船戸 匠*; 山影 相*; 松尾 衛
Physical Review B, 106(21), p.214420_1 - 214420_7, 2022/12
We theoretically study the generation of spin current due to a surface acoustic wave (SAW) in a superconductor. We model an -wave superconductor as the mean-field Hamiltonian and calculate spin current generated via spin-vorticity coupling based on quantum kinetic theory. The results suggest that the spin current can be driven in a single-superconductor layer, and our estimation suggests that the detectable magnitude of the spin current can be generated in aluminum. Our proposal may contribute to the advancement of spin transport in superconductors from application and fundamental physics aspects.
大上 能悟*; 松尾 衛
Physical Review A, 106(4), p.L041501_1 - L041501_7, 2022/10
Mechanical rotation and oscillation have far lower frequencies than light does; thus they are not coupled to each other conventionally. In this Letter, we show that the torsional mechanical oscillation of an optical cavity can be coupled to the optical modes by introducing birefringence, which produces nondegenerate modes in the cavity: ordinary and extraordinary rays. Twisting the cavity mixes them and modulates the electromagnetic energy. We find the torsional optomechanical Hamiltonian by quantizing the total energy and reveal that the torsional oscillation can be resonantly driven by light.
船戸 匠*; 加藤 岳生*; 松尾 衛
Physical Review B, 106(14), p.144418_1 - 144418_10, 2022/10
We study spin pumping into an anisotropic Dirac electron system induced by microwave irradiation to an adjacent ferromagnetic insulator theoretically. We formulate the Gilbert damping enhancement due to the spin current flowing into the Dirac electron system using second-order perturbation with respect to the interfacial exchange coupling. As an illustration, we consider the anisotropic Dirac system realized in bismuth to show that the Gilbert damping varies according to the magnetization direction in the ferromagnetic insulator. Our results indicate that this setup can provide helpful information on the anisotropy of the Dirac electron system.
田島 裕之*; 大上 能悟*; 松尾 衛
Physical Review A, 106(3), p.033310_1 - 033310_8, 2022/09
We elucidate the multiparticle transport of pair tunneling and spin tunneling in strongly correlated interfaces. Not only usual single-particle tunneling but also interaction-induced multiparticle tunneling processes naturally arise from a conventional microscopic model without any empirical parameters, through the overlap of the many-body wave functions around the interface. We demonstrate how anomalous tunneling currents occur in a strongly interacting system due to the pair-tunneling process which we derived microscopically. Our formulation is useful for junction systems in various disciplines, including atomtronics, spintronics, and nuclear reactions.
大湊 友也*; 山影 相*; 加藤 岳生*; 松尾 衛
Physical Review B, 105(20), p.205406_1 - 205406_7, 2022/05
We investigate ferromagnetic resonance (FMR) modulation in -wave superconductor (SC)/ferromagnetic insulator (FI) bilayer systems theoretically. The modulation of the Gilbert damping in these systems reflects the existence of nodes in the -wave SC and shows power-law decay characteristics within the low-temperature and low-frequency limit. Our results indicate the effectiveness of the use of spin pumping as a probe technique to determine the symmetry of unconventional SCs with high sensitivity for nanoscale thin films.
大湊 友也*; 大上 能悟*; 松尾 衛
Physical Review B, 105(19), p.195409_1 - 195409_9, 2022/05
Angular momentum conversion between mechanical rotation and the valley degree of freedom in 2D Dirac materials is investigated theoretically. Coupling between the valley and vorticity of dynamic lattice distortions is derived by applying the method to 2D Dirac materials with an inertial effect. Lattice strain effects are also incorporated. Valley transfer and valley-dependent carrier localization are predicted using the dynamic lattice distortions. The transport properties are found to be controllable, allowing the system to be insulating and to generate pulsed charge current. Our formalism offers a route toward mechanical manipulation of valley dynamics in 2D Dirac materials.
船戸 匠*; 松尾 衛
Physical Review Letters, 128(7), p.077201_1 - 077201_6, 2022/02
The spin-motive force (SMF) in a simple ferromagnetic monolayer caused by a surface acoustic wave is studied theoretically via spin-vorticity coupling (SVC). The SMF has two mechanisms. The first is the SVC-driven SMF, which produces the first harmonic electromotive force, and the second is the interplay between the SVC and the magnetoelastic coupling, which produces the dc and second harmonic electromotive forces. We show that these electric voltages induced by a Rayleigh-type surface acoustic wave can be detected in polycrystalline nickel. No sophisticated device structures, noncollinear magnetic structures, or strong spin-orbit materials are used in our approach. Consequently, it is intended to broaden the spectrum of SMF applications considerably.
大上 能悟*; 松尾 衛
Physical Review B, 105(2), p.L020302_1 - L020302_5, 2022/01
We propose a spin transport induced by inertial motion. Our system is composed of two host media and a narrow vacuum gap in between. One of the hosts is sliding at a constant speed relative to the other. This mechanical motion causes the Doppler effect, which shifts the density of states and the nonequilibrium distribution function in the moving medium. Those shifts induce the difference in the distribution function between the two media, and they result in tunneling spin current. The spin current is calculated from the Schwinger-Keldysh formalism with a spin tunneling Hamiltonian. This scheme does not require temperature difference, voltage, or chemical potential.
泉田 渉*; 奥山 倫*; 佐藤 健太郎*; 加藤 岳生*; 松尾 衛
Physical Review Letters, 128(1), p.017701_1 - 017701_6, 2022/01
We propose a nanoscale rotor embedded between two ferromagnetic electrodes that is driven by spin injection. The spin-rotation coupling allows this nanorotor to continuously receive angular momentum from an injected spin under steady current flow between ferromagnetic electrodes in an antiparallel magnetization configuration. We develop a quantum theory of this angular-momentum transfer and show that a relaxation process from a precession state into a sleeping top state is crucial for the efficient driving of the nanorotor by solving the master equation. Our work clarifies a general strategy for efficient driving of a nanorotor.
中堂 博之; 今井 正樹; 松尾 衛; 前川 禎通; 齊藤 英治
Journal of the Physical Society of Japan, 90(8), p.081003_1 - 081003_11, 2021/08
We demonstrate observation of the angular momentum compensation temperature , at which the net angular momentum is quenched in ferrimagnets. Using the Barnett effect, in which an object is magnetized by mechanical rotation owing to spin-rotation coupling, we measure in the HoDyFeO system. We determine to be 240 K in HoFeO (HoIG). We find that increases with Dy content and show that of HoDyFeO corresponds with room temperature. We also demonstrate that Fe-NMR measurements can be applied to explore domain wall dynamics in HoIG. We find that the NMR intensity exhibits a maximum at in the multi-domain state. We provide a simple model for describing this NMR signal enhancement caused by enhancement of domain-wall mobility at .
立野 翔真*; 栗宗 勇貴*; 松尾 衛; 山野井 一人*; 能崎 幸雄*
Physical Review B, 104(2), p.L020404_1 - L020404_5, 2021/07
We demonstrate the phase shift of the Rayleigh-type surface acoustic wave (RSAW) in a ferromagnetic NiFe film based on the gyromagnetic Einstein-de Haas (EdH) torque that can be attributed to the back action of spin-wave excitation. The EdH torque modulates the transverse velocity of the bulk acoustic wave that causes the bipolar phase shift of RSAW. When compared with a magnetoelastic torque, the phase shift can be more prominently enhanced by increasing the frequency. The gyromagnetic torque in the RSAW paves the way.
藤本 純治*; 小椎八重 航*; 松尾 衛; 前川 禎通
Physical Review B, 103(22), p.L220402_1 - L220402_5, 2021/06
The Dzyaloshinskii-Moriya interaction, i.e., antisymmetric exchange interaction, combined with a Zeeman magnetic field gives rise to various magnetic states such as chiral, helical, and skyrmionic states. This interaction conventionally originates from spin-orbit coupling and thus needs somewhat heavy elements. In contrast, we here show a Dzyaloshinskii-Moriya interaction which is driven by electric current vorticity. We also find that the vorticity acts on localized spins as a Zeeman field, which may explain a recent experiment on current-driven magnetic skyrmion creation and annihilation without an external magnetic field in a device with a notch structure in FeGe. The theory explains the control of skyrmion creation and annihilation by current direction and opens different possibilities for studies of magnetic textures by using structural settings.
中堂 博之; 松尾 衛*; 前川 禎通*; 齊藤 英治
Physical Review B, 103(17), p.174308_1 - 174308_10, 2021/05
We report the observation of the Barnett field, rotational Doppler effect, and Berry phase using the rotating nuclear quadrupole resonance (NQR) methods. We have developed coil-spinning techniques that enable us to systematically study the effects of rotation in setups involving rotation of the signal detector, rotation of the sample, and simultaneous rotation of both the signal detector and sample. Applying these setups to NQR measurements, we observe NQR line splittings in which the spectral structures are clearly distinct among the setups. By analyzing these structures, we clarify the origin of the NQR line splittings and discuss the relationship between the rotational Doppler effect, Barnett field effect, and Berry phase in terms of the rotational degrees of freedom, such as the relative rotation and the sample rotation itself, and the observation frame of reference. We also provide clear evidence of the difference between the rotational Doppler effect and the Barnett field, and the equivalence of the Barnett field and the Berry phase.
栗宗 勇貴*; 松尾 衛; 前川 禎通; 能崎 幸雄*
Physical Review B, 102(17), p.1174413_1 - 1174413_7, 2020/11
A nonuniform vorticity of lattice deformation in a surface acoustic wave (SAW) can generate a spin current (SC) in nonmagnetic metals via spin-vorticity coupling (SVC). We demonstrated a strong enhancement of SVC derived SC generated in Cu and Pt films with increasing the frequency of the SAW by observing the spin-wave resonance (SWR) in an adjacent NiFe film. The comparative amplitudes and high-order frequency variations of SWR in NiFe/Cu and NiFe/Pt bilayers imply that the amplitude of the SC generated via SVC in a SAW is robust against the strength of spin-orbit interaction in nonmagnetic metals.
中堂 博之; 松尾 衛*; 針井 一哉*; 前川 禎通*; 齊藤 英治
Applied Physics Express, 13(10), p.109102_1 - 109102_2, 2020/10
バーネット効果とは物体を回転させるとその物体が磁化する現象である。これは磁気モーメントに対して回転座標系での慣性力として磁場が働くと解釈できる。その磁場をバーネット磁場と呼ぶ。バーネット磁場は慣性力であるために、高速で回転する物体と同じ速度で回転する測定系が必要である。我々は新規にコイル回転法を独自開発し、その共振回路を用いて核磁気共鳴法(NMR)測定を行ったところ、核磁気モーメントに働くバーネット磁場によってNMRの共鳴周波数が回転数に比例してシフトするという結果を得た。この成果は2014年にApplied Physics Expressに掲載されたが、2018年の夏、Jean Jeener人物から、我々が測定したものはバーネット磁場ではなく、コイルを回転させた事による人為的な効果であるとの批判が出た。本論文はこれに対する我々の返答である。
立野 翔真*; 岡野 元基*; 松尾 衛; 能崎 幸雄*
Physical Review B, 102(10), p.104406_1 - 104406_9, 2020/09
In many interacting-electron systems, the microscopic spin angular momentum of electrons is conserved along with the macroscopic angular momentum in electrical current flows, i.e., vorticity. Such spin-vorticity coupling expands the choice of materials for spintronics devices. In this paper, we evaluate the magnitude of an alternating spin current generated by the spin-vorticity coupling with a gigahertz-order surface acoustic wave in a Cu thin film. We measure the gigahertz alternating spin current by an electrical method based on the inverse spin-Hall effect. From the amplitude of the spin current, we can determine the conversion efficiency of the angular momentum between local lattice rotation and electron spin in the Cu film. The conversion efficiency is four orders of magnitude larger than the case of spin current generation via kilohertz-order vorticity in turbulent flow of liquid mercury.
加藤 岳生*; 大沼 悠一*; 松尾 衛
Physical Review B, 102(9), p.094437_1 - 094437_10, 2020/09
We consider a microscopic theory for the spin Hall magnetoresistance (SMR). We generally formulate a spin conductance at an interface between a normal metal and a magnetic insulator in terms of spin susceptibilities. We reveal that SMR is composed of static and dynamic parts. The static part, which is almost independent of the temperature, originates from spin flip caused by an interfacial exchange coupling. However, the dynamic part, which is induced by the creation or annihilation of magnons, has an opposite sign from the static part. By the spin-wave approximation, we predict that the latter results in a nontrivial sign change in the SMR signal at a finite temperature. In addition, we derive the Onsager relation between spin conductance and thermal spin-current noise.
洞口 泰輔*; 松尾 衛; 能崎 幸雄*
Journal of Magnetism and Magnetic Materials, 505, p.166727_1 - 166727_5, 2020/07
Spin-torque ferromagnetic resonance (ST-FMR) is a powerful tool to evaluate spin-torque efficiency (), the efficiency of current-induced torque in metallic bilayers consisting of ferromagnetic and nonmagnetic materials. In general, can be evaluated from the amplitude ratio between symmetric and anti-symmetric components of the ST-FMR spectrum. However, the ratio is also affected by an unnecessary magnetic field owing to the asymmetric alternating current flow used to excite the ST-FMR. In this article, we demonstrate that can be precisely evaluated from the full-angular dependence of the ST-FMR spectrum even though the distribution of alternating current flow is highly asymmetric.
今井 正樹; 中堂 博之; 松尾 衛; 前川 禎通; 齊藤 英治
Physical Review B, 102(1), p.014407_1 - 014407_5, 2020/07
The angular momentum compensation temperature of ferrimagnets has attracted much attention because of high-speed magnetic dynamics near . We show that NMR can be used to investigate domain wall dynamics near in ferrimagnets. We performed Fe-NMR measurements on the ferrimagnet HoFeO with = 245 K. In a multi-domain state, the NMR signal is enhanced by domain wall motion. We found that the NMR signal enhancement shows a maximum at in the multi-domain state. The NMR signal enhancement occurs due to increasing domain-wall mobility toward . We develop the NMR signal enhancement model involves domain-wall mobility. Our study shows that NMR in multi-domain state is a powerful tool to determine , even from a powder sample and it expands the possibility of searching for angular momentum-compensated materials.
高橋 遼*; 中堂 博之; 松尾 衛; 針井 一哉*; 大沼 悠一*; 前川 禎通; 齊藤 英治
Nature Communications (Internet), 11, p.3009_1 - 3009_6, 2020/06
Hydrodynamic motion can generate a flux of electron-spin's angular momentum via the coupling between fluid rotation and electron spins. Such hydrodynamic generation, called spin hydrodynamic generation (SHDG), has recently attracted attention in a wide range of fields, especially in spintronics. Spintronics deals with spin-mediated interconversion taking place on a micro or nano scale because of the spin-diffusion length scale. To be fully incorporated into the interconversion, SHDG physics should also be established in such a minute scale, where most fluids exhibit a laminar flow. Here, we report electric voltage generation due to the SHDG in a laminar flow of a liquid-metal mercury. The experimental results show a scaling rule unique to the laminar-flow SHDG. Furthermore, its energy con- version efficiency turns out to be about 105 greater than of the turbulent one. Our findings reveal that the laminar-flow SHDG is suitable to downsizing and to extend the coverage of fluid spintronics.