研究等業績 - 原著論文 - 菊池 伸明
-
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）
-
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）
-
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）
-
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
-
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
-
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
-
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）
-
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）
-
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.
-
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）
-
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.
-
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.
-
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.
-
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.
-
N. Kikuchi, T. Yomogita, K. Sato, S. Okamoto, O. Kitakami, H. Osawa, M. Suzuki, K. Toyoki, Y. Kotani, T. Nakamura
Japanese Journal of Applied Physics ( IOP Publishing ) 59 ( SE ) SEED03 - SEED03 2020年04月 [査読有り]
研究論文（学術雑誌）
-
Hirofumi Suto, Nobuaki Kikuchi, Satoshi Okamoto, Kentaro Toyoki, Yoshinori Kotani, Tetsuya Nakamura
APPLIED PHYSICS EXPRESS ( IOP PUBLISHING LTD ) 13 ( 4 ) 043002 - 043002 2020年04月 [査読有り]
研究論文（学術雑誌）
We present an X-ray magnetic circular dichroism (XMCD) study on a nanoscale nonlocal spin injection structure consisting of a perpendicular polarizer and an in-plane free layer. We perform XMCD measurements while applying an operating current to the sample, and observe that the free layer magnetization tilts to the perpendicular direction owing to injection of perpendicular spin. A photoelectron collection method utilizing an order-sorting-aperture (OSA) is developed for this measurement. This OSA collection method enables XMCD measurement on a sample under current application, and its measurement quality is comparable to that recorded with the conventional total electron yield method.
-
Detection of elemental magnetization reversal events in a micro-patterned Nd-Fe-B hot-deformed magnet
Takahiro Yomogita, Nobuaki Kikuchi, Satoshi Okamoto, Osamu Kitakami, Hossein Sepehri-Amin, Tadakatsu Ohkubo, Kazuhiro Hono, Keiko Hioki, Atsushi Hattori
AIP ADVANCES ( AMER INST PHYSICS ) 9 ( 12 ) 125052-1 - 125052-4 2019年12月 [査読有り]
研究論文（学術雑誌）
Magnetization reversal in a permanent magnet takes place through multiple and simultaneous events of nucleation and domain wall depinning. Thus, detection and analysis of elemental magnetization reversal events are essentially important to understand the coercivity mechanism of a permanent magnet. In this study, we have fabricated a micro-patterned Nd-Fe-B hot-deformed magnet using mechanical polishing and focused ion beam, and anomalous Hall effect (AHE) detection has been adopted to measure the magnetization reversal of the sample. During the micro-patterning process, the degradation of magnetic property is carefully evaluated. Consequently, the micro-patterned Nd-Fe-B hot-deformed magnet with the thick of 5 mu m and the width of 13 mu m is fabricated, and subsequently, the discrete steps on the AHE curve are successfully detected. The magnetization reversal unit size estimated from the step height is similar to 1 mu m(2), which is almost the same as observed in the magneto-optical Kerr microscopy. We have clearly demonstrated that this technique has significant potential to study the physical nature of elemental magnetization reversal events in permanent magnets. (C) 2019 Author(s).
-
Temperature dependent magnetization reversal process of a Ga-doped Nd-Fe-B sintered magnet based on first-order reversal curve analysis
Satoshi Okamoto, Kazunori Miyazawa, Takahiro Yomogita, Nobuaki Kikuchi, Osamu Kitakami, Kentaro Toyoki, David Billington, Yoshinori Kotani, Tetsuya Nakamura, Taisuke Sasaki, Tadakatsu Ohkubo, Kazuhiro Hono, Yukio Takada, Takashi Sato, Yuji Kaneko, Akira Kato
ACTA MATERIALIA ( PERGAMON-ELSEVIER SCIENCE LTD ) 178 90 - 98 2019年10月 [査読有り]
研究論文（学術雑誌）
A Ga-doped Nd-Fe-B sintered magnet has attracted significant attention as a heavy-rare-earth-free high-performance magnet. We have studied the temperature dependent magnetization reversal process of a Ga-doped Nd-Fe-B sintered magnet based on the first-order reversal curve (FORC) analysis. The FORC diagram pattern of the Ga-doped Nd-Fe-B sintered magnet changes from single spot in the high field region at room temperature to double spots in the low and high field regions at 200 degrees C, indicating that the dominant magnetization reversal process changes from single domain type to multidomain type. The single domain magnetization reversal at room temperature is well confirmed by using the soft X-ray magnetic circular dichroism microscopy observation. This change in the magnetization reversal process is well discussed by the temperature dependent local demagnetization field and the saturation field of multidomain state. Moreover, we have demonstrated the quantitative analysis of the FORC diagram pattern, which makes a deeper understanding of the magnetization reversal process of the Ga-doped Nd-Fe-B sintered magnet. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
-
Microwave-assisted switching in CoCrPt granular medium under continuous microwave fields
Kikuchi Nobuaki, Sato Katsunari, Kikuchi Shun, Okamoto Satoshi, Shimatsu Takehito, Kitakami Osamu, Osawa Hitoshi, Suzuki Motohiro
JOURNAL OF APPLIED PHYSICS ( AMER INST PHYSICS ) 126 ( 8 ) 2019年08月 [査読有り]
研究論文（学術雑誌）
We have fabricated a device for microwave-assisted switching (MAS) experiments with perpendicularly magnetized CoCrPt-SiO2 granular film with anisotropy field H-k=18.8 kOe. The device was carefully designed to be able to apply a microwave field of 0.95 kOe in amplitude continuously, allowing direct evaluation of microwave-assisted switching properties of the medium without any assumptions about time-dependent switching behavior. The coercivity of CoCrPt-SiO2 medium linearly decreases with increasing microwave frequency, and the maximum coercivity reduction ratio reaches 80% for h(rf)=0.95kOe at f(rf)=25 GHz after subtracting the effect of temperature rise due to microwave field application. The linear slope of coercivity against the frequency becomes steeper with increasing microwave field amplitude, which is a unique frequency and amplitude dependencies in granular media. Correlation length obtained by X-ray magnetic circular dichroism microscopy measurement decreases by applying a microwave field with higher frequency, suggesting that the magnetic clustering dimensions of the medium possibly depend on microwave frequency during microwave-assisted switching. Published under license by AIP Publishing.
-
Microwave-magnetic-field-induced magnetization excitation and assisted switching of antiferromagnetically coupled magnetic bilayer with perpendicular magnetization
Hirofumi Suto, Taro Kanao, Tazumi Nagasawa, Koichi Mizushima, Rie Sato, Nobuaki Kikuchi, Satoshi Okamoto
JOURNAL OF APPLIED PHYSICS ( AMER INST PHYSICS ) 125 ( 15 ) 2019年04月 [査読有り]
研究論文（学術雑誌）
Antiferromagnetically coupled (AFC) magnetic bilayer is a candidate media structure for high-density magnetic recording. Because the stray fields from the two magnetic layers of the AFC bilayer cancel each other out, switching field distribution originating from the stray fields from the adjacent data bits can be suppressed. Furthermore, in microwave-assisted magnetic recording (MAMR), which utilizes ferromagnetic resonance (FMR) excitation in a microwave field to reverse a high-anisotropy magnetic material, AFC media can suppress the distribution in FMR frequency originating from the stray fields and improve MAMR performance. In this study, we fabricate an AFC magnetic bilayer consisting of two Co/Pt multilayers with perpendicular magnetization. We use anomalous-Hall-effect-FMR in combination with a circularly polarized microwave field and carry out layer-selective analysis of FMR excitation of the two magnetic layers. We then investigate the switching behavior of an AFC bilayer nanodot in a microwave magnetic field. The switching field decreases with increasing microwave field frequency and increases abruptly at the critical frequency, and a large switching field reduction by applying a microwave field is demonstrated. This switching behavior is similar to that of a single-layer perpendicular magnetic nanodot, showing that the AFC structure does not hinder the microwave assist effect. Published under license by AIP Publishing.
-
First-order reversal curve analysis of a Nd-Fe-B sintered magnet with soft X-ray magnetic circular dichroism microscopy
Kazunori Miyazawa, Satoshi Okamoto, Takahiro Yomogita, Nobuaki Kikuchi, Osamu Kitakami, Kentaro Toyoki, David Billington, Yoshinori Kotani, Tetsuya Nakamura, Taisuke Sasaki, Tadakatsu Ohkubo, Kazuhiro Hono
ACTA MATERIALIA ( PERGAMON-ELSEVIER SCIENCE LTD ) 162 1 - 9 2019年01月 [査読有り]
研究論文（学術雑誌）
First-order reversal curve (FORC) diagram, which visualizes the variation of magnetic susceptibility on a field plane, has been applied to a Nd-Fe-B sintered magnet. The FORC diagram exhibits the characteristic behavior of two remarkable spots in low-field and high-field regions. The high-field FORC spot corresponds to the irreversible magnetization reversal at a coercive field, whereas the low-field FORC spot indicates the appearance of a large magnetic susceptibility state during the demagnetization process. Moreover, this low-field FORC spot becomes dominant at high temperature, accompanied by a significant reduction in coercivity. These results suggest that the low-field FORC spot has a strong correlation with the degradation of magnetic properties of a Nd-Fe-B sintered magnet. To clarify the actual magnetization reversal processes corresponding to these two FORC spots, soft X-ray magnetic circular dichroism (XMCD) microscopy observation was employed with similar field sequences of the FORC measurements. Consequently, the low-field FORC spot is mainly attributed to the domain wall motion in multi-domain grains, whereas the high-field FORC spot corresponds to the magnetization reversal of single-domain grains. These indicate that a FORC diagram is a powerful evaluation method for the magnetization reversal processes of permanent magnets. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
-
Variation of Magnetization Dynamics of Co/Ni Multilayer by Capturing Magnetic Nanoparticles
Y. Otaki, T. Kato, S. Okamoto, N. Kikuchi, D. Oshima, S. Iwata, O. Kitakami
IEEE Transactions on Magnetics 54 ( 11 ) 6100804-1 - 6100804-4 2018年11月 [査読有り]
研究論文（学術雑誌）
-
Frequency dependence of microwave-assisted switching in CoCrPt granular perpendicular media
Kikuchi Nobuaki, Shimada Kyohei, Shimatsu Takehito, Okamoto Satoshi, Kitakami Osamu
JAPANESE JOURNAL OF APPLIED PHYSICS ( IOP PUBLISHING LTD ) 57 ( 9 ) 09TE02-1 - 4 2018年09月 [査読有り]
研究論文（学術雑誌）
The microwave-assisted switching properties of a CoCrPt-TiO2 granular perpendicular medium are investigated by detecting the anomalous Hall effect. We improved the device structure to be able to generate a linearly polarized microwave field of 950 Oe without a significant increase in temperature. Coercivity decreases linearly with increasing microwave frequency and the microwave assistance effect is most enhanced by the application of microwaves of 18 GHz frequency with a coercivity reduction ratio of 55%. (C) 2018 The Japan Society of Applied Physics
-
Spin transfer torque switching of Co/Pd multilayers and Gilbert damping of Co-based multilayers
Kimura Takumi, Dong Xiayin, Adachi Kanta, Oshima Daiki, Kato Takeshi, Sonobe Yoshiaki, Okamoto Satoshi, Kikuchi Nobuaki, Kawato Yoshiaki, Kitakami Osamu, Iwata Satoshi
JAPANESE JOURNAL OF APPLIED PHYSICS ( IOP PUBLISHING LTD ) 57 ( 9 ) 09TD01-1 - 5 2018年09月 [査読有り]
研究論文（学術雑誌）
The critical current density (J(c0)) and thermal stability factor Delta for the spin transfer torque switching of [Pt/Co](6)/Cu/[Co/Pd](3) nanopillars with various Co/Pd layer compositions were investigated. Moreover, the Gilbert damping constants a of Co/Pd and Co/Pt multilayers (MLs) with various layer compositions were also studied to understand the variations in the J(c0) and Delta of the nanopillars. The J(c0) and Delta of Co/Pd MLs were found to be almost independent of the pillar diameters, and the effective reversal size contributing to Delta was found to be much smaller than the physical pillar diameter. From the spin transfer torque (STT) switching of Co/Pd MLs with various layer compositions, J(c0) was found to gradually increase with increasing thickness ratio of Pd and Co layers, t(Pd)/t(Co), up to t(Pd)/t(Co) = 2, and further increase in t(Pd)/t(Co) resulted in the decrease in J(c0). On the other hand, Delta was roughly independent of the thickness ratio. The Gilbert damping constants a of Co/Pd and Co/Pt MLs increased with increasing thickness ratio of noble metal and Co layers, t(NM)/t(Co). The large damping constant of the Co/Pt ML compared with that of the Co/Pd ML means that Co/Pt and Co/Pd MLs are appropriate for use as reference and memory layers, respectively. The increase in a with t(Pd)/t(Co) is considered to be responsible for the increase in J(c0) up to t(Pd)/t(Co) = 2. (C) 2018 The Japan Society of Applied Physics
-
Microwave Spectroscopy of a Single Permalloy Chiral Metamolecule on a Coplanar Waveguide
Toshiyuki Kodama, Yusaku Kusanagi, Satoshi Okamoto, Nobuaki Kikuchi, Osamu Kitakami, Satoshi Tomita, Nobuyoshi Hosoito, Hisao Yanagi
Physical Review Applied ( American Physical Society ) 9 ( 5 ) 054025-1 - 6 2018年05月 [査読有り]
研究論文（学術雑誌）
We investigate the microwave spectroscopies of a micrometer-sized single permalloy (Py) chiral structure on coplanar waveguides (CPWs). Under an external dc magnetic field applied in a direction perpendicular to the microwave propagation, the Py chiral structure loaded on the center of the CPW signal line shows Kittel-mode ferromagnetic resonance. Contrastingly, the structure on the signal-line edge highlights two additional resonances: spin-wave resonance at a higher frequency, and unique resonance at a lower frequency of approximately 7.8 GHz. The resonance signal at 7.8 GHz originates from magnetically induced, geometry-driven resonance, although the resonance frequency does not depend on the external magnetic field. Moreover, the displacement of the Py structures on the signal line results in nonreciprocal microwave transmission, which is traced back to the edge-guide mode.
-
Layer-selective microwave-assisted magnetization switching in a dot of double antiferromagnetically coupled (AFC) layers
Yuming Lu, Satoshi Okamoto, Nobuaki Kikuchi, Osamu Kitakami, Takehito Shimatsu
Applied Physics Letters ( American Institute of Physics Inc. ) 112 ( 16 ) 162404-1 - 4 2018年04月 [査読有り]
研究論文（学術雑誌）
Layer-selective magnetization switching is a key technology for three-dimensional magnetic recording. In this study, layer-selective magnetization switching is demonstrated in a dot of double antiferromagnetically coupled (AFC) layers under an assistance of an rf field. Each AFC layer consists of two Co/Pt multilayers with a Ru interlayer. By optimally tuning the rf frequency, the layer-selective switching of each AFC layer is clearly confirmed. However, this layer-selective switching is probabilistic, with a maximum probability of about 65%. The stability of the layer-selective switching is improved if the dipolar field from the first switched AFC layer is suppressed.
-
Magnetization Reversal of Nd-Fe-B Thin Film under a Nanosecond Pulse Magnetic Field
Kazuya Kadonosawa, Nobuaki Kikuchi, Ryota Goto, Satoshi Okamoto, Osamu Kitakami
ELECTRICAL ENGINEERING IN JAPAN ( WILEY ) 203 ( 1 ) 3 - 8 2018年04月 [査読有り]
研究論文（学術雑誌）
The magnetization reversal process of Nd-Fe-B magnets has been long discussed. Since the elemental steps of the magnetization reversal process such as a nucleation of reversed embryo and a wall depinning are the events as small as nanometer scale and as fast as nano/sub-nanosecond time scale, the study on the magnetization reversal with these space and time regions is essentially important. In this work, a generation of large amplitude nanosecond pulse field and a fabrication of Nd-Fe-B nanodot were examined, and then the magnetization reversal process of the Nd-Fe-B nanodot under the nanosecond pulse field was investigated. Combination of a Blumlein transmission line and a micro-coil enables to generate a pulse field of 0.87 T in amplitude and 6 ns in width. It was clearly demonstrated that the reversed domain evolution in the dot under the pulse field was quite different from that under the static field, probably owing to the dispersion of energy barrier of each elemental magnetization reversal step.
-
Novel torque magnetometry for uniaxial anisotropy constants of thin films and its application to FePt granular thin films
Takuya Ono, Nobuaki Kikuchi, Satoshi Okamoto, Osamu Kitakami, Takehito Shimatsu
APPLIED PHYSICS EXPRESS ( IOP PUBLISHING LTD ) 11 ( 3 ) 033002-1 - 3 2018年03月 [査読有り]
研究論文（学術雑誌）
A technique based on torque magnetometry is proposed for precise determination of the uniaxial magnetic anisotropy constant, K-u, of perpendicularly magnetized thin films, in particular for K-u larger than 10(6) J/m(3), by electrical detection of the anomalous Hall effect and exact determination of the magnetic field direction. This technique can measure the K-u value of several-nanometer-thick polycrystalline films. Further, the K-u value of FePt-C granular films, which are expected to be useful for high-density magnetic recording media, is evaluated. The intrinsic K-u value of 4-nm-thick FePt-C thin films fabricated at 450 degrees C is approximately 3 MJ/m(3) for carbon contents up to 34 vol% C. (C) 2018 The Japan Society of Applied Physics
-
Temperature and field direction dependences of first-order reversal curve (FORC) diagrams of hot-deformed Nd-Fe-B magnets
Takahiro Yomogita, Satoshi Okamoto, Nobuaki Kikuchi, Osamu Kitakami, Hossein Sepehri-Amin, Tadakatsu Ohkubo, Kazuhiro Hono, Takahiro Akiya, Keiko Hioki, Atsushi Hattori
JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS ( ELSEVIER SCIENCE BV ) 447 110 - 115 2018年02月 [査読有り]
研究論文（学術雑誌）
First-order reversal curve (FORC) diagram has been previously adopted for the analyses of magnetization reversal process and/or quantitative evaluation of coercivity and interaction field dispersions in various magnetic samples. Although these kinds of information are valuable for permanent magnets, previously reported FORC diagrams of sintered Nd-Fe-B magnets exhibit very complicated patterns. In this paper, we have studied the FORC diagrams of hot-deformed Nd-Fe-B magnets under various conditions. Contrary to the previous reports on sintered Nd-Fe-B magnets, the FORC diagram of the hot-deformed Nd-Fe-B magnet exhibits a very simple pattern consisting of a strong spot and a weak line. From this FORC diagram pattern, it is revealed that the coercivity dispersion of the hot-deformed Nd-Fe-B magnets is surprisingly small. Moreover, this feature of the FORC diagram pattern is very robust and unaffected by changes in various conditions such as grain boundary diffusion process, temperature, and field direction, whereas these conditions significantly change the coercivity and the shape of magnetization curve. This fact indicates that the magnetization reversal process of the hot-deformed Nd-Fe-B magnets is almost unchanged against these conditions. (c) 2017 Elsevier B.V. All rights reserved.
-
Time and Spatially Resolved Hard X-Ray MCD Measurement on a Co/Pt Multilayer Dot Excited by Pulsed RF Field
Nobuaki Kikuchi, Hitoshi Osawa, Motohiro Suzuki, Osamu Kitakami
IEEE Transactions on Magnetics ( Institute of Electrical and Electronics Engineers Inc. ) 54 ( 2 ) 6100106-1 - 6 2018年02月 [査読有り]
研究論文（学術雑誌）
Understanding of magnetization behavior in magnetic nanostructures is a key issue for future magnetic/spintronic devices. Time-resolved X-ray magnetic circular dichroism (XMCD) is a powerful measurement technique with potential of nanometer spatial resolution, picosecond time resolution, and element selectivity. We have carried out time and spatially resolved XMCD microscopy measurement on a Co/Pt multilayer dot by detecting XMCD at the Pt L-{3} edge at BL39XU of SPring-8 using hard X-ray. Transient magnetic response of Co/Pt multilayer dots with diameter of 2.6~\\mu \\text{m} was investigated against pulsed RF field with frequency around 2.5 GHz and the maximum amplitude of about 400 Oe. By synchronizing excitation RF field pulse with X-ray pulses, the growth of magnetization precession of the induced Pt magnetic moment in the dot was clearly observed with sub-100 ps time resolution and submicrometer spatial resolution. The developed measurement setup can be applicable for a wide range of studies for time-resolved hard X-ray experiments.
-
Significant modification of perpendicular magnetic anisotropy of W/Fe(001) multilayer by controlling in-plane lattice constant
Yuki Matsumoto, Yoshio Miura, Satoshi Okamoto, Nobuaki Kikuchi, Osamu Kitakami
APPLIED PHYSICS EXPRESS ( IOP PUBLISHING LTD ) 10 ( 6 ) 063005-1 - 063005-4 2017年06月 [査読有り]
研究論文（学術雑誌）
First-principles calculation predicts a large negative to positive change in the intrinsic perpendicular magnetic anisotropy (PMA) on the order of 10(7) erg/cm(3) of a W/Fe(001) multilayer upon reducing the in-plane lattice constant. This PMA arises at the W/Fe interface, and the second interfacial W site plays an important role. To experimentally verify this theoretical prediction, we have grown W/Fe/W(001) epitaxial trilayers on Cr(001) underlayers. By varying the W layer thickness, the in-plane lattice constant of W can be widely controlled, and the large change in PMA from negative to positive is successfully demonstrated. (C) 2017 The Japan Society of Applied Physics
-
Influence of intergrain interactions and thermal agitation on microwave-assisted magnetization switching behavior of granular magnetic film
Satoshi Okamoto, Nobuaki Kikuchi, Osamu Kitakami, Takehito Shimatsu
APPLIED PHYSICS EXPRESS ( IOP PUBLISHING LTD ) 10 ( 2 ) 023004-1 - 023004-4 2017年02月 [査読有り]
研究論文（学術雑誌）
Microwave-assisted magnetization switching (MAS) in a granular magnetic film is examined by computer simulation. Contrary to the macrospin calculation and the experiments on single magnetic dots reported so far, in which the switching field linearly decreases with increasing rf frequency and then sharply increases at the critical frequency, the granular film exhibits considerably broad MAS behavior against the rf frequency. This broad MAS behavior is mainly caused by the dispersion of magnetic properties and thermal agitation. On the other hand, intergrain dipolar and exchange interactions enhance the MAS effect in the granular film and suppress the MAS broadening. (C) 2017 The Japan Society of Applied Physics
-
Magnetic characteristics and nanostructures of FePt granular films with GeO2 segregant
Takuya Ono, Tomohiro Moriya, Masatoshi Hatayama, Kaoru Tsumura, Nobuaki Kikuchi, Satoshi Okamoto, Osamu Kitakami, Takehito Shimatsu
APPLIED PHYSICS LETTERS ( AMER INST PHYSICS ) 110 ( 2 ) 2017年01月 [査読有り]
研究論文（学術雑誌）
To realize a granular film composed of L1(0)-FePt grains with high uniaxial magnetic anisotropy energy, K-u, and segregants for energy-assisted magnetic recording, a FePt-GeO2/FePt-C stacked film was investigated in the engineering process. The FePt-GeO2/FePt-C stacked film fabricated at a substrate temperature of 450 degrees C realized uniaxial magnetic anisotropy, K-u(grain), of about 2.5 x 10(7) erg/cm(3), which is normalized by the volume fraction of FePt grains, and a granular structure with an averaged grain size of 7.7 nm. As the thickness of the FePt-GeO2 upper layer was increased to 9 nm, the K-u values were almost constant. That result differs absolutely from the thickness dependences of the other oxide segregant materials such as SiO2 and TiO2. Such differences on the oxide segregant are attributed to their chemical bond. The strong covalent bond of GeO2 is expected to result in high K-u of the FePt-GeO2/FePt-C stacked films. Published by AIP Publishing.