$$|$$ferrite$$|$$cuprate-superconductor trilayer films

塩見 雄毅*; Lustikova, J.*; 齊藤 英治

Scientific Reports (Internet), 7, p.5358_1 - 5358_8, 2017/07

 被引用回数:4 パーセンタイル:22.81(Multidisciplinary Sciences)

Although magnetism and superconductivity hardly coexist in a single material, recent advances in nanotechnology and spintronics have brought to light their interplay in magnetotransport in thin-film heterostructures. Here, we found a periodic oscillation of Nernst voltage with respect to magnetic fields in Pt$$|$$LiFe$$_5$$O$$_8$$ (Pt$$|$$LFO) bilayers grown on a cuprate superconductor YBa$$_2$$Cu$$_3$$O$$_{7-x}$$ (YBCO). At high temperatures above the superconducting transition temperature ($$T_C$$) of YBCO, spin Seebeck voltages originating in Pt$$|$$LFO layers are observed. As temperature decreases well below $$T_C$$, the spin Seebeck voltage is suppressed and unconventional periodic voltage oscillation as a function of magnetic fields appears; such an oscillation emerging along the Hall direction in the superconducting state has not been observed yet. Dynamics of superconducting vortices pinned by surface precipitates seems responsible for the oscillatory Nernst effect.

論文

Linear magnetoresistance in a topological insulator Ru$$_2$$Sn$$_3$$

塩見 雄毅*; 齊藤 英治

AIP Advances (Internet), 7(3), p.035011_1 - 035011_6, 2017/03

 被引用回数:5 パーセンタイル:28.01(Nanoscience & Nanotechnology)

We have studied magnetotransport properties of a topological insulator material Ru$$_2$$Sn$$_3$$. Bulk single crystals of Ru$$_2$$Sn$$_3$$ were grown by a Bi flux method. The resistivity is semiconducting at high temperatures above 160 K, while it becomes metallic below 160 K. Nonlinear field dependence of Hall resistivity in the metallic region shows conduction of multiple carriers at low temperatures. In the high-temperature semiconducting region, magnetoresistance exhibits a conventional quadratic magnetic-field dependence. In the low-temperature metallic region, however, high-field magnetoresistance is clearly linear with magnetic fields, signaling a linear dispersion in the low-temperature electronic structure. Small changes in the magnetoresistance magnitude with respect to the magnetic field angle indicate that bulk electron carriers are responsible mainly for the observed linear magnetoresistance.

論文

Fabrication and magnetic control of Y$$_3$$Fe$$_5$$O$$_{12}$$ cantilevers

Seo, Y.-J.*; 針井 一哉; 高橋 遼*; 中堂 博之; 大柳 洸一*; Qiu, Z.*; 小野 崇人*; 塩見 雄毅*; 齊藤 英治

Applied Physics Letters, 110(13), p.132409_1 - 132409_4, 2017/03

 被引用回数:12 パーセンタイル:55.52(Physics, Applied)

フェリ磁性体であるY$$_3$$Fe$$_5$$O$$_{12}$$を用いたサブミクロンカンチレバーを収束イオンビーム法によって作成し、その振動特性を磁場によって制御した。カンチレバーは縦横2つの振動モードを示し、それぞれのモードは外部磁場の印加によって共鳴周波数が減少、増大した。この結果は磁気力の磁場変化を取り扱った数値シミュレーションでよく再現された。周波数変化は数%に及ぶことから、磁場による高効率な振動制御が行われたと結論できる。

論文

One-dimensional spinon spin currents

廣部 大地*; 佐藤 正寛*; 川股 隆行*; 塩見 雄毅*; 内田 健一*; 井口 亮*; 小池 洋二*; 前川 禎通; 齊藤 英治

Nature Physics, 13(1), p.30 - 34, 2017/01

 被引用回数:81 パーセンタイル:96.62(Physics, Multidisciplinary)

Quantum spin fluctuation in a low-dimensional or frustrated magnet breaks magnetic ordering while keeping spin correlation. Such fluctuation has been a central topic in magnetism because of its relevance to high-Tc superconductivity and topological states. However, utilizing such spin states has been quite difficult. In a one-dimensional spin-1/2 chain, a particle-like excitation called a spinon is known to be responsible for spin fluctuation in a paramagnetic state. Spinons behave as a Tomonaga-Luttinger liquid at low energy, and the spin system is often called a quantum spin chain. Here we show that a quantum spin chain generates and carries spin current, which is attributed to spinon spin current. This is demonstrated by observing an anisotropic negative spin Seebeck effect along the spin chains in Sr$$_{2}$$CuO$$_{3}$$. The results show that spin current can flow even in an atomic channel owing to long-range spin fluctuation.

論文

Universal scaling for the spin-electricity conversion on surface states of topological insulators

山本 敬太*; 塩見 雄毅*; 瀬川 耕司*; 安藤 陽一*; 齊藤 英治

Physical Review B, 94(2), p.024404_1 - 024401_6, 2016/05

 被引用回数:16 パーセンタイル:62.97(Materials Science, Multidisciplinary)

スピンポンピング技術を用いてバルク絶縁トポロジカルインシュレータ(TI)材料の表面状態のスピン-電気変換を調べた。サンプル構造はNi-Fe/Cu/TI三層であり、TI表面上の磁気近接効果はCu層の挿入により無視できるほど小さい。スピン-電気変換によって生成される電圧信号は、より低い温度で支配的な表面輸送に沿って、観察され、温度が低下するにつれて増強される。スピン-電気変換の効率は、表面スピン-電気変換と一致して、バルク状態の抵抗率が高く、表面状態の平均自由行程が長いTIサンプルの方が大きい。

論文

Dispersion-type Hall resistance in InSb$$|$$Pt hybrid systems

塩見 雄毅*; 齊藤 英治

Scientific Reports (Internet), 6, p.22085_1 - 22085_6, 2016/02

 被引用回数:3 パーセンタイル:29.89(Multidisciplinary Sciences)

In nonmagnetic semiconductors and metals, most of Hall resistance exhibits a linear dependence with applied magnetic fields. In this work, by combining conduction in a metal and a semiconductor under external magnetic fields, we realize a dispersion-type magnetic-field dependence of Hall resistance. The dispersion-type Hall resistance appears in a broad temperature range below 150K, where quantum linear magnetoresistance is noticeable in the semiconductor substrate. This unconventional Hall response in metal$$|$$semiconductor hybrid systems is explained by a change in dominant conduction from the semiconductor to the metal with increasing magnetic field strength.

論文

Vector spectroscopy for spin pumping

Lustikova, J.*; 塩見 雄毅*; 齊藤 英治

Physical Review B, 92(22), p.224436_1 - 224436_8, 2015/12

 被引用回数:14 パーセンタイル:55.18(Materials Science, Multidisciplinary)

We propose a method to separate the inverse spin Hall effect (ISHE) from galvanomagnetic effects in spin pumping experiments on metallic bilayer systems by measuring the dc electromotive force in two orthogonal directions. Calculations of dc voltages in longitudinal and Hall directions induced in Ni$$_{81}$$Fe$$_{19}$$ and Ni$$_{81}$$Fe$$_{19}$$/Pt films at ferromagnetic resonance in a microwave cavity predict that contributions from ISHE and from the galvanomagnetic effects, i.e., the anisotropic magnetoresistance and the anomalous Hall effect. exhibit distinct signal symmetry as well as angular dependence when changing the direction of the external field with respect to the film plane. According to measurements on Ni$$_{81}$$Fe$$_{19}$$/Pt, only that dc voltage component which includes ISHE is more than five times larger than purely galvanomagnetic components. This is corroborated by results on La$$_{0.67}$$Sr$$_{0.33}$$MnO$$_{3}$$/Pt samples, demonstrating universality of this method.

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