研究者詳細 - 能崎　幸雄

論文 - 能崎　幸雄

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Observation of Gyromagnetic Spin Wave Resonance in NiFe Films

Kurimune Y., Matsuo M., Nozaki Y.

Physical Review Letters （Physical Review Letters） 124 （ 21 ） 217205-1 - 217205-6 2020年05月

査読有り, ISSN 00319007

© 2020 American Physical Society. This Letter demonstrates spin wave resonance (SWR) owing to the gyromagnetic effect by propagating a Rayleigh-type surface acoustic wave (R-SAW) through ferromagnetic thin films. The SWR amplitude in a NiFe film shows a higher-order frequency variation than in a magnetoelastic Ni film. This frequency dependence is well understood in terms of the presence of a gyromagnetic field attributable to the local lattice rotation in the R-SAW. From the frequency dependence of the SWR amplitude, the gyromagnetic SWR could be separated from another SWR caused by a magnetoelastic effect of the ferromagnet.

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Highly nonreciprocal spin waves excited by magnetoelastic coupling in a ni / si bilayer

Tateno S., Nozaki Y., Nozaki Y.

Physical Review Applied （Physical Review Applied） 13 （ 1 ） 034074-1 - 034074-5 2020年03月

査読有り

© 2020 American Physical Society. We demonstrate a largely enhanced nonreciprocal propagation of spin waves (SWs) magnetoelastically excited in Ni/Si bilayer films. The nonreciprocity is attributed to the shear component of the strain tensor, which can be enlarged by increasing the thickness of the Si layer. By evaporating a 400-nm-thick Si layer on a Ni film, we enhanced the nonreciprocal ratio by up to 1200%. This finding paves the way for rectification of SWs in magnonic devices.

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Spin waves parametrically excited via three-magnon scattering in narrow NiFe strips

Okano G., Nozaki Y.

Physical Review B （Physical Review B） 100 （ 10 ） 104424-1 - 104424-10 2019年09月

査読有り, ISSN 24699950

© 2019 American Physical Society. Three-magnon scattering can be used to evaluate the lowest frequency of the magnon band in a ferromagnet. Both the ferromagnetic-resonant (FMR) frequency fFMR and the lowest frequency of the magnon band fmin in narrow-shaped NiFe strips were electrically measured using the anisotropic-magnetoresistance effect. The comparison with a micromagnetic simulation shows that fmin of the magnon band can be controlled independent of fFMR by varying the width w and thickness t of the NiFe strip while maintaining a constant t/w ratio. In addition, we found that the frequency difference, fFMR-fmin, can be greatly increased in thicker NiFe strips. Our results show that narrow-shaped ferromagnets allow us to tune the magnon-band structures by varying their w and t. This ability is important for designing magnon circuits in integrated magnonic devices and for improving the quantitative study on the Bose-Einstein condensation of magnons.

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Nonreciprocal Spin Current Generation in Surface-Oxidized Copper Films

Okano G., Matsuo M., Ohnuma Y., Maekawa S., Nozaki Y.

Physical Review Letters （Physical Review Letters） 122 （ 21 ） 217701-1 - 217701-6 2019年05月

査読有り, ISSN 00319007

© 2019 American Physical Society. We experimentally demonstrate the nonreciprocal generation of spin current (Js) in a surface-oxidized copper film. The efficiency of conversion is at least 320 times larger than the inverse conversion. This nonreciprocity is due to a novel type of Js generation, which relies on the transfer of angular momentum from the velocity field of free electrons. A gradient in the electrical mobility in the film produces vorticity in the in-plane drift velocity of the free electrons. The inverse process can hardly occur when Js is collinear with the gradient in the electrical mobility.

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Evaluating the nonadiabatic spin-transfer torque using coupled oscillation of magnetic vortices

Horaguchi T., Tanazawa T., Nozaki Y.

Physical Review B （Physical Review B） 98 （ 6 ） 2018年08月

査読有り, ISSN 24699950

© 2018 American Physical Society. In this paper, we detect the coupled oscillation of magnetic vortices in Ni81Fe19 gourd-shaped thin films electrically to evaluate the nonadiabatic spin-transfer torque (STT) coefficient β, which is important for understanding a variety of STT-related phenomena. We excite a translational mode in coupled magnetic vortices by applying an alternating electrical current that causes both STT and Oersted field torque. Since the coupled oscillation modes due to the STT and Oersted field torque have totally different eigenfrequencies, the coupled oscillation spectrum can be used to evaluate β, which is generally difficult to distinguish from the Oersted field contributions. Based on the ratio between the amplitudes of the spectrum's Lorentzian and anti-Lorentzian components, the value of β is found to be 0.085±0.016, which is 4.3 times larger than the Gilbert damping constant α.

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Excitation of Ferromagnetic Resonance Using Surface Acoustic Waves

Nozaki Y., Yanagisawa S.

Electrical Engineering in Japan (English translation of Denki Gakkai Ronbunshi) （Electrical Engineering in Japan (English translation of Denki Gakkai Ronbunshi)） 204 （ 3 ） 3 - 9 2018年08月

査読有り, ISSN 04247760

© 2018 Wiley Periodicals, Inc. Mechanical rotation of a crystal lattice in ferromagnetic materials can be energetically coupled with its magnetization via magnetoelastic coupling or spin rotation coupling. Surface acoustic wave (SAW) in piezoelectric materials is one of the promising candidates to realize the mechanical excitation of magnetization dynamics. In order to understand the mechanical rotation induced magnetization dynamics quantitatively, we examined the ferromagnetic resonance in an Ni film using a SAW in a LiNbO3 substrate. By decreasing a period of interdigital transducer as short as 4 μm which is one-fifth of the previous work by Weiler [M. Weiler and co-workers, Elastically driven ferromagnetic resonance in Nickel thin films. Phys Rev Lett 106, 117601 (2011)], the fundamental frequency of SAW could be higher than 800 MHz. From the dependence of microwave absorption on the angle between the magnetization and the wave vector of SAW, it was confirmed that the Rayleigh type SAW, which was significant to obtain a large mechanical coupling with the magnetization, was dominantly excited in the 800-MHz-SAW device.

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Modulation of magnetization dynamics in an epitaxial Heusler ferromagnet due to pure spin current in a laterally configured structure

Oki S., Sasaki Y., Kasatani Y., Yamada S., Mizukami S., Nozaki Y., Hamaya K.

Journal of Physics Condensed Matter （Journal of Physics Condensed Matter） 30 （ 25 ） 2018年05月

査読有り, ISSN 09538984

© 2018 IOP Publishing Ltd. Using a laterally configured structure, we demonstrate modulation of the magnetization dynamics of an epitaxial Heusler ferromagnet, Fe3Si, by nonlocally injecting pure spin currents. From the ferromagnetic resonance signals, the effective Gilbert damping constant () of the epitaxial Fe3Si nanomagnet is estimated to be ∼0.003 at room temperature. By using the lateral and nonlocal spin injection, the value of the epitaxial Fe3Si nanomagnet can be modulated from 0.0034 to 0.0024. This study will open a way for control of the magnetization dynamics of high-performance ferromagnetic Heusler alloys due to pure spin currents even in laterally configured structures.

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Evaluation of the effective potential barrier height in nonlinear magnetization dynamics excited by ac magnetic field

Okano G., Nozaki Y.

Physical Review B （Physical Review B） 97 （ 1 ） 2018年01月

査読有り, ISSN 24699950

© 2018 American Physical Society. An alternating current (ac) magnetic field or spin current can reduce the switching field of a ferromagnet through resonance excitation of a large-angle precession of magnetization. The nonlinear magnetization dynamics of this switching scheme completely differ from the general ferromagnetic resonance phenomenon, which is linearly excited by a small ac magnetic field. To understand these dynamics, it is necessary to evaluate the effective potential barrier height for switching, ΔUeff. However, most previous studies have measured the consequent precession angle in the nonlinear dynamics by magneto-optical methods and/or by applying a magneto-resistive effect. Here, we applied the cooperative switching method, which evaluates the ΔUeff of the nonlinear dynamics under a sub-ns-wide magnetic field impulse, and observed a nontrivial reduction of ΔUeff in a submicron-wide NiFe strip. The strong reduction of ΔUeff under a negative magnetic field was caused by a saddle-node bifurcation in the nonlinear dynamics. In a micromagnetics simulation, we also confirmed that the magnetization is nonuniformly excited at the shallowest ΔUeff.

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Micromagnetic Simulation of Microwave-Assisted Magnetization Switching Process for Granular Films

Terumitsu Tanaka, Yukio Nozaki, Kimihide Matsuyama

IEEE TRANSACTIONS ON MAGNETICS （IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC） 53 （ 11 ） 2017年11月

筆頭著者, 最終著者, 責任著者, 査読有り, ISSN 0018-9464

Microwave-assisted magnetization switching (MAS) processes for granular films were demonstrated by time integration of the Landau Lifshitz Cilbert equation. At low microwave frequencies, hysteresis loops become narrower, and at higher frequencies, an extra step occurs in the curves. In the case when the extra step occurs, saturation field increases to be comparable to that for non-MAS case, although switching field decreases. The occurrence of the extra step possibly originates from "unstable switching process" predicted by Bertotti et al., and the magnetization contour maps indicated large un-switched agglomerates remains when MATS proceeds through the unstable switching process. The unstable switching processes are also observed even when dc incident angles are large.

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Spin Current Generation Using a Surface Acoustic Wave Generated via Spin-Rotation Coupling」

D. Kobayashi, T. Yoshikawa, M. Matsuo, R. Iguchi, S. Maekawa, E. Saitoh, Y. Nozaki

Physical Review Letters （The American Physical Society） 119 （ 7 ） 077202-1 - 077202-5 2017年08月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0031-9007

We demonstrate the generation of alternating spin current (SC) via spin-rotation coupling (SRC) using a surface acoustic wave (SAW) in a Cu film. Ferromagnetic resonance caused by injecting SAWs was observed in a Ni-Fe film attached to a Cu film, with the resonance further found to be suppressed through the insertion of a SiO2 film into the interface. The intensity of the resonance depended on the angle between the wave vector of the SAW and the magnetization of the Ni-Fe film. This angular dependence is explicable in terms of the presence of spin transfer torque from a SC generated via SRC.

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表面弾性波を用いた強磁性共鳴の励起

能崎幸雄, 柳澤翔吾

電気学会論文誌Ａ（基礎・材料・共通部門誌）（電気学会） 137 （ 7 ） 386 - 391 2017年07月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0385-4205

Circularly Polarized Magnetic Field Generated by Two Microfabricated Crossed Coplanar Waveguides」

I. Kan, Y. Soeno, T. Roppongi, Y. Nozaki

Applied Physics Letters （The American Institute of Physics） 110 （ 20 ） 202404-1 - 202404-4 2017年05月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0003-6951

We generate a circularly polarized (CP) magnetic field using two crossed coplanar waveguides (TCCWs). By using the CP field, we selectively excite a ferromagnetic resonance between positively and negatively magnetized Co/Ni multilayers. The CP field arises from a superposition of Ampere fields generated by microwaves orthogonally propagating along the crossed arms of the TCCWs. The polarity of the magnetic field can be tuned continuously from circular to linear by exploiting a phase delay between the orthogonal microwaves. From a local measurement of the magneto-optical Kerr effect, we also find that the ellipticity of the CP field is distributed over the intersection of the TCCWs. These results indicate that TCCWs have great potential as CP-field generators that can be integrated into micron-scale electrical devices. Published by AIP Publishing.

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Magneto-optic Kerr effect CCD imaging with polarization modulation technique

Satoru Nakayama, Makoto Okano, Yukio Nozaki, Shinichi Watanabe

AIP ADVANCES （AMER INST PHYSICS） 7 （ 5 ） 2017年05月

筆頭著者, 最終著者, 責任著者, 査読有り, ISSN 2158-3226

We have developed a magneto-optic Kerr effect (MOKE) imaging system with a charge-coupled-device (CCD) camera by using the rotating compensator technique. We chose optimal conditions of the rotation frequency of the compensator with stable rotation along with a CCD camera frame rate that allowed precise control of the exposure timing in order to link with the angle of the compensator. Precise timing management of the CCD exposure enables us to carry out repeated experiments, which greatly improves the signal-to-noise ratio of the longitudinal MOKE signal. We applied the technique to the material characterization of the Ni-81 Fe-19 thin film and its microstructure, and succeeded in evaluating the spatial variation of the complex magneto-optic constant Q of the sample. Because of its attractive advantages such as high-speed and compactness, the present method provides a novel platform for investigating the domain structures in various magnetic materials. (C) 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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交差型コプレーナ線路上に生じた円偏向磁場の空間分布測定

菅　逸樹, 能崎幸雄

電気学会研究会資料（電気学会） MAG-16-217 1-6 2016年12月

研究論文（その他学術会議資料等）, 共著

FMRスペクトル形状によるマイクロ波アシスト磁化反転の安定性の定量的解析

能崎幸雄, 田中輝光

電気学会研究会資料（電気学会） MAG-16-230 1-6 2016年12月

研究論文（その他学術会議資料等）, 共著

Spin-transfer-torque-assisted magnetization reversal in spin-valve strips using the spin-pumping effect

TAKAHASHI Mao, NOZAKI Yukio

Applied Physics Express （Japanese Society of Applied Physics） 9 （ 6 ） 063002-1 - 063002-4 2016年04月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1882-0778

The modulation of a switching field by a spin-transfer torque generated by the spin-pumping effect is demonstrated in spin-valve strips. A spin current pumped into a spin valve prefers an antiparallel configuration of magnetization, so that the hysteresis loop of a soft magnetic layer is shifted by applying an alternating-current magnetic field with a frequency that matched the resonant condition of a hard magnetic layer. Furthermore, we confirmed that an alternating spin current generated by the spin pumping in the hard magnetic layer produces an orthogonal magnetic torque that can reduce the coercive field of the soft magnetic layer.

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Microwave-assisted magnetization reversal using transient precession of magnetization in permalloy hexagons

OKANO Genki, NOZAKI Yukio

Applied Physics Express （Japanese Society of Applied Physics） 9 （ 6 ） 063001-1 - 063001-4 2016年04月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1882-0778

Microwave-assisted magnetization reversal utilizing a transient precession of magnetization was demonstrated in a permalloy hexagon by applying a 25-ns-wide microwave field and a 500-ps-wide pulsed field with a tunable delay to the microwave field. The switching field in a combination of these two fields becomes smaller than that in only the microwave field, and this additional reduction in switching field oscillates relative to the delay time. From the comparison with the results of micromagnetic simulations, we found that the oscillatory behavior is attributed to the beats in transient precession that occurs in the early stage of microwave-field-induced magnetization excitation.

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Current-driven antivortex core resonance measured by the rectifying effect

GOTO Minori, NOZAKI Yukio

AIP Advances （AIP） 6 （ 2 ） 025313-1 - 025313-6 2016年02月

研究論文（学術雑誌）, 共著, 査読有り

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Real-space observation of magnetic vortex core gyration in a magnetic disc both with and without a pair tag

A. Yamaguchi, H. Hata, M. Goto, M. Kodama, Y. Kasatani, K. Sekiguchi, Y. Nozaki, T. Ohkochi, M. Kotsugi, T. Kinoshita

Japanese Journal of Applied Physics （IOP Science） 55 （ 2 ） 023002-1 - 023002-13 2016年01月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0021-4922

We demonstrate the time-resolved real-space observation of vortex gyration in a magnetic circular disc by X-ray magnetic circular dichroism photoemission microscopy at the SPring-8 beamline BL25SU, both with and without an additional structure called a pair tag, which serves to control the magnetic curling direction. By comparing the gyration orbits of the vortices, we found that the gyration properties are dependent on the presence of the additional structure in spite of the discs having the same diameter. Furthermore, we reproduced the vortex gyration by micromagnetic simulation. The confinement potential of the vortex can be controlled by the additional structure.

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パラメトリック励起スピン波を活用した磁化反転実験

岡野元基, 能崎幸雄

日本物理学会講演概要集（一般社団法人日本物理学会） 71 892 - 892 2016年

筆頭著者, 最終著者, 責任著者, 査読有り

<p>ある閾値強度よりも大強度のマイクロ波印加の下では、Suhl instabilityという不安定性が現れ、パラメトリックにスピン波が励起される。一方で大振幅歳差運動を励起して磁性体の保磁力を低減するマイクロ波アシスト磁化反転(MAMR)という技術が注目されている。本研究では、ナノスケール金属磁性体中でのSuhl instabilityの性質について調べ、パラメトリックに励起されたスピン波を活用したMAMR実験を行った。その結果、不安定性がMAMRにおいてどのような役割を果たすのかについて明らかにした。</p>

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能崎 幸雄

論文 - 能崎 幸雄

能崎　幸雄

論文 - 能崎　幸雄