研究等業績 - 原著論文 - 菊池 伸明
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Ying R, T. Taniguchi, K. Nabeta, K. Ishigami, N. Kikuchi, S. Okamoto
Japanese Journal of Applied Physics ( IOP ) 63 ( 11 ) 11SP04 2024年11月 [査読有り]
研究論文（学術雑誌） 国内共著
Abstract
In our energy-consuming society, understanding magnetization reversal in permanent magnets is crucial for improving energy conversion efficiency between electric energy and mechanical energy. First-order reversal curves (FORCs) have enabled qualitative studies of reversal mechanisms, however, further understanding based on quantitative analysis is still difficult due to the complexity of FORC diagrams. We introduce a machine-learning based approach combining the Gaussian mixture model and the Davies-Bouldin index to separate characteristic features in FORC diagrams of Nd-Fe-B magnets. The clustering method is evaluated using several FORC diagrams obtained at different temperatures, and we also demonstrated hysteresis loop reconstruction using the clustered FORC diagrams. -
Multimodal iron loss analyses based on magnetization processes for various soft magnetic toroidal cores
Nobuhisa Ono, Yuji Uehara, Tomoyuki Onuma, Takuya Taniguchi, Nobuaki Kikuchi, Satoshi Okamoto
Journal of Magnetism and Magnetic Materials ( Elsevier BV ) 603 172222 - 172222 2024年08月
研究論文（学術雑誌）
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Direct observation of current-induced nonlinear spin torque in Pt-Py bilayers
Toshiyuki Kodama, Nobuaki Kikuchi, Takahiro Chiba, Satoshi Okamoto, Seigo Ohno, Satoshi Tomita
Physical Review B ( American Physical Society (APS) ) 109 ( 21 ) 2024年06月
研究論文（学術雑誌）
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Minh Van Nguyen, Nobuaki Kikuchi, Toshiyuki Kodama, Taiyu Okatani, Naoki Inomata, Yoshiaki Kanamori
Japanese Journal of Applied Physics ( IOP Publishing ) 63 ( 3 ) 03SP72 2024年02月
研究論文（学術雑誌）
Abstract
Here, we proposed two delay lines consisting of spoof Surface Plasmon Polariton (sSPP) waveguides and C-shaped metamaterials (C-MMs). The delay lines, namely OFF and ON devices, were designed and fabricated. On the OFF device, an sSPP waveguide is capacitively coupled to the C-MMs via an air gap on a high-resistivity silicon substrate. On the ON device, a connection is established between the C-MMs and the sSPP waveguide by metal connectors. The difference in the electrical properties in the coupling between the C-MMs and the sSPP waveguide creates a large phase contrast between the ON and OFF delay lines. The structural design was performed using a numerical calculation based on a commercial finite element solver. We successfully fabricated and characterized delay lines with phase differences equal to tens of degrees between the ON and OFF devices in the target frequency range of 2-6 GHz, while maintaining the original transmittance properties. The promising applications of the delay lines are a phase shifter or modulator when integrating with suitable switches. -
Analysis on bimodal complex permeability spectrum of a noise suppression sheet and single constituent flakes
Toshiyuki Igarashi, Shingo Tamaru, Nobuaki Kikuchi, Shigeyoshi Yoshida, Satoshi Okamoto
Journal of Magnetism and Magnetic Materials ( Elsevier BV ) 590 171640 - 171640 2024年01月
研究論文（学術雑誌）
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時間変調磁性メタマテリアルに向けたスピン流誘起透磁率変調の評価
児玉 俊之, 菊池 伸明, 岡本 聡, 大野 誠吾, 冨田 知志
日本磁気学会論文特集号 8 ( 1 ) 15 - 23 2024年01月 [査読有り]
研究論文（学術雑誌） 国内共著
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Analysis on Iron Loss of Sendust Dust Cores Based on Magnetization Reversal Processes
Nobuhisa Ono, Yuji Uehara, Yasushi Endo, Shigeyoshi Yoshida, Nobuaki Kikuchi, Hidehiko Oikawa, Satoshi Okamoto
IEEE Transactions on Magnetics ( Institute of Electrical and Electronics Engineers (IEEE) ) 59 ( 11 ) 1 - 5 2023年11月 [査読有り]
研究論文（学術雑誌） 国内共著
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Spin-Current-Driven Permeability Variation for Time-Varying Magnetic Metamaterials
Toshiyuki Kodama, Nobuaki Kikuchi, Satoshi Okamoto, Seigo Ohno, Satoshi Tomita
Physical Review Applied ( American Physical Society (APS) ) 19 ( 4 ) 2023年04月 [査読有り]
研究論文（学術雑誌） 国内共著
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Satoshi Tomita, Nobuaki Kikuchi, Satoshi Okamoto
Japanese Journal of Applied Physics ( IOP Publishing ) 62 ( SB ) SB1010 - SB1010 2023年03月 [査読有り]
研究論文（学術雑誌）
Abstract
We study cobalt/platinum (Co/Pt) superlattices by means of ferromagnetic resonance spectroscopy in the frequency domain. The magnetization easy-axis of superlattices, which lies in a plane of the film with Co layers of 1.5 and 1.2 nm thickness, changes to the direction perpendicular to the film surface with Co layers of 0.9 and 0.6 nm thickness. As the Co layer thickness decreases from 0.9 to 0.6 nm, the Gilbert damping parameter significantly increases while the g-value is almost constant. As the Pt layer thickness decreases from 0.6 to 0.3 nm in the superlattices with 0.9 nm thick Co layers, the damping parameter slightly decreases. The present study demonstrates that the Co/Pt superlattices with 0.9 nm thick Co layers and 0.3 nm thick Pt layers are suitable for time-varying magnetic metamaterials realizing a larger magnetic permeability modulation. -
Nobuaki Kikuchi, Katsunari Sato, Takehito Shimatsu, Satoshi Okamoto
Japanese Journal of Applied Physics ( IOP Publishing ) 62 ( SB ) SB1015 - SB1015 2023年03月 [査読有り]
研究論文（学術雑誌） 国内共著
Abstract
Optimizing switching behavior under a microwave field is a key issue for microwave-assisted magnetic recording. We study the effect of inter-grain exchange coupling induced by a continuous magnetic layer on microwave-assisted switching of CoCrPt-SiO<sub>2</sub> granular media. The microwave assistance effect is improved in the low-frequency region by introducing the inter-grain exchange coupling, but the coupling reduces the critical frequency. The maximum assistance effect is almost independent of the film structure since the dual effect cancels each other. Micromagnetic simulations also show similar results on exchange coupling between grains. These results indicate that the inter-grain exchange coupling can be used to optimize the critical frequency of the media in practical applications. -
Thermal activation on microwave-assisted magnetization switching in Co/Pt nanodot arrays
Satoshi Mizutani, Nobuaki Kikuchi, Masatoshi Hatayama, Takehito Shimatsu, Satoshi Okamoto
Japanese Journal of Applied Physics ( IOP Publishing ) 62 ( SB ) SB1012 - SB1012 2023年03月 [査読有り]
研究論文（学術雑誌） 国内共著
Abstract
The magnetization switching field is efficiently reduced by exciting precession with a microwave field of GHz frequency. Analytical calculations based on the Landau–Lifshitz–Gilbert equation have revealed that the effect of the thermal activation process plays an important role in magnetization switching behavior under a microwave field. In this study, we experimentally investigated the microwave-assisted magnetization switching (MAS) behavior of Co/Pt nanodot arrays under various microwave field conditions. Experimental results were compared with the calculated effective energy barrier height of MAS. Consequently, all the experimental MAS behaviors can be explained by the effect of thermal activation, but quantitative discussion will require accurate experimental studies. -
Makoto Takeuchi, Motohiro Suzuki, Shintaro Kobayashi, Yoshinori Kotani, Tetsuya Nakamura, Nobuaki Kikuchi, Anton Bolyachkin, Hossein Sepehri-Amin, Tadakatsu Ohkubo, Kazuhiro Hono, Yasuhiro Une, Satoshi Okamoto
NPG Asia Materials ( Springer Science and Business Media LLC ) 14 ( 1 ) 2022年12月 [査読有り]
研究論文（学術雑誌） 国内共著
Abstract
In the long history of permanent magnet research for more than 100 years, three-dimensional magnetic microscopy has been eagerly awaited to elucidate the origin of the magnetic hysteresis of permanent magnets. In this study, we succeeded in observing the three-dimensional magnetic domain structure of an advanced high-coercivity Nd-Fe-B-based permanent magnet throughout the magnetic hysteresis curve using a recently developed hard X-ray magnetic tomography technique. Focused-ion-beam-based three-dimensional scanning electron microscopy was employed to study the relationship between the observed magnetic domains and the microstructure of the magnet for the same observing volume. Thermally demagnetized and coercivity states exhibit considerably different magnetic domain structures but show the same periodicity of 2.3 μm, indicating that the characteristic length of the magnetic domain is independent of the magnetization states. Further careful examination revealed some unexpected magnetic domain behaviors, such as running perpendicular to the magnetic easy axis and reversing back against the magnetic field. These findings demonstrate a wide variety of real magnetic domain behaviors along the magnetic hysteresis inside a permanent magnet. -
Nobuaki Kikuchi, Katsunari Sato, Satoshi Okamoto, Osamu Kitakami, Takehito Shimatsu, Hirofumi Suto
Physical Review B ( American Physical Society (APS) ) 105 ( 5 ) 054430-1 - 054430-11 2022年02月 [査読有り]
研究論文（学術雑誌）
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M. Takeuchi, T. Yomogita, N. Kikuchi, K. Toyoki, S. Kobayashi, Y. Kotani, T. Nakamura, K. Hioki, S. Okamoto
Journal of the Japan Institute of Metals and Materials ( Japan Institute of Metals ) 86 ( 1 ) 1 - 7 2022年01月 [査読有り]
研究論文（学術雑誌）
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Development of a high-sensitivity VNA-FMR spectrometer with field modulation detection and its application to magnetic characterization
Shingo Tamaru, Tatsuya Yamamoto, Tomoyuki Onuma, Nobuaki Kikuchi, Satoshi Okamoto
Electronics and Communications in Japan 104 ( 3 ) e12320-1 - e12320-8 2021年09月 [査読有り]
研究論文（学術雑誌）
We have developed a vector network analyzer ferromagnetic resonance (VNA-FMR) spectrometer with field modulation detection. This system provides broadband coverage and very high sensitivity to resolve the FMR signal on a ultra-thin (∼ 1 nm thick) magnetic film. This paper first presents the detail of the VNA-FMR spectrometer built in this work, including its operation principle and design guideline, and second various magnetic properties obtained by the VNA-FMR measurement results.
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磁場変調検出を用いた高感度 VNA 強磁性共鳴測定装置の開発とその磁化測定への応用
田丸慎吾, 山本竜也, 大沼智幸, 菊池伸明, 岡本聡
電気学会論文誌 A ( Institute of Electrical Engineers of Japan (IEE Japan) ) 141 ( 5 ) 295 - 300 2021年05月 [査読有り]
研究論文（学術雑誌）
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Direct detection and stochastic analysis on thermally activated domain-wall depinning events in micropatterned Nd-Fe-B hot-deformed magnets
Takahiro Yomogita, Satoshi Okamoto, Nobuaki Kikuchi, Osamu Kitakami, Hossein Sepehri-Amin, Yukiko K. Takahashi, Tadakatsu Ohkubo, Kazuhiro Hono, Keiko Hioki, Atsushi Hattori
ACTA MATERIALIA ( PERGAMON-ELSEVIER SCIENCE LTD ) 201 7 - 13 2020年12月
研究論文（学術雑誌）
Although the magnetization reversal process in the permanent magnets has long been studied, it has not been fully revealed. The recent progress of the computational science realizes the atomistic calculations for the thermally activated magnetization reversal process in permanent magnets. In contrast, the experimental study on the magnetization reversal has remain to be the macroscopic evaluation. In this study, Nd-Fe-B hot-deformed magnets are micropatterned, and a staircase-like magnetization curve corresponding to the elemental domain-wall depinning event at each grain boundary is successfully observed. Each elemental domain-wall depinning event fluctuates with respect to thermal activation, and the stochastic analysis based on the Neel-Arrhenius model gives the energy barrier parameters H-0 and E-0 of each depinning event, which are the intrinsic domain-wall depinning field and energy barrier height, respectively. Three types of Nd-Fe-B hot-deformed magnets with different coercivities are adopted for this stochastic analysis. As a result, E-0 exhibits very little dependence on H-0, and its slope becomes steeper for the lower-coercivity magnet. The stochastic micromagnetics simulation based on the Landau-Lifshitz-Gilbert equation for the two-grain model with various inter-grain exchange coupling reproduces the experimentally observed relationship between E-0 and H-0. Moreover, the behavior for the lower-coercivity magnet can be explained by assuming the presence of a low magnetic anisotropy layer on the grain surface. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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Nguyen Tat Trung, Mohammad Ikram Hossain, Md Iftekharul Alam, Atsushi Ando, Osamu Kitakami, Nobuaki Kikuchi, Tsuyoshi Takaoka, Yasuyuki Sainoo, Ryuichi Arafune, Tadahiro Komeda
ACS Omega ( American Chemical Society (ACS) ) 5 ( 43 ) 28108 - 28115 2020年11月
研究論文（学術雑誌）
We report a precise measurement of the sensor behavior of the field effect transistor (FET) formed with the MoS2 channel when the channel part is exposed to Cl2 gas. The gas exposure and the electrical measurement of the MoS2 FET were executed with in situ ultrahigh-vacuum (UHV) conditions in which the surface analysis techniques were equipped. This makes it possible to detect how much sensitivity the MoS2 FET can provide and understand the surface properties. With the Cl2 gas exposure to the channel, the plot of the drain current versus the gate voltage (Id-Vg curve) shifts monotonically toward the positive direction of Vg, suggesting that the adsorbate acts as an electron acceptor. The Id-Vg shifts are numerically estimated by measuring the onset of Id (threshold voltage, Vth) and the mobility as a function of the dosing amounts of the Cl2 gas. The behaviors of both the Vth shift and the mobility with the Cl2 dosing amount can be fitted with the Langmuir adsorption kinetics, which is typically seen in the uptake curve of molecule adsorption onto well-defined surfaces. This can be accounted for by a model where an impinging molecule occupies an empty site with a certain probability, and each adsorbate receives a certain amount of negative charge from the MoS2 surface up to the monolayer coverage. The charge transfer makes the Vth shifts. In addition, the mobility is reduced by the enhancement of the Coulomb scattering for the electron flow in the MoS2 channel by the accumulated charge. From the thermal desorption spectroscopy (TDS) measurement and density functional theory (DFT) calculations, we concluded that the adsorbate that is responsible for the change of the FET property is the Cl atom that is dissociated from the Cl2 molecule. The monotonic shift of Vth with the coverage suggests that the MoS2 device sensor has a good sensitivity to detect 10-3 monolayers (ML) of adsorption corresponding to the ppb level sensor with an activation time of 1 s.
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Time-resolved imaging of an operating hard-disk-drive write head using nano-beam x-ray magnetic circular dichroism
Hirofumi Suto, Akira Kikitsu, Yoshinori Kotani, Tomoyuki Maeda, Kentaro Toyoki, Hitoshi Osawa, Nobuaki Kikuchi, Satoshi Okamoto, Tetsuya Nakamura
JOURNAL OF APPLIED PHYSICS ( AMER INST PHYSICS ) 128 ( 13 ) 2020年10月
研究論文（学術雑誌）
We studied the dynamic magnetization behavior of a hard-disk-drive (HDD) write head by using nano-beam x-ray magnetic circular dichroism (XMCD). The write head was operated by applying a square-wave write current with a frequency of 211.9MHz to the coil of the write head, which was in synchronization with the fivefold frequency of the x-ray pulses. Snapshot images of the magnetization on the air-bearing surface of the write head were obtained with spatial and time resolutions of 100nm and 50ps, respectively, and temporal changes in the magnetization of the main pole and the shield were observed. When a square-wave write current with a transition time of 0.2ns was applied, the magnetization reversal of the main pole was completed in 0.7ns, and this reversal time was determined by the dynamic property of the write head. During the main pole reversal, the shield area showed spatially non-uniform magnetization patterns, and its response was slower than that of the main pole. These results show that the XMCD measurement setup developed in this study is a powerful tool for understanding the dynamic behavior of an HDD write head and contributes to the development of higher capacity HDDs.
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Broadband and high-sensitivity permeability measurements on a single magnetic particle by transformer coupled permeameter
S. Tamaru, N. Kikuchi, T. Igarashi, S. Okamoto, H. Kubota, S. Yoshida
Journal of Magnetism and Magnetic Materials ( Elsevier BV ) 501 166434 - 166434 2020年05月 [査読有り]
研究論文（学術雑誌）
We have developed a system to measure permeability over a wide frequency range with high sensitivity, which we call the transformer coupled permeameter. In this system, the magnetic sample is sandwiched by two short-terminated coplanar waveguides that form a loosely coupled transformer. In this way, the stimulus signal acting as a background becomes smaller as the frequency decreases, thus significantly suppressing the deterioration of the signal-to-noise ratio (SNR) at low frequencies that most conventional permeameters suffer from. It has been demonstrated that this system can provide a sensitivity high enough to measure the permeability of a single ferromagnetic flake particle comprising a noise suppression sheet over 10 MHz–20 GHz with a high SNR.