研究等業績 - 原著論文 - 菊池 伸明
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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）
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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.
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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).
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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.
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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.
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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.
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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.
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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）
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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
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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
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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.
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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.
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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.
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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
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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.
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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.
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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
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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
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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.