論文 - 能崎　幸雄

Reversal of Spin-Torque Polarity with Inverting Current Vorticity in Composition-Graded Layer at the Ti/W Interface

Nakayama H., Horaguchi T., Uzuhashi J., He C., Sukegawa H., Ohkubo T., Mitani S., Yamanoi K., Nozaki Y.

Advanced Electronic Materials （Wiley）     2400797-1 - 2400797-9 2025年04月

筆頭著者, 最終著者, 責任著者, 査読有り,  ISSN  2199-160X

　概要を見る

While compositional gradient-induced spin-current generation is explored, its microscopic mechanisms remain poorly understood. Here, the contribution of polarity of compositional gradient on spin-current generation is explored. A nanoscale compositional gradient, formed by in situ atomic diffusion of ultrathin Ti and W layers, is introduced between 10-nm-thick W and Ti layers. Spin-torque ferromagnetic resonance in ferromagnetic Ni95Cu5 deposited on this gradient reveals that a moderate compositional gradient suppresses negative spin torque from the spin Hall effect in W. In contrast, reversing the Ti/W stacking order, which inverts the gradient, suppresses positive spin torque from the orbital Hall effect in Ti. These findings suggest that the sign of spin torque is governed by the polarity of compositional gradient, providing a novel strategy for efficient spin-torque generation without relying on materials with strong spin or orbital Hall effect.

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Gyro-spintronic material science using vorticity gradient in solids

Nozaki Y., Sukegawa H., Watanabe S., Yunoki S., Horaguchi T., Nakayama H., Yamanoi K., Wen Z., He C., Song J., Ohkubo T., Mitani S., Maezawa K., Nishikawa D., Fujii S., Matsuo M., Fujimoto J., Maekawa S.

Science and Technology of Advanced Materials （Informa UK Limited）  26 （ 1 ） 2428153-1 - 2428153-39 2025年

査読有り,  ISSN  14686996

　概要を見る

We present a novel method for generating spin currents using the gyromagnetic effect, a phenomenon discovered over a century ago. This effect, crucial for understanding the origins of magnetism, enables the coupling between various macroscopic rotational motions and electron spins. While higher rotational speeds intensify the effect, conventional mechanical rotations, typically, below 10,000 RPM, produce negligible results comparable to geomagnetic fluctuations, limiting applied research. Our studies demonstrate that spin current generation comparable to that of rare metals can be achieved through atomic rotations induced by GHz-range surface acoustic waves and the rotational motion of conduction electrons in metallic thin films with nanoscale gradient modulation of electrical conductivity. These effects, termed the acoustic gyromagnetic effect and the current-vorticity gyromagnetic effect, are significant in different contexts. The acoustic gyromagnetic effect is notable in high-conductivity materials like aluminum and copper, which are more abundant than conventional spintronics materials with strong spin–orbit interactions (SOIs). Conversely, the current-vorticity gyromagnetic effect requires a large conductivity gradient to produce current vorticity efficiently. This is achieved by using composition gradient structures from highly conductive metals to poorly conductive oxides or semiconductors. Consequently, unlike traditional strong-SOI materials, we can create highly efficient spin current generators with low energy dissipation due to reduced Joule loss.

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Spatiotemporal visualization of a surface acoustic wave coupled to magnons across a submillimeter-long sample by pulsed laser interferometry

Maezawa K., Fujii S., Yamanoi K., Nozaki Y., Watanabe S.

Physical Review Applied （American Physical Society (APS)）  21 （ 4 ） 044047-1 - 044047-13 2024年04月

査読有り

　概要を見る

Surface acoustic waves (SAWs) coupled to magnons have attracted much attention because they allow for the long-range transport of magnetic information that cannot be achieved by magnon alone. We employed pulsed laser interferometry to visualize the entire spatiotemporal dynamics of a SAW that travels on a nickel (Ni) thin film and is coupled to magnons. It was possible to trace the coupling-induced amplitude reduction and phase shift that occurs as the SAW propagates over a distance of 0.4 mm. The observed changes are consistent with results obtained from conventional radio-frequency transmission measurements, which probe the total SAW absorption due to magnon-phonon coupling. This result verifies that our method can accurately capture the spatiotemporal dynamics of a SAW coupled to magnons across the entire length of the sample. Additionally, we validated our time-resolved profiles by comparing them with theoretical results that take the echo wave due to reflection into account. The impact of the echo wave is significant even when it has propagated over a distance of the order of millimeters. Our imaging results highlight the visualization of the long-range propagation of the SAW coupled to magnons and offer more information about the surface vibration profiles in such devices.

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Enhanced orbital torque efficiency in nonequilibrium Ru<inf>50</inf>Mo<inf>50</inf>(0001) alloy epitaxial thin films

Tang K., He C., Wen Z., Sukegawa H., Ohkubo T., Nozaki Y., Mitani S.

APL Materials （AIP Publishing）  12 （ 3 ） 031131-1 - 031131-7 2024年03月

査読有り

　概要を見る

Epitaxial thin films of fully nonequilibrium hcp-Ru50Mo50(0001) nanoalloys were prepared as a chemically disordered alloy, in which the intrinsic spin Hall effect is expected to be negligible. Structural analyses confirmed the epitaxial growth and atomic scale alloying of the films. In contrast to a tiny torque efficiency (ξDL) of ∼0.4% for Ru50Mo50/CoFeB, the ξDL for the Ru50Mo50/Ni heterostructure reached ∼30% with a long-range relaxation length. The apparent dependence of ξDL on the ferromagnetic layer can be attributed to the orbital Hall effect (OHE). Interestingly, a smaller ξDL was observed for Ru/Ni, suggesting that the nonequilibrium Ru50Mo50 enhances its OHE. Furthermore, the enhanced ξDL is maintained by inserting a Ru layer between the Ru50Mo50 and Ni layers, showing orbital transport through Ru. This finding illustrates potential applications of nonequilibrium nanoalloy films in spin orbitronics and contributes to getting insights into the understanding of the interrelationships between nanostructures and orbital transport properties.

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Y<inf>3</inf>Fe<inf>5</inf>O<inf>12</inf> film with multi-domain epitaxy on single-crystalline LiNbO<inf>3</inf> substrate

Yamanoi K., Hase K., Komori S., Taniyama T., Nozaki Y.

APL Materials （AIP Publishing）  12 （ 2 ）  2024年02月

査読有り

　概要を見る

Y3Fe5O12 is one of the magnetic insulators that can realize high-speed and low-power-consuming spintronics devices. However, it is hard to prepare a high-quality Y3Fe5O12 film via a conventional sputtering process owing to its low crystalline texture, which leads to a crucial increase in the Gilbert damping constant. Moreover, a single crystalline Gd3Ga5O12 substrate, whose lattice constant is well matched with Y3Fe5O12, is indispensable to improve the crystallinity of the Y3Fe5O12 film. In this article, we demonstrated an epitaxial growth of multiple domains for a 30-nm-thick Y3Fe5O12 film by means of magnetron sputtering on a single crystalline 128° Y-X LiNbO3 substrate , which has been widely utilized in surface acoustic wave devices. From the pole figure of x-ray diffraction, an oblique epitaxial growth of Y3Fe5O12(400) is successfully observed on the 128° Y-X LiNbO3 substrate after a high-temperature post-annealing. The saturation magnetization is equivalent to the value of the epitaxial Y3Fe5O12 film on the Gd3Ga5O12 substrate. The relatively low effective Gilbert damping constant of 0.0039 also supports the high crystalline texture of the Y3Fe5O12 film. The developed growth technique will pave the way for the application of the Y3Fe5O12 film on magneto-acoustic devices.

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Charge-to-spin conversion in fully epitaxial Ru/Cu hybrid nanolayers with interface control

Song J., He C., Scheike T., Wen Z., Sukegawa H., Ohkubo T., Nozaki Y., Mitani S.

Nanotechnology 34 （ 36 ） 365704-1 - 365704-10 2023年09月

査読有り,  ISSN  09574484

　概要を見る

The demonstration of the charge-to-spin conversion, especially with enhanced spin Hall conductivity, is crucial for the development of energy-efficient spintronic devices such as spin-orbit torque (SOT) based magnetoresistive random access memories. In this work, fully epitaxial Ru/Cu heterostructures were fabricated with interface engineering and nanolayer insertions consisting of Cu (1 nm)/Ru (1 nm) structures with different numbers of periods. The atomically controlled interface was confirmed by the high-resolution high-angle annular dark-field scanning transmission electron microscopy, and the epitaxial relationship persists even in the hybrid nanolayer insertion structures. The spin current generation was detected by the measurement of unidirectional spin Hall magnetoresistance, and the effective damping-like spin Hall efficiency (ξ DL) was further quantitatively evaluated by the spin-torque ferromagnetic resonance with thickness dependence of the ferromagnetic layer. It is found that the sharp interface Ru/Cu film has a sizeable ξ DL of −2.2% and the insertion of Cu/Ru nanolayers at the interface can increase the ξ DL value to −3.7%. The former could be attributed to the interface spin-orbit filtering effect and the latter may be further understood by the intrinsic contribution from the local electronic structure tuning due to the lattice distortion near the interface. A large effective spin Hall conductivity is achieved to be (3∼5) × 105 ℏ 2 e Ω−1 m−1 in the epitaxial Ru/Cu hybrid nanolayers, which is in the same range as that of platinum. This work indicates that the interfacial control with hybrid nanolayer structures can extend the SOT-based materials to highly conductive metals, even with weak spin-orbit interactions, toward high stability, low cost, and low energy consumption for spintronic applications.

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A large modulation of spin pumping using magnetic phase transitions in single crystalline dysprosium

Yamanoi K., Sakakibara Y., Fujimoto J., Matsuo M., Nozaki Y.

Applied Physics Express （IOP Publishing）  16 （ 6 ） 063004-1 - 063004-6 2023年06月

査読有り,  ISSN  18820778

　概要を見る

We report a large modulation of spin pumping using temperature-induced magnetic phase transitions in c-axis oriented single crystalline dysprosium (Dy). From the temperature variation of the magnetic susceptibility, transitions from paramagnetic (PM) to ferromagnetic (FM) phases via antiferromagnetic (AFM) phase are clearly observed in the Dy. Unlike polycrystalline Dy, the spin pumping of both PM- and AFM-Dy are strongly suppressed owing to the increased non-dissipative backflow of spin current by the long-range spin transport, although two orders of magnitude difference exist between FM- and AFM-phases.

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Spin-torque generation using a compositional gradient at the interface between titanium and tungsten thin films

Nakayama H., Horaguchi T., He C., Sukegawa H., Ohkubo T., Mitani S., Yamanoi K., Nozaki Y.

Physical Review B （AMER PHYSICAL SOC）  107 （ 17 ） 174416-1 - 174416-9 2023年05月

査読有り,  ISSN  24699950

　概要を見る

We experimentally demonstrate spin-torque generation using a compositional gradient at the interface between titanium and tungsten thin films. The width of the compositional gradient interface (CGI) between films is varied from 1.4 to 2.0 nm via sputtering. Spin-torque ferromagnetic resonance is observed in the ferromagnetic Ni95Cu5 alloy fabricated on a Ti/W bilayer with the CGI. The positive spin torque increases with decreasing CGI width, but a negative spin torque is superimposed owing to the negative spin Hall effect in bulk tungsten. A structural undulation in the CGI eliminates this variation in positive spin torque. The CGI width dependence of the spin torque is associated with the generation of spin and/or orbital angular momentum flow at the CGI. Spin-torque generation using a CGI expands the range of material choice for magnetic nonvolatile memory applications.

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Epitaxial Co<inf>2</inf>FeSi/LiNbO<inf>3</inf> multiferroic heterostructures with a low damping constant

Yamada S., Usami T., Komori S., Nagata S., Nozaki Y., Taniyama T., Hamaya K.

2023 IEEE International Magnetic Conference - Short Papers, INTERMAG Short Papers 2023 - Proceedings （IEEE）  2023年

査読有り

　概要を見る

Piezoelectric LiNbO3 has been widely utilized to generate and detect spin waves in surface acoustic wave devices. Here we demonstrate an epitaxial Co-based Heusler alloy on a LiNbO3 128° Y-cut (LN-128Y) substrate by molecular beam epitaxy. Although there is no matching between the lattice length and symmetry, the insertion of a bcc metal buffer layer enables to grow L21 - ordered epitaxial Co2FeSi films on the LN-128Y substrate. The epitaxial Co2FeSi/LN- 128Y multiferroic heterostructures show a high saturation magnetization of ~1235 eμcc and a low damping constant of ~6 × 10-3. This study will open a way for an efficient electric-field control of the spin waves in surface acoustic wave devices.

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Optical Profiling of Surface-Acoustic-Wave Absorption due to Elastically Driven Ferromagnetic Resonance in Nickel

Maezawa K., Fujii S., Yamanoi K., Nozaki Y., Watanabe S.

2023 IEEE International Magnetic Conference - Short Papers, INTERMAG Short Papers 2023 - Proceedings 2023年

　概要を見る

We optically characterized the local changes in the amplitude and phase of a surface vibration that are associated with the magnetoelastic effect. Pulsed laser interferometry was used to directly probe the amplitude and phase of a surface acoustic wave (SAW) propagating on a nickel thin film in a magnetic field, and a conversion from the angular momentum of the SAW to that of the macroscopic spin system was clearly observed. Pulsed laser interferometry opens the door to local probe and spatial mapping of magnon-phonon conversion in various magnetic materials, which can lead to further progress in fundamental and applied research related to magnetism.

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Spin current generation in highly conductive Ru/Cu epitaxial heterostructures

Wen Z., Song J., He C., Scheike T., Sukegawa H., Ohkubo T., Nozaki Y., Mitani S.

2023 IEEE International Magnetic Conference - Short Papers, INTERMAG Short Papers 2023 - Proceedings 2023年

　概要を見る

Significant spin current generation from highly conductive materials is promising for spintronic applications, such as spin-orbit-torque magnetic random-access memories. In this work, we fabricated epitaxial Ru/Cu heterostructures with interface engineering where 1-nm-thick Ru and Cu layers were alternately deposited at the interface. The spin current generation in the heterostructures was evaluated by unidirectional spin Hall magnetoresistance and spin-torque ferromagnetic resonance. A sizable spin Hall efficiency (∼-2%) was achieved in the Ru/Cu sample with a sharp interface which may result from the interface spin filtering effect. Increased spin Hall efficiency (∼-4%) was observed in the interface-engineered samples which could be attributed to the intrinsic contribution from lattice distortion and local band structure near the interface. The effective spin Hall conductivity was estimated to be 3∼ 5 × 105h/2eΩ - 1m - 1, which is comparable to that of Platinum.

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Enhancement of room-temperature unidirectional spin Hall magnetoresistance by using a ferromagnetic metal with a low Curie temperature

Yamanoi K., Semizu H., Nozaki Y.

Physical Review B （American Physical Society (APS)）  106 （ 14 ） L140401-1 - L140401-5 2022年10月

査読有り,  ISSN  24699950

　概要を見る

The unidirectional spin Hall magnetoresistance (USMR) effect is useful to detect the direction of magnetization in a ferromagnetic metal/nonmagnetic metal bilayer, which is the typical geometry used in a spin-orbit torque switching device. In this Letter, we demonstrate the enhancement of the USMR effect by reducing the Curie temperature TC of the ferromagnetic (FM) layer in a FM/platinum bilayer. The USMR ratio was maximized when the thickness of each FM layer was consistent with the spin diffusion length. We found that the maximum USMR ratio can be doubled by replacing Ni81Fe19 (TC=854 K) with Ni85Cu15 (TC=527 K) as the FM. This enhancement of the USMR effect is attributed to an increase of electron-magnon scattering in accordance with Bloch law. We also found that the use of ferromagnets with face-centered-cubic structures, such as Ni, Ni81Fe19, and Ni85Cu15, increased the USMR effect relative to that of body-centered-cubic Fe.

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Temporal-offset dual-comb vibrometer with picometer axial precision

Iwasaki A., Nishikawa D., Okano M., Tateno S., Yamanoi K., Nozaki Y., Watanabe S.

APL Photonics （AIP Publishing）  7 （ 10 ） 106101-1 - 106101-8 2022年10月

査読有り

　概要を見る

We demonstrate a dual-comb vibrometer where the pulses of one frequency-comb are split into pulse pairs. We introduce a delay between two pulses of each pulse pair in front of the sample, and after the corresponding two consecutive reflections at the vibrating sample surface, the initially introduced delay is canceled by a modified Sagnac geometry. The remaining phase difference between the two pulses corresponds to the change in the axial position of the surface during the two consecutive reflections. The Sagnac geometry reduces the effect of phase jitter since both pulses propagate through nearly the same optical path (in opposite directions), and spurious signals are eliminated by time gating. We determine the amplitude of surface vibration on a surface-acoustic-wave device with an axial precision of 4 pm. This technique enables the highly accurate determination of extremely small displacements.

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Quantitative Measurement of Absolute Displacement of Surface Acoustic Wave with ~26 pm Axial Resolution Using Time-Division Dual-Comb Interferometry

Iwasaki A., Nishikawa D., Okano M., Tateno S., Yamanoi K., Nozaki Y., Watanabe S.

2022 Conference on Lasers and Electro-Optics, CLEO 2022 - Proceedings 2022年

　概要を見る

We demonstrate a quantitative evaluation of absolute displacement of surface acoustic wave by the time-division dual-comb interferometry with the Sagnac geometry. Due to an ultrahigh stability of the Sagnac geometry, achieved resolution is -26 pm.

Quantitative Measurement of Absolute Displacement of Surface Acoustic Wave with ~26 pm Axial Resolution Using Time-Division Dual-Comb Interferometry

Iwasaki A., Nishikawa D., Okano M., Tateno S., Yamanoi K., Nozaki Y., Watanabe S.

Optics InfoBase Conference Papers 2022年

　概要を見る

We demonstrate a quantitative evaluation of absolute displacement of surface acoustic wave by the time-division dual-comb interferometry with the Sagnac geometry. Due to an ultrahigh stability of the Sagnac geometry, achieved resolution is ~26 pm.

Einstein-de Haas phase shifts in surface acoustic waves

Tateno S., Kurimune Y., Matsuo M., Yamanoi K., Nozaki Y.

Physical Review B （AMER PHYSICAL SOC）  104 （ 2 ） L020404-1 - L020404-5 2021年07月

査読有り,  ISSN  24699950

　概要を見る

We demonstrate the phase shift of the Rayleigh-type surface acoustic wave (RSAW) in a ferromagnetic Ni81Fe19 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 for controlling spin-mechatronics devices without magnetoelastic torque which is specific to a certain material.

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Experimental method for quantitative evaluation of nonlinear magnetization dynamics

Okano G., Nozaki Y.

IEEJ Transactions on Fundamentals and Materials （Institute of Electrical Engineers of Japan）  141 （ 2 ） 99 - 104 2021年02月

査読有り,  ISSN  03854205

　概要を見る

We have developed an experimental technique to investigate a nonlinear magnetization dynamics resonantly excited by strong microwave fields. In this technique, a magnetization switching was observed when a sub-nanosecond wide pulsed field was applied to the ferromagnetic resonance-excited ferromagnets. Such a cooperative switching with simultaneous application of microwave- and pulsed-fields can evaluate an effective energy barrier for magnetization switching in various nonlinear magnetization dynamics. In this review, we show the experimental results on (1) the temporal evolution of the energy barrier in the transient of magnetization precession growth and (2) the observation of saddle-node bifurcation in the microwave-excited nonlinear magnetization dynamics.

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Highly nonlinear frequency-dependent spin-wave resonance excited via spin-vorticity coupling

Yuki Kurimune, Mamoru Matsuo, Sadamichi Maekawa, Yukio Nozaki

Physical Review B （American Physical Society (APS)）  102 （ 17 ） 174413-1 - 174413-7 2020年11月

筆頭著者, 最終著者, 責任著者, 査読有り,  ISSN  2469-9950

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Electrical evaluation of the alternating spin current generated via spin-vorticity coupling

Shoma Tateno, Genki Okano, Mamoru Matsuo, Yukio Nozaki

Physical Review B （American Physical Society (APS)）  102 （ 10 ） 104406-1 - 104406-9 2020年09月

筆頭著者, 最終著者, 責任著者, 査読有り,  ISSN  2469-9950

　概要を見る

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 [M. Matsuo et al., Phys. Rev. B 96, 020401(R) (2017)]. Such a huge conversion efficiency is attributable to a smaller inconsistency of energy scale between lattice rotation and electron spin than the case of liquid vorticity.

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Highly accurate evaluation of spin-torque efficiency by measuring in-plane angular dependence of spin-torque ferromagnetic resonance

Horaguchi T., Matsuo M., Nozaki Y.

Journal of Magnetism and Magnetic Materials （Journal of Magnetism and Magnetic Materials）  505   166727-1 - 166727-5 2020年07月

査読有り,  ISSN  03048853

　概要を見る

© 2020 Spin-torque ferromagnetic resonance (ST-FMR) is a powerful tool to evaluate spin-torque efficiency (ξFMR), the efficiency of current-induced torque in metallic bilayers consisting of ferromagnetic and nonmagnetic materials. In general, ξFMR 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 ξFMR 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.

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