Research Achievements - Original paper -
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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 2018.09 [Refereed]
Research paper (journal)
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 2018.05 [Refereed]
Research paper (journal)
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 2018.04 [Refereed]
Research paper (journal)
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 2018.04 [Refereed]
Research paper (journal)
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 2018.03 [Refereed]
Research paper (journal)
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 2018.02 [Refereed]
Research paper (journal)
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 2018.02 [Refereed]
Research paper (journal)
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 2017.06 [Refereed]
Research paper (journal)
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 2017.02 [Refereed]
Research paper (journal)
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 ) 2017.01 [Refereed]
Research paper (journal)
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.