Authorship：Corresponding author   Publishing type：Research paper (international conference proceedings)  

Magnetic skyrmions are used as binary digits of a domain motion-type magnetic memory such as racetrack memory. When two or more skyrmions are aligned on a track, a space must exist between each skyrmion. Eliminating this space would increase the storage density of racetrack memory. Thus, this study proposed the use of connected skyrmions as binary digits in racetrack memory and novel domain structures to remove this space. We proposed bi-skyrmion, i.e., two connected skyrmions, and tri-skyrmions, i.e., three connected skyrmions, having skyrmion numbers of 2 and 3, respectively. The connected skyrmions do not have a space between them. We investigate 2-, 3-, and 100-connected skyrmions produced in the nanowire, which is perpendicularly magnetized, using the micromagnetic simulation. The 2-, 3-, and 100-connected skyrmions can be stabilized in the wire. They were moved by the spin-polarized current in a manner similar to the skyrmion.

DOI： 10.1109/INTERMAG50591.2023.10265067

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We fabricated two types of cobalt-ferrite (001) thin films, insulative Fe-rich cobalt-ferrite CoxFe3-xO4+δ (I-CFO) and conductive Fe-rich cobalt-ferrite CoyFe3-yO4 (C-CFO), with perpendicular magnetic anisotropy (PMA) on MgO (001) substrates. Although the stoichiometric cobalt ferrite is known as an insulating material, it is found that the conductivity of Fe-rich cobalt ferrites can be controlled by changing the source materials and deposition conditions in the pulsed laser deposition technique. The I-CFO and C-CFO films exhibit PMA through the in-plane lattice distortion. We investigated the Fe-ion-specific valence states in both I-CFO and C-CFO films by Mössbauer spectroscopy and x-ray magnetic circular dichroism and found that the difference in conductivity corresponds to the abundance ratio of the Fe2+ state at the octahedral B-site (Oh) in the inverse spinel structure. Furthermore, first-principles calculations reproduce the changes in the density of states at the Fermi level depending on the cation vacancies at the B-site, which explains the difference in conductivity between I-CFO and C-CFO.

DOI： 10.1103/PhysRevMaterials.7.054402

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DOI： 10.1063/5.0131390

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The effect of interlayer exchange coupling (IEC) in {Tb/Co}4/Rh/{Co/Tb}4/Pt multilayered wires on the domain wall (DW) structure and DW velocity was investigated. It was found that the stability of the Néel-type DWs was enhanced and the velocity of the current-induced DW movement increased with an increase in the antiferromagnetic IEC strength. The micromagnetic simulation based on the Landau–Lifshitz equation clarifies that the direction of the magnetic moment in the DWs depends on the antiferromagnetic IEC strength, and the stability of the Néel-type DW increases with an increase in the antiferromagnetic IEC strength. As a consequence of this stability, the spin-orbital torque affects effectively on the magnetic moment of the DWs and the DW velocity increases. These results imply that the DW velocity in magnetic domain-based memories can be controlled by the antiferromagnetic IEC strength.

DOI： 10.1016/j.jmmm.2022.170218

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Magnetic-skyrmion-based racetrack memory is a high-density memory for storing information driven by low-power, wherein the arrangement of skyrmions is stored in the form of binary digit combinations. This arrangement is altered based on the skyrmion motion resulting from the influence of the spin-transfer torque. We propose a novel layered structure of a ferromagnetic metal (FM) wire/nonmagnetic insulator wire/FM wire in which the skyrmions move while maintaining the arrangement. It consists of two FM wires that are perpendicularly magnetized. One of these comprises several skyrmions, which act as binary digits (data skyrmions). The other wire constitutes a sufficient number of skyrmions that are closely packed and allow the spin-polarized current to flow. A dipole-dipole interaction acts between these two FM wires. The motion of the data skyrmion is induced by the current-induced motion of the packed skyrmion. The motion of skyrmions is simulated via micromagnetic approaches. The data skyrmions move while maintaining the distance between each skyrmion. The analog position of the data skyrmion is controlled by the current flowing through another wire. The stable data skyrmion motion has a threshold current. When the current is larger than the threshold current, the skyrmion breaks, and/or the distance between skyrmions breaks. The threshold current density is found to decrease with an increasing vacuum layer thickness and increase with increasing thickness of the FM wire flowing current. Our results show that the skyrmion motion in the proposed structure can drive skyrmions with a spin-current density lower than that of the coupled granular/continuous structure and the edge-notched nanowire structure. This structure can be used in developing low-power operational devices based on skyrmion motion.

DOI： 10.1088/1361-6463/ac941d

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We focus on rare-earth-free anti-perovskite Mn4-xNixN epitaxial films, which can be used for ultrafast current-induced domain wall motion (CIDWM) in magnetic strips. The magneto-transport properties of these materials are very important for a deep understanding of CIDWM. In this study, we investigated the magneto-transport properties of Mn4-xNixN epitaxial films grown on SrTiO3(001) and MgO(001) substrates through anisotropic magnetoresistance (AMR) measurements at temperatures between 2 and 300 K. In samples with a small Ni composition such as x = 0.05-0.1, the AMR ratio of Mn4-xNixN drastically decreased with increasing temperature. We also analyzed the twofold and fourfold symmetries in the AMR curves. Fourfold symmetry is caused by tetragonal crystal fields and is unique to anti-perovskite 3d-metal nitrides such as Mn4N and Fe4N. Only slight fourfold symmetry was observed in Mn4-xNixN. We also performed first-principles calculations with the Vienna ab initio simulation package (VASP) to obtain the projected density of states (PDOS) of d orbitals in Mn4-xNixN, which is responsible for the magnetism of these materials. We conclude that these results are due to the Ni atoms, which function as magnetic impurities and lead to a noticeable change in PDOS, as proved by VASP calculation.

DOI： 10.1063/5.0107172

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The Stoner criterion allows only three single elements possessing room-temperature (RT) ferromagnetism: cobalt (Co), nickel (Ni), and iron (Fe). Although these three elements have played central roles in magnetism-based materials, their large work function (4.5∼5.2eV) is becoming a non-negligible obstacle for realization of spin devices using nonmetallic materials with finite energy gaps, because injection of electron spins into these nonmetallic materials is strongly hampered due to the large Schottky barrier height. Hence, a novel ferromagnetic or ferrimagnetic material simultaneously possessing RT ferromagnetism or ferrimagnetism and high Fermi energy is strongly required. Here, we show that an Fe-based alloy, iron-gadolinium (FeGd), allows circumvention of the obstacle. Surprisingly, only 20% of Gd incorporation in Fe dramatically modulates the Fermi energy from -4.8 to -3.0 eV, which is the largest modulation in all metallic alloys reported thus far. The coexistence of ferrimagnetism and nonzero spin polarization at RT of FeGd supports its abundant potential for future applications in low-carrier-density materials such as monolayer, organic, and nondegenerate inorganic semiconductors.

DOI： 10.1103/PhysRevMaterials.6.104405

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.1088/1361-6463/ac80dd

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Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

The ferrimagnet Mn4N forms a family of compounds useful in spintronics. In a compound comprising non-magnetic and magnetic elements, one basically expects the compound to become ferromagnetic when the proportion of the magnetic element increases. Conversely, one does not expect ferromagnetism when the proportion of the non-magnetic element increases. Surprisingly, Mn4N becomes ferromagnetic at room temperature when the Mn content is decreased by the addition of In atoms, a non-magnetic element. X-ray magnetic circular dichroism measurement reveals that the magnetic moment of Mn atoms at face-centered sites, Mn(II), reverses between x= 0.15 and 0.27 and aligns parallel to that of Mn atoms at corner sites, Mn(I), at x = 0.27 and 0.41. The sign of the anomalous Hall resistivity also changes between x = 0.15 and 0.27 in accordance with the reversal of the magnetic moment of the Mn(II) atoms. These results can be interpreted using first-principles calculations, showing that the magnetic moment of Mn(II) sites which are the nearest neighbors to the In atom align to that of Mn(I) sites.

DOI： 10.1088/1361-6463/ac3e90

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Authorship：Corresponding author  

J-GLOBAL

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Authorship：Corresponding author  

J-GLOBAL

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.1088/1361-6463/ac3454

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing Ltd  

Anti-perovskite transition metal nitrides exhibit a variety of magnetic properties-such as ferromagnetic, ferrimagnetic, and paramagnetic-depending on the 3d transition metal. Fe4N and Co4N are ferromagnetic at room temperature (RT), and the minority spins play a dominant role in the electrical transport properties. However, Mn4N is ferrimagnetic at RT and exhibits a perpendicular magnetic anisotropy caused by tensile strain. Around the magnetic compensation in Mn4N induced by impurity doping, researchers have demonstrated ultrafast current-induced domain wall motion reaching 3000 m s(-1) at RT, making switching energies lower and switching speed higher compared with Mn4N. In this review article, we start with individual magnetic nitrides-such as Fe4N, Co4N, Ni4N, and Mn4N; describe the nitrides' features; and then discuss compounds such as Fe(4-x )A (x) N (A = Co, Ni, and Mn) and Mn4-x B (x) N (B = Ni, Co, and Fe) to evaluate nitride properties from the standpoint of spintronics applications. We pay particular attention to preferential sites of A and B atoms in these compounds, based on x-ray absorption spectroscopy and x-ray magnetic circular dichroism.

DOI： 10.1088/1361-6528/ac2fe4

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Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

DOI： 10.1063/5.0045233

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.1088/1361-6463/abd060

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Spinel-type magnetite Fe3O4 is predicted to be a half-metal material with negative spin polarization. However, magnetic tunnel junctions (MTJs) using an Fe3O4 electrode exhibit a small tunnel magnetoresistance (TMR) effect, the sign of which has not been established experimentally. The development of Fe3O4 as an excellent TMR material requires a better understanding of the characteristics of the interface and the phase transition of Fe3O4 called the Verwey transition. We fabricate MTJs using epitaxial Fe3O4-based stacks on MgO(001) substrates and find a large inverse TMR ratio of -55.8% at 80 K, which corresponds to 126% by the optimistic definition of the TMR ratio. The temperature dependence of the TMR ratio is significantly affected by the Verwey transition. Moreover, we investigate the dependence of TMR on oxygen partial pressure during Fe3O4 deposition. It is found that the magneto-transport properties of the MTJs show different behaviors depending on the oxygen partial pressure because the Verwey transition is sensitive to the oxygen concentration. Furthermore, the electronic and magnetic properties at the interfacial regions are investigated by x-ray magnetic spectroscopy and first-principles calculation. These findings greatly support the use of Fe3O4 in spintronic devices and should lead to further developments in oxide spintronics.

DOI： 10.1103/PhysRevApplied.15.034042

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Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

DOI： 10.1063/9.0000066

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Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers ({IEEE})  

DOI： 10.1109/TMAG.2020.3010328

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevb.102.174442

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.102.174442/fulltext

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Magnetic domains are used to represent binary digits in magnetic memories. In domain-motion magnetic memories, the domain must move longitudinally along a ferromagnetic nanowire. Antiferromagnetically coupled magnetic skyrmions, which are domains meeting this requirement, can be stabilized via the Dzyaloshinskii-Moriya interaction (DMI) in an antiferromagnetically coupled bi-layered perpendicularly magnetized thin wire. We find that antiferromagnetically coupled skyrmion-like bubble domains (BDs) may be produced in such a wire without DMI. The stability and the dynamics of such domains are studied by micromagnetic simulations based on the Landau-Lifshitz-Gilbert equation. The BDs in the two layers have clockwise and anti-clockwise Bloch type domain walls, so the BD cores have anti-parallel magnetic moments and are antiferromagnetically coupled. Domain size decreases with decreasing wire width and/or increasing antiferromagnetic coupling strength of the wire. The minimum value of the BD radius is about 10 nm. When a spin-polarized current flows through the wire, the BD moves with the current.

DOI： 10.1088/1361-6463/aba14f

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Ferrimagnets in the vicinity of the magnetic compensation point are of particular interest for spin-torquebased spintronics applications, because they require lower switching currents and thus smaller switching energies. Room-temperature compensation is rather uncommon, but can be obtained by tuning the chemical concentrations of compounds such as Co0.79Tb0.21 or Co0.56Gd0.44, in order to find the single concentration at which one sublattice compensates the other. Here we study the ferrimagnetic Mn4-xCoxN films, showing that they presumably possess at room temperature not only one but two different Co concentrations at which the magnetic compensation occurs. Using x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements, we show that this startling behavior can be explained by the existence of two nonequivalent sites for the Co, when it is inserted in the antiperovskite structure of epitaxial Mn4-xCoxN films.

DOI： 10.1103/PhysRevMaterials.4.094401

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We investigated the relation between the domain wall (DW) configuration and the strength of the interlayer exchange coupling (IEC) in multilayered magnetic wires, where a Ru spacer layer mediates the IEC between two ferromagnetic layers, and a Pt cap layer contributes to the interfacial Dzyaloshinskii-Moriya interaction. It was found that the antiferromagnetic IEC stabilizes the Neel-type DWs in the wires and that the stability varies with the strength of the antiferromagnetic IEC. The micromagnetic simulations imply that the Neel-type DWs are stabilized because the rotation of the magnetic moments in the DWs is restricted in the antiferromagnetically coupled ferromagnetic layers.

DOI： 10.1063/5.0013481

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

© 2019 American Physical Society. Passivation of barium disilicide (BaSi2) films is very important for their use in solar cell applications. In this paper, we demonstrated the effect of hydrogen (H) passivation on both the photoresponsivity and minority-carrier lifetime of BaSi2 epitaxial films grown by molecular beam epitaxy. First, we examined the growth conditions of a 3-nm-thick hydrogenated amorphous silicon (a-Si) capping layer formed on a 500-nm-thick BaSi2 film and found that an H supply duration (ta-Si:H) of 15 min at a substrate temperature of 180 °C sizably enhanced the photoresponsivity of the BaSi2 film. We next supplied atomic H to BaSi2 epitaxial films at 580 °C and changed supply duration (tBaSi;H) in the range of 1-30 min, followed by capping with an a-Si layer. The photoresponsivity of the films changed considerably depending on tBaSi;H and reached a maximum of 2.5 A/W at a wavelength of 800 nm for the sample passivated for tBaSi;H=15 min under a bias voltage of 0.3 V applied to the front-surface indium-tin-oxide electrode with respect to the back-surface aluminum electrode. This photoresponsivity is approximately one order of magnitude higher than the highest value previously reported for BaSi2. Microwave photoconductivity decay measurements revealed that the minority-carrier lifetime of the BaSi2 film with the highest photoresponsivity was 14 μs, equivalent to its bulk carrier lifetime ever reported. We performed theoretical analyses based on a rate equation including several recombination mechanisms and reproduced the experimentally obtained decay curves. We also calculated the total density of states of BaSi2 by ab initio studies when one Si vacancy existed in a unit cell and one, two, and three H atoms occupied Si vacancy or interstitial sites. A Si vacancy caused a localized state with two energy bands to appear close to the middle of the band gap. In certain cases, H passivation of the Si dangling bonds can markedly decrease trap concentration. From both experimental and theoretical viewpoints, we conclude that an atomic H supply is beneficial for BaSi2 solar cells.

DOI： 10.1103/PhysRevMaterials.3.065403

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

A water streamer was designed for the purpose of enhancing cost-efficient circulation between the water surface and bottom of shrimp aquaculture ponds. We took direct measurements of the water current field induced by the designed aerator in a large rectangular reservoir of dimensions (L)50 x (W)19 x (D)1.3 m(3) and compared the results with those of a standard Taiwanese paddle-wheel aerator. Vertical circulation between the surface and the bottom induced by the paddle wheel aerator was less than that by the designed aerator. Furthermore, the paddle-wheel aerator consumed more electric power than the designed aerator. The structures of water current induced by the two aerators are elucidated, and the advantages and disadvantages of the aerators are discussed.Auxiliary employment of the designed aerators may contribute to delivery of high DO water throughout the pond, which is more efficient than a paddle wheel aerator.

DOI： 10.1016/j.aquaeng.2019.03.006

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© 2018 Elsevier B.V. We successfully fabricated perpendicularly magnetized cobalt-ferrite CoxFe3-xO4+δ (0 0 1) films on nonmagnetic metal TiN (0 0 1) films. We found that the in-plane lattice constant of the CoxFe3-xO4+δ (0 0 1) films was larger than the out-of-plane lattice constant. This lattice strain induced perpendicular magnetic anisotropy, and square hysteresis curves were observed in out-of-plane magnetic fields for the CoxFe3-xO4+δ (x = 0.4 ∼ 0.6) films. With the decrease of the Co composition from x = 1 to x = 0, the CoxFe3-xO4+δ films approached not to conductive Fe3O4 but to insulating γ-Fe2O3 accompanying cation vacancies in the cubic spinel structure. The good tunneling conductivity character was observed for the CoxFe3-xO4+δ tunnel junctions. The results open up a possibility that insulating CoxFe3-xO4+δ films can be applied to elements of perpendicularly magnetized spintronics devices.

DOI： 10.1016/j.jmmm.2018.12.025

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers ({IEEE})  

DOI： 10.1109/TMAG.2018.2866500

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We show evident crystal orientation effect on pure spin current transport in Si-based lateral spin-valve (LSV) devices with epitaxially grown CoFe/MgO tunnel contacts. When we compare nonlocal spin signals between LSV devices along < 1 0 0 > (Si < 1 0 0 >) and < 1 1 0 > (Si < 1 1 0 >), we find that the magnitude of the spin signals for Si < 100 > LSV devices is always larger than that for Si < 1 1 0 > LSV devices. The analyses based on the one-dimensional spin diffusion model reveal that the spin diffusion length and spin lifetime between Si < 1 0 0 > and Si < 1 1 0 > LSV devices are comparable, while the spin injection/detection efficiency in Si < 1 0 0 > LSV devices is evidently larger than that in Si < 1 1 0 > ones. Possible origins of the difference in the spin injection/detection efficiency between Si < 1 0 0 > and Si < 1 1 0 > LSV devices are discussed.

DOI： 10.1088/1361-6463/aaf37c

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Bubble domain (BD) motion is expected to be useful for domain motion magnetic memory. We investigate the current-induced motion of BDs produced in a perpendicularly magnetised thin wire with edge defects by using micro-magnetic simulation. We produce a stable BD in the wire and find that its size increases with the wire width. Upon application of a spin-polarised current, the BD both with and without Bloch points moves with a velocity similar to the theoretical velocity of domain wall motion while maintaining its structure. In the presence of edge defects in the wire, the motion of the BD is slightly affected by the magnetisation of the uneven edge structure, whereas a straight domain wall is pinned strongly by the edge defects. The effect of edge defects on BD motion is much smaller than that in straight domain wall motion.

DOI： 10.1088/1361-6463/aae8cd

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Herein, 30 nm-thick Fe4−xMnxN (x = 0, 1, 2, 3, and 4) epitaxial films were grown on MgO(001) substrates by molecular beam epitaxy, and the anisotropic magnetoresistance (AMR) properties were measured at temperatures (T) between 10 and 300 K. A negative AMR effect was observed in the Fe4N and FeMn3N films at T ≤ 300 K and in the Mn4N film at T ≤ 100 K. In contrast, a positive AMR effect was observed in the Fe3MnN and Fe2Mn2N films at T ≤ 300 K. Using the relationship between the AMR ratio, the spin polarization of the density of states at the Fermi level, and the spin polarization of the electrical conductivity (Pσ), we derived the sign of Pσ to be negative in Fe4N, Fe3MnN, and Fe2Mn2N and to be positive in FeMn3N and Mn4N. These results show that the minority spin transport is dominant in Fe4−xMnxN for x = 0, 1, and 2 at lower temperatures, whereas the majority spin transport is dominant with increasing x in Fe4−xMnxN for x = 3 and 4.

DOI： 10.1063/1.5051721

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.1088/2053-1591/aad14b

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Authorship：Lead author,　Corresponding author  

© 2018 Kansai University. All rights reserved. Single-layer molybdenum disulfide, which is expected to be used in electronics devices base on two-dimensional materials, is a direct bandgap semiconductor with a bandgap of 1.8 eV. A single layer of molybdenum disulfide substitutional-doped chromium atoms (CMo!) with 2H-phase is also a direct bandgap semiconductor. Its own bandgap decreases as the number of chromium atoms increases. We investigated the electronic structure of CrMoi- in the IT and distorted-lT phases by means of the first-principle calculation. We found that Cr.Mo! w&#039;th lT-phase changes to a semimetal, metal, and a semiconductor as the doping concentration of chromium atoms changes. CrMoi- in the distorted-lT phase is a semiconductor with an indirect small bandgap of about 0.1 eV.

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© 2018 Kansai University. All rights reserved. Water circulation induced by a standard Taiwanese paddle-wheel aerator in a rectangular reservoir with the dimensions (L, W, D) 50 × 19 × 1.3 m3 was studied to evaluate of the efficiency of the aeration process in aquaculture. The water velocity was measured using an electromagnetic water current meter located at 12 grid points selected downstream of the aerator. We found that the water velocity field is unidirectional. This suggests that fluid mixing in the vertical direction may not be comparable to what would be expected from the intense splashes and loud noise produced by the operating aerator.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We investigated head-to-head domain walls in nanostrips of epitaxial Fe4N(001) thin films, displaying a fourfold magnetic anisotropy. Magnetic force microscopy and micromagnetic simulations show that the domain walls have specific properties, compared to soft magnetic materials. In particular, strips aligned along a hard axis of magnetization are wrapped by partial flux-closure concertina domains below a critical width, while progressively transforming to zigzag walls for wider strips. Transverse walls are favored upon the initial application of a magnetic field transverse to the strip, while transformation to vortex walls is favored upon motion under a longitudinal magnetic field. In all cases, the magnetization texture of such fourfold anisotropy domain walls exhibits narrow micro-domain walls, which may give rise to peculiar spin-transfer features. Published by AIP Publishing.

DOI： 10.1063/1.4989991

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We demonstrate generation and detection of pure spin currents by combining a two-terminal spin-injection technique and Co2FeSi (CFS) spin injectors in lateral spin valves (LSVs). We find that the two-terminal spin injection with CFS has the robust dependence of the nonlocal spin signals on the applied bias currents, markedly superior to the four-terminal spin injection with permalloy reported previously. In our LSVs, since the spin transfer torque from one CFS injector to another CFS one is large, the nonlocal magnetoresistance with respect to applied magnetic fields shows large asymmetry in high bias-current conditions. For utilizing multi-terminal spin injection with CFS as a method for magnetization reversals, the terminal arrangement of CFS spin injectors should be taken into account. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

DOI： 10.1063/1.4972852

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We investigated the electric-current-induced dynamics of bubble domains in a perpendicularly magnetized ferrimagnetic {Tb/Co}(7) multilayer wire with a heavy-metal Pt cap layer. The {Tb/Co}(7) wire with the transition-metal-dominant and rare-earth-dominant magnetizations was obtained by changing temperature. We found that the bubble domains moved to the electric current direction with growing in oblique angles when electric current pulses were applied. The oblique directions of the bubble-domain's growth in the {Tb/Co}(7) wire with the transition-metal-dominant and rare-earth-dominant magnetizations were opposite with each other. The micromagnetic simulations imply that these oblique growths are accounted by the spin injection from the Pt layer via the spin Hall effect. (C) 2017 Author(s).

DOI： 10.1063/1.4974067

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Language：Japanese   Publisher：一般社団法人 日本物理学会  

&lt;p&gt;プラチナ層と接する垂直磁化フェリ磁性体多層膜Tb/Coの{Tb/Co}_7_/Pt細線上にバブル磁区を生成し，電流印加による挙動を調べた．電流印加によりバブル磁区は細線の端方向への傾斜をもって成長した．フェリ磁性体の磁化の主体となる原子がTbとCoの場合で傾斜方向が逆になることがわかった．プラチナ層からのスピンホール効果を考慮したマイクロマグネティスシミュレーションでも同様の結果が得られ，このバブル磁区の傾斜はスピンホール効果によって説明できることがわかった&lt;/p&gt;

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Language：Japanese   Publisher：The Magnetics Society of Japan  

<p>  We calculated the conductance and magnetoresistance (MR) ratio in the ferromagnetic metal/ non-magnetic metal/ ferromagnetic insulator/ non-magnetic metal junction by using a free electron model. The spin-filter in the ferromagnetic insulator results in the MR effect. The value of the MR ratio oscillates as the thickness of the non-magnetic metal layer is increased. The period of the oscillation is given by the Fermi wave length of the non-magnetic metal and therefore the oscillation is caused by the interference of the electron wave function in the non-magnetic metal. The extremum of the MR ratio saturates as the insulator thickness is increased. This saturated value does not depend on the material parameters of the ferromagnetic insulator if the insulator is sufficiently thick. Furthermore, the dispersion of the MR ratio originating from the unevenness in the thickness of the non-magnetic metal layer, which could occur during fabrication, is suppressed by using the non-magnetic metal with longer Fermi wave length.</p>

DOI： 10.20819/msjtmsj.17TR118

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

We investigate the spin-injection driving of skyrmion domains on a wire using micromagnetic simulations. We find that both Néel- and Bloch-type skyrmion domains are driven in oblique directions when the spin current is injected into the vertical axis of the skyrmion domains. The driving direction is analyzed from the magnetized direction and the domain-wall structure of the skyrmion domains.

DOI： 10.7567/JJAP.56.098001

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

We study magnetic properties and interfacial characteristics of all-epitaxial D03-Fe3Si/L21- Fe3-xMnxSi/L21-Co2FeSi Heusler-compound trilayers grown on Ge(111) by room-temperature molecular beam epitaxy. We find that the magnetization reversal processes can be intentionally designed by changing the chemical composition of the intermediate Fe3-xMnxSi layers because of their tunable ferromagnetic-paramagnetic phase-transition temperature. From first-principles calculations, interfacial half metallicity in the Co2FeSi layer is nearly expected when the sequence of stacking layers along (111) of the Fe2MnSi/Co2FeSi interface includes the atomic row of L21- or B2-ordered structures. We believe that Co2FeSi/Fe2MnSi/Co2FeSi trilayer systems stacked along (111) will open a new avenue for high-performance current-perpendicular-to-plane giant magnetoresistive devices with Heusler compounds.

DOI： 10.1103/PhysRevB.94.094435

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The magnetic structure of the domain wall (DW) of a 30-nm-thick Fe4N epitaxial film with a negative spin polarization of the electrical conductivity is observed by magnetic force microscopy and is well explained by micromagnetic simulation. The Fe4N film is grown by molecular beam epitaxy on a SrTiO3(001) substrate and processed into arc-shaped ferromagnetic nanostrips 0.3 mu m wide by electron beam lithography and reactive ion etching with Cl-2 and BCl3 plasma. Two electrodes mounted approximately 12 mu m apart on the nanostrip register an electrical resistance at 8K. By changing the direction of an external magnetic field (0.2 T), the presence or absence of a DW positioned in the nanostrip between the two electrodes can be controlled. The resistance is increased by approximately 0.5 Omega when the DW is located between the electrodes, which signifies the negative anisotropic magnetoresistance effect of Fe4N. The electrical detection of the resistance change is an important step toward the electrical detection of current-induced DW motion in Fe4N. Published by AIP Publishing.

DOI： 10.1063/1.4962721

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Current-induced magnetic domain wall (DW) motion in ferromagnetic ribbons is utilized in spintronic devices. The direction of the motion changes in response to the sign of the spin-polarizability of the current through the ribbon. The DW motion is expected to measure the sign. In this study, we investigate the magnetic structures of chamfered L-shaped nano-ribbons using micro-magnetic simulations, and show that the position at which the DW is produced can be controlled by applying an external magnetic field with a low spin-polarized current (SPC). In particular, we use the material parameters of Fe4N and permalloy to simulate the magnetic structure of the ribbon. The DW can be produced at either of two locations in a chamfered corner of the ribbon, and disappears upon applying an external magnetic field. From this point, after the field is removed, a new DW is produced at either of two locations, and its position can be controlled by adjusting the low SPC.

DOI： 10.1088/0022-3727/49/38/385002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Surface Green's functions for semi-infinite systems play an important role in the design of nano-scale electronic devices. In particular, functions in multilayered periodic structures are useful for practical applications. This paper describes numerical issues associated with the computation of these functions and offers an approach to addressing these issues. The proposed approach is based on the observation that numerical errors, which can occur in the existing algorithm to compute the functions, are caused by inaccurate computation of eigenpairs. We consider an alternative approach to eigenpairs to derive an algorithm to compute the functions. Numerical experiments show that the proposed algorithm can compute functions that satisfy physical requirements.

DOI： 10.1007/s10665-015-9794-x

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Authorship：Lead author,　Last author,　Corresponding author  

We examined the conductance in metallic electrode/graphene/metallic electrode junctions, accounting for both the n- And the cr-bands of the graphene. Although the cr-band has been neglected in previous theoretical studies concerned with transport properties, we found that the cr-band of graphene significantly affects electrical transport. In our calculations, we used an atomistic tight-binding model and the Kubo-Landauer formula. The conductance values we obtained taking the cr-band of the graphene into account are three orders of magnitude larger than those obtained without considering the cr-band. The cr-band has a band gap at the Fermi level and does not affect conductance in bulk graphene, but it does indirectly hybridize with the rc-band of the graphene via metallic electrodes and contribute to conductance in junctions.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The electronic structures of single-layer Mo1-xCrxS2 are calculated by the spin-polarized density functional method. Single-layer Mo1-xCrxS2 exhibits a direct band gap for any Cr concentration (x). The energy band gap monotonically decreases as a function of x and takes an ideal value for photovoltaic applications at x similar or equal to 0.23. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.028003

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Language：Japanese   Publisher：一般社団法人 日本物理学会  

DOI： 10.11316/jpsgaiyo.71.1.0_1156

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

Current-driven magnetic domain wall (DW) motion has been extensively studied not only theoretically, but also experimentally. The DW motion is induced by spin-transfer torque, that is, the transfer of spin angular momentum from conduction electrons to localized electrons. The velocity of DW motion is proportional to the spin polarization [P&lt
inf&gt
a&lt
/inf&gt
= (σ&lt
inf&gt
↑&lt
/inf&gt
- σ&lt
inf&gt
↓&lt
/inf&gt
)/(σ&lt
inf&gt
↑&lt
/inf&gt
+ σ&lt
inf&gt
↓&lt
/inf&gt
)] of electrical conductivity (σ) and its direction is the same as electron current when P&lt
inf&gt
σ&lt
/inf&gt
&gt
0. The reverse DW motion is thus expected in ferromagnetic materials with negative spin polarization (P&lt
inf&gt
σ&lt
/inf&gt
&lt
0) compared to those with positive spin polarization, because minority spin dominates the electrical conduction. Thereby, spintronics devices composed of both a positive P&lt
inf&gt
σ&lt
/inf&gt
material and a negative P&lt
inf&gt
σ&lt
/inf&gt
material, are of fundamental interest. We have paid a lot of attention to ferromagnetic Fe&lt
inf&gt
4&lt
/inf&gt
N epitaxial films for application to spintronics devices because it is theoretically expected to have a large negative spin polarization (P&lt
inf&gt
σ&lt
/inf&gt
= -1.0).&lt
sup&gt
2&lt
/sup&gt
Very recently, we confirmed its negative spin polarization by experiment.&lt
sup&gt
3,4&lt
/sup&gt

DOI： 10.1109/INTMAG.2015.7157071

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We investigated the bubble-domain dynamics in TbFeCo wires with perpendicular magnetization under electric current flows. In TbFeCo wires with relatively low saturation magnetization (M-s), the current pulse caused the bubble domains to collapse without moving. In TbFeCo wires with relatively high M-s, however, the bubble domains grew in the current direction. We explain these shape changes by assessing the M-s-dependent forces caused by the exchange field and magnetostatic field. We also found that we could control the current-induced behaviors of the bubble domains by using an external magnetic field. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/APEX.8.073002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigated the strong antiferromagnetic (AF) interlayer exchange coupling (IEC) at junctions between Fe(001) and Fe3O4(001) epitaxial films and the effects of an insertion layer of Mn or Co. In addition, we developed a nonlinear least-squares-fit technique to analyze the details of the magnetization processes and to distill the IEC constants. The analyses were performed by taking into account bilinear and biquadratic exchange couplings with a twisted magnetization state. The fitting results suggest the existence of significant distributions in bilinear and/or biquadratic IECs, which can originate from nonuniform stacking of atoms at the interface. The insertion of a Co layer with a thickness in the range of zero to two monolayers between the Fe3O4(001) and the Fe(001) layers significantly affects the magnetic-hysteresis (M-H) processes, whereas the insertion of a Mn layer does not. A quantitative fitting analysis of M-H curves suggests that the inserted thin Mn layer does not affect the AF-IEC significantly, whereas the Co layer suppresses the AF coupling and in fact is preferable for achieving ferromagnetic coupling, which is consistent with the tendency predicted by our theoretical model regarding impurity effects for the IEC of Fe/Fe3O4(001).

DOI： 10.1103/PhysRevB.91.174423

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We grow a 15-nm-thick ferromagnetic Fe4N epitaxial film on a SrTiO3(001) substrate by molecular beam epitaxy, and process it into approximately 0.5-mu m-wide and 24-mu m-long L-shaped ferromagnetic narrow wires by electron-beam lithography and Cl-2 reactive ion etching. Their longitudinal directions are set in parallel to the magnetic easy axes, Fe4N[100] and [010]. With applying external magnetic field in the direction parallel to Fe4N[1001 or [010], the position of domain wall is controlled either on the upper side or lower side of the corner. This experiment is the preliminary step toward current-driven domain wall motion in Fe4N having a negative spin polarization. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.54.028003

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Language：English   Publisher：Institute of Physics  

We grow a 15-nm-thick ferromagnetic Fe<inf>4</inf>N epitaxial film on a SrTiO<inf>3</inf>(001) substrate by molecular beam epitaxy, and process it into approximately 0.5-µm-wide and 24-µm-long L-shaped ferromagnetic narrow wires by electron-beam lithography and Cl<inf>2</inf>reactive ion etching. Their longitudinal directions are set in parallel to the magnetic easy axes, Fe<inf>4</inf>N[100] and [010]. With applying external magnetic field in the direction parallel to Fe<inf>4</inf>N[100] or [010], the position of domain wall is controlled either on the upper side or lower side of the corner. This experiment is the preliminary step toward current-driven domain wall motion in Fe<inf>4</inf>N having a negative spin polarization.

DOI： 10.7567/JJAP.54.028003

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.2.0_842

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

We analyzed the M-H loop observed for Fe3O4/bcc-Fe(001) junctions with thin insertion layers of Co or Mn by using several models that include bilinear and biquadratic coupling and twisting of magnetization near the interface. Both biquadratic and twisting of the magnetization are found to be important for explaining the M-H loop. To clarify the mechanism of antiparallel (AP) coupling in the Fe3O4/bcc-Fe junction, we performed first-principles calculations for junctions with and without extra Fe atoms on hollow sites of the spinel lattice at the interface. It was shown that parallel coupling is stable for junctions without extra Fe atoms whereas AP coupling becomes stable when the number of extra Fe atoms increases. The results suggest that magnetic frustration may occur when a nonuniform distribution of the extra Fe atoms exists at the interface.

DOI： 10.1016/j.phpro.2015.12.178

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

We investigate the spatial correlation effects of ionized impurities on electron transport properties under quasi-1D nanowire structures. The impurity-limited resistance is evaluated with the Landauer formula under various spatial distributions of impurities and the simulation results are theoretically analyzed. We show that phase interference is significant even at room temperature when the separation between impurities along the axis direction is small, whereas strong phase randomization takes place as the separation increases.

DOI： 10.1088/1742-6596/647/1/012028

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The magnetization and spin transport of asymmetric zigzag-edge graphene nanoribbons, terminated by hydrogen on one edge while unterminated on the other edge, were investigated by a combination of first-principles calculations and a tight-binding approach. At the unterminated edge, a spin-polarized sigma edge state of minority spin appears near the Fermi level and contributes to spin transport. This state enters the band gap for ribbon widths of less than 15 chains, dominating the spin-polarized current. This indicates the importance of the sigma edge states in the design of spintronic devices using graphene nanoribbons. We also examined the case where the 'unterminated' edge is partially terminated by hydrogen.

DOI： 10.1088/0022-3727/47/48/485004

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3379/msjmag.1402R006

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Language：English   Publisher：American Institute of Physics  

DOI： 10.1063/1.4896760

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We evaluated electronic structures and magnetic moments in Co3FeN epitaxial films on SrTiO3(001). The experimentally obtained hard x-ray photoemission spectra of the Co3FeN film have a good agreement with those calculated. Site averaged spin magnetic moments deduced by x-ray magnetic circular dichroism were 1.52 mu(B) per Co atom and 2.08 mu(B) per Fe atom at 100 K. They are close to those of Co4N and Fe4N, respectively, implying that the Co and Fe atoms randomly occupy the corner and face-centered sites in the Co3FeN unit cell. (C) 2013 AIP Publishing LLC.

DOI： 10.1063/1.4836655

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Language：Japanese  

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Language：English   Publishing type：Research paper (international conference proceedings)  

In material physics, surface Green's functions are needed to analyze the electronic structures of nanoscale junctions. The algorithm for computing the functions consists of three steps. First, a matrix is generated by solving two equations. Then, the eigenvectors of the matrix are computed. Finally, another equation, which is generated by using the eigenvectors, is solved to produce a surface Green's function. In numerical computations, a perturbation will be added into the matrix at the first step. As a result, by computing the eigenvectors of the perturbed matrix at the second step, a considerable numerical error of the function will emerge at the third step. In this paper, we modify the algorithm in order to successfully compute surface Green's functions. We replace the first and second steps in the algorithm by an alternative step so that we can compute the eigenvectors of the matrix without computing the matrix. To show the effect of the modification, we report numerical experiments for computing the surface Green's functions at GdAs surface using a full orbitals tight-binding model. © 2013 The JJIAM Publishing Committee and Springer Japan.

DOI： 10.1007/s13160-013-0116-2

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Understanding the effect of the interface on electrical spin injection is of great importance for the development of semiconductor spintronics. Fe/GaAs(001) is one of the leading systems for exploring these effects due to the small lattice mismatch. We report on the correlation between the experimentally observed Fe/GaAs(001) interface with the spin-transport properties. Using high-angle annular dark-field scanning transmission electron microscopy, we observe a predominantly abrupt interface with some regions of partial mixing also observed in the same film. We report that reproducible behavior with no bias-dependent polarization inversion was achieved for three-terminal devices. Using ab initio calculations of the experimentally observed interfaces, we show that the contribution to the transport from minority carriers is strongly dependent on the interface structure. © 2013 American Physical Society.

DOI： 10.1103/PhysRevB.87.024401

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Understanding the effect of the interface on electrical spin injection is of great importance for the development of semiconductor spintronics. Fe/GaAs(001) is one of the leading systems for exploring these effects due to the small lattice mismatch. We report on the correlation between the experimentally observed Fe/GaAs(001) interface with the spin-transport properties. Using high-angle annular dark-field scanning transmission electron microscopy, we observe a predominantly abrupt interface with some regions of partial mixing also observed in the same film. We report that reproducible behavior with no bias-dependent polarization inversion was achieved for three-terminal devices. Using ab initio calculations of the experimentally observed interfaces, we show that the contribution to the transport from minority carriers is strongly dependent on the interface structure. DOI: 10.1103/PhysRevB.87.024401

DOI： 10.1103/PhysRevB.87.024401

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Language：Japanese   Publisher：公益社団法人 日本磁気学会  

The dynamics of the magnetization reversal process in permalloy films by the pure spin current injection has been analyzed. We have performed micromagnetics simulations based on the Landau-Lifshitz equation taking into account both the relaxation term and spin transfer torque term caused by the pure spin current which is injected into the small permalloy thin films laterally. We found that the magnetization switching time is decreased and the switching probability is increased by optimizing the spin polarization direction of the injecting pure spin current.

DOI： 10.3379/msjmag.1308R001

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

D0(3)-ordered Fe3Si, which is one of the ferromagnetic Heusler compounds, has so far been formed by high-temperature heat treatments. Here, we demonstrate room-temperature D0(3) ordering of Fe3Si films grown on Ge(111) by using a molecular beam epitaxy (MBE) technique. In our MBE conditions, higher growth temperatures are not effective to obtain highly ordered D0(3) structures because of the interfacial reactions between Fe3Si and Ge. Even for the room-temperature growth, the degree of the D0(3) ordering can be improved with increasing film thickness. Considering the experimental data and the calculated results based on molecular dynamics, we can understand that the main structural disorder is derived from a specific Fe site in the Fe3Si film near the interface. We also discuss the room-temperature D0(3) ordering in terms of the local stoichiometry of the supplied atoms in MBE conditions.

DOI： 10.1103/PhysRevB.86.174406

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER SCIENTIFIC PUBLISHERS  

Pure spin current injection has recently been developed as an effective method of controlling magnetization in spintronic devices. In order to investigate the dynamics of the magnetization reversal process in magnetic films with pure spin current injection, we performed micromagnetics simulations based on the Landau-Lifshitz equation and taking into account a term representing spin transfer torque caused by the pure spin current. We studied the time evolution of magnetization in permalloy films and estimated the probability of magnetization switching to show how the probability depends on the width of the film and the intensity of the injected spin current. We found that, in narrow films, a transverse domain wall is formed in the film and that this wall moves outside the film when magnetization switching occurs. The switching time becomes shorter as the intensity of the injected spin current is increased. In wide films, on the other hand, magnetization switching does not occur even if the intensity of injected spin current is increased. In such cases, either a magnetic vortex core is formed or the configuration of the moments hardly changes from the initial magnetic state.

DOI： 10.1166/jnn.2012.6481

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the electrical conductance in a ferromagnetic Ni constriction, changing its size using a mechanically controllable break junction technique. A Fano resonance with a zero-bias anomaly, likely due to the Kondo effect, appears in Ni atomic-scale contacts and changes shape as the size of the contact changes. Moreover, the zero-bias anomaly persists in large size constrictions with nearly 50-atom configurations where the bulk ferromagnetic properties should be retained, despite the decrease of the signal intensity. The results suggest that the Kondo effect and ferromagnetism could coexist in the ferromagnetic nanoconstrictions.

DOI： 10.1103/PhysRevB.86.064404

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Exchange coupling (EC) in Fe/Fe3O4 junctions containing magnetic impurities and in-gap states at the interface is calculated using a formula obtained by a cleaved layer method. The model for EC is constructed by performing first-principles calculations of the electronic and magnetic states of Co, Mn, and Cr impurities on the Fe surface and those of in-gap states in a bulk gamma-Fe2O3, which has the same lattice structure as Fe3O4 but contains Fe defects. We show that the effect of Co impurities on EC is opposite to that of Cr and Mn impurities and that in-gap states tend to cause parallel magnetization alignment of two ferromagnets. These results are attributed to the change in electronic states caused by the presence of impurities. Further, we compare calculated results with experimental ones obtained in Fe/Fe3O4 junctions and suggest that doping magnetic impurities at the interface could be a useful way to control the magnitude and sign of the EC.

DOI： 10.1103/PhysRevB.85.184431

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

DOI： 10.1103/PhysRevB.85.184431

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The spin-dependent conductance and magnetoresistance (MR) in fcc Ni/graphene/fcc Ni (111) junctions are theoretically investigated. Assuming the Fermi energy at the Dirac points, we focus on conditions at the contact between the graphene and the fcc Ni layer to realize high MR ratios. Our findings are as following: band mixing between graphene and Ni should be rather weak, the effect of dopants on the MR is small for Ni electrodes, and a contact region of a few nanometers in size is sufficient to obtain high MR ratios. We further show that Ni-Co and Ni-Cu alloys are useful for electrode materials. The effect of roughness is also discussed. The increase in MR has been attributed to matching/mismatching of the Dirac point with the band structure of fcc Ni at the contact. The importance of the d orbitals of fcc Ni is also emphasized.

DOI： 10.1103/PhysRevB.85.094420

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Language：Japanese   Publisher：公益社団法人 日本磁気学会  

&amp;nbsp;&amp;nbsp;The dynamics of the magnetization reversal process in permalloy films by pure spin current injection has been analyzed. We have performed micromagnetics simulations based on the Landau-Lifshitz equation taking into account both the relaxation term and spin transfer torque term caused by the pure spin current which is injected into permalloy thin films laterally. For spin injection from one side of the film sample, the magnetization reversal occurs in film samples less than 70 nm wide. This size is independent of the magnitude of the injected spin current. We also found that spin injection from two sides can reverse the magnetization in permalloy films that are several times wider.

DOI： 10.3379/msjmag.1210R002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We have proposed a method for controlling the vortex chirality in a squared permalloy dot by using the circular Oersted field locally induced by flowing a DC current across a small Py/Cu junctions. The reliability of the chirality control has been evaluated by measuring the nonlocal spin valve signal. The desired vortex chirality has been obtained when the injecting DC current has a moderate magnitude. However, the large DC current is found to reduce the control reliability. Another possibility for controlling the vortex structure using the large DC current injection was also discussed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3669410]

DOI： 10.1063/1.3669410

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Material dependence of the spin polarization of the tunneling current in the spin injection process is calculated for ferromagnetic metal alloy/GaAs contacts with several values of Schottky barrier height. It is shown that calculated results of bias dependence of are qualitatively the same as those in Fe/GaAs contacts, indicating weak dependence of on materials. We have confirmed our previous result that the sign change in with bias voltage is attributed to interfacial resonant states within the Schottky barrier in the GaAs layer.

DOI： 10.1109/TMAG.2011.2158807

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We calculate the spin-dependent conductance in ferromagnetic metal (FM)/graphene(G)/FM lateral junctions using a numerical simulation and study the effects of roughness and electronic states at the FM/G contact on magnetoresistance (MR) in detail. It is shown that the MR ratio can be extremely high in fcc Ni alloy/G/fcc Ni alloy junctions because of a spin-dependent change in the electronic states at the contact. The MR ratio, however, is reduced by introducing roughness into junctions. Conditions for a high MR ratio in FM/G/FM junctions are discussed.

DOI： 10.1109/TMAG.2011.2158808

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：TRANS TECH PUBLICATIONS LTD  

The electronic structures of Co-based Heusler alloys with nonstoichiometric atomic compositions as well as those at the interface of semiconductor junctions are investigated using first principles band calculations. It is shown that the electronic structure of a Co-based Heusler alloy is half-metallic, even for nonstoichiometric but Co-rich compositions, whereas the half-metallicity is lost for Co-poor compositions. It is also shown that magnetic moments at the interface of Co2MnSi/ Si junctions are sensitive to the growth direction and interface structure of the junctions. Efficient spin-injection into Si can be achieved by using a (111)-oriented Co-rich Heusler alloy and controlling the layer-by-layer stacking sequence at the interface.

DOI： 10.4028/www.scientific.net/KEM.470.54

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Influence of the linear energy-momentum relationship in graphene on conductance and magnetoresistance (MR) in ferromagnetic metal (FM)/graphene/FM lateral junctions is studied in a numerical simulation formulated using the Kubo formula and recursive Green's function method in a tight-binding model. It is shown that the contribution of electron tunneling through graphene should be considered in the electronic transport in metal/graphene/metal junctions, and that the Dirac point (DP) is effectively shifted by the band mixing between graphene and metal electrodes. It is shown that MR appears due to spin-dependent shift of DP or spin-dependent change in the electronic states at DPs. It is shown that the MR ratio caused by the latter mechanism can be very high when certain transition metal alloys are used for electrodes. These results do not essentially depend on the shape of the junction structure. However, to obtain high MR ratios, the effects of roughness should be small. © 2011 Materials Research Society.

DOI： 10.1557/opl.2011.646

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The electronic and magnetic structures and magnetic moments of Co2MnSi (CMS)/Cr/(CoMnSi)-Mn-2 (001) trilayers are calculated by the first-principles method and compared to those calculated for Fe/Cr/Fe trilayers. The magnetic moments of Co are found to be parallel to those of Cr, whereas those of Mn are antiparallel to Cr moments. The results suggest that an antiphase structure in CMS layers may also bring about a frustration in the interlayer exchange coupling in multilayers. We also study the effects of antisite atoms at interfaces on the electronic and magnetic structures and find that the antiferromagnetic ordering of Cr moments is more robust than that in Fe/Cr/Fe trilayers. We propose a possible scenario in which the robustness of the antiferromagnetic ordering of Cr moments and the frustration in the interlayer exchange coupling, which is caused by antiphase or step structures, are responsible for the biquadratic coupling observed in CMS/Cr multilayers.

DOI： 10.1103/PhysRevB.82.014413

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Based on a numerical study of a full-orbital tight-binding model, we show that magnetoresistance (MR) in ferromagnetic metal (FM)/graphene/FM lateral junctions originates from a spin-dependent shift of the Dirac points (DPs) caused by the presence of FM electrodes. Graphene with finite length L is in contact with FM at a zigzag edge and the spin-dependent conductance is calculated in the ballistic transport regime. We find that the tunneling conductance near DPs influences the features of the MR considerably. We further show that a slight change in the electronic structure near the DPs strongly affects the spin-dependent conductance resulting in a large MR ratio for an FM made of certain ferromagnetic transition metal alloys.

DOI： 10.1103/PhysRevB.82.033402

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Spin polarization of the tunnel current under finite bias voltage is studied theoretically for Fe/GaAs junctions with Schottky barriers and Fe/GaAs/Fe tunnel junctions, using a realistic tight-binding model and linear response theory. Calculating the spin polarization of the tunnel current across the interface as a function of bias voltage, we show that the spin-polarized interface resonant states within the Schottky barrier significantly influence spin-dependent tunnelling. The position of the resonant states depends on the Schottky barrier height, and the spin polarization of the tunnel current can be negative for a rather wide range of bias voltage, depending on the Schottky barrier height. The magnetoresistance of Fe/GaAs/Fe tunnel junctions is also negative under high bias voltage, because of the interfacial resonant states in the GaAs layer. The present results demonstrate the importance of controlling Schottky barrier formation to obtain high spin polarization in Fe/GaAs junctions.

DOI： 10.1088/0022-3727/43/13/135002

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We perform numerical simulations of conductance and conductivity for magnetic multilayers and magnetic granular films which show giant magnetoresistance (GMR). The system size to which the recursive Green&apos;s function method is applied is sufficiently large to eliminate incidental effects caused by the system boundary reported previously. It is shown that the resistivity change in the magnetic granular films is proportional to the inverse of the grain size.

DOI： 10.1103/PhysRevB.81.054445

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Language：Japanese   Publisher：公益社団法人 日本磁気学会  

&amp;nbsp;&amp;nbsp;We studied the spin dependence of conductance and magnetoresistance for clean ferromagnet/graphene/ferromagnet junctions by making use of both simple and realistic band models for ferromagnetic leads. We found that conductance decays monotonically with increasing graphene length for pure graphene, but oscillates for doped graphenes. Magnetoresistance had non-zero values when ferromagnetic leads with a realistic band structure were attached. For doped graphene magnetoresistance also oscillates with graphene length. The oscillation period was determined by the Fermi wave length of the doped graphene.

DOI： 10.3379/msjmag.0912RE0013

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Language：Japanese   Publisher：The Magnetics Society of Japan  

&amp;nbsp;&amp;nbsp;The Magnetic structure of magnetic multilayers composed of a full Heusler Co&lt;sub&gt;2&lt;/sub&gt;MnSi(CMS) alloy and Cr was calculated by using first-principles band calculation. We found that the magnetic moments of Co and Cr were parallel, while those of Mn and Cr were anti-parallel in CMS/Cr/CMS multilayers. By comparing the calculated results with those in Fe/Cr/Fe multilayers, it could be argued that the magnetic structure calculated for Co, Mn, and Cr atoms is crucial for the biquadratic coupling observed in these multilayers.

DOI： 10.3379/msjmag.1003R004

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Spin polarization of the tunnel conductivity has been studied for Fe/GaAs junctions with Schottky barriers. It is shown that band matching of resonant interface states within the Schottky barrier defines the sign of spin polarization of electrons transported through the barrier. The results account very well for experimental results including the tunneling of photoexcited electrons and suggest that the spin polarization (from -100% to 100%) is dependent on the Schottky barrier height. They also suggest that the sign of the spin polarization can be controlled with a bias voltage.

DOI： 10.1103/PhysRevB.78.245316

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Language：Japanese  

DOI： 10.3379/msjmag.32.351

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

Tunnel conductance and magnetoresistance of (Ga-Mn)As/AlAs/(Ga-Mn)As are calculated as a function of AlAs barrier thickness in the linear response regime! using a realistic tight-binding model. It is shown that the barrier thickness dependence of the tunnel magnetoresistance ratio is quite weak in contrast to experimental results. It is also shown that the tunnel magnetoresistance ratio is decreased by GaAs layer inserted at the interfaces between (Ga-Mn)As and AlAs. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/pssb.200777263

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

In order to clarify the tunneling spectroscopy in hole-doped high-T-c cuprates, we study the strongly correlated electron systems with surface using the 2D Hubbard model with fluctuation exchange (FLEX) approximation from the side of weak coupling theory. We have found that (i) LDOS near the (1, 1, 0) surface has a zero energy peak, and (ii) LDOS is asymmetric, reflecting the asymmetry of band structure and electron correlation in the hole-doped high-T-c cuprates. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.05.019

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The interfacial density of states and conductance of GdAs/GaAs/GdAs junctions, where GdAs and GaAs are a semimetal and a semiconductor, respectively, are calculated using a full-orbital tight-binding model. The tunnel conductance of the junction is calculated using the Kubo formula at zero temperature assuming coherent tunneling of electrons. Emphasizing the role of interfacial states on the conductance, the following results are obtained. The interfacial states extend 2-4 atomic layers from the interface; however, the effect of the interfacial states on the conductance extends up to nearly 10 atomic GaAs layers, and nonmonotonic dependence of the tunnel conductance on the barrier thickness appears for thin GaAs layers. The height of the interfacial density of states does not necessarily correlate with the magnitude of the tunnel conductance. The matching between the Fermi surface of the leads and interfacial states governs the magnitude of the tunnel conductance. It is also found that the nonmonotonic dependence of the tunnel conductance is caused by an interference effect of the interface states existing at the two interfaces of GaAs layer.

DOI： 10.1103/PhysRevB.74.155329

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Language：English   Publishing type：Research paper (scientific journal)  

The interfacial density of states and conductance of GdAs/GaAs/GdAs junctions, where GdAs and GaAs are a semimetal and a semiconductor, respectively, are calculated using a full-orbital tight-binding model. The tunnel conductance of the junction is calculated using the Kubo formula at zero temperature assuming coherent tunneling of electrons. Emphasizing the role of interfacial states on the conductance, the following results are obtained. The interfacial states extend 2-4 atomic layers from the interface
however, the effect of the interfacial states on the conductance extends up to nearly 10 atomic GaAs layers, and nonmonotonic dependence of the tunnel conductance on the barrier thickness appears for thin GaAs layers. The height of the interfacial density of states does not necessarily correlate with the magnitude of the tunnel conductance. The matching between the Fermi surface of the leads and interfacial states governs the magnitude of the tunnel conductance. It is also found that the nonmonotonic dependence of the tunnel conductance is caused by an interference effect of the interface states existing at the two interfaces of GaAs layer. © 2006 The American Physical Society.

DOI： 10.1103/PhysRevB.74.155329

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：WILEY-V C H VERLAG GMBH  

Conductance and magnetoresistance of spin-polarized semi-metal(GdAs)/seniiconductor(GaAs) junctions are calculated by using a full-orbital tight-binding scheme and the linear response theory. It is shown that the magnetoresistance as well as conductance are strongly influenced by the presence of interfacial states. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/pssc.200672845

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Numerical results of electrical conductance and magnetoresistance of ferromagnetic tunnel junctions and semiconductor/semimetal junctions are presented, putting an emphasis on effects of disorder on the conductance. It is shown that the disorder strongly affects the tunnelling conductance and magnetoresistance. It is discussed how the interfacial resonant states appearing in semiconductor/semimetal junctions influence the conductance. Numerical simulation of diffusive conductivity of a two-dimensional electron gas with spin-orbit interaction is also performed, and the results are compared with the analytical result.

DOI： 10.1088/0953-8984/16/48/012

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Language：Japanese   Publisher：一般社団法人 日本物理学会  

DOI： 10.11316/jpsgaiyo.71.1.0_1156

CiNii Books

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Language：Japanese  

CiNii Books

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Language：Japanese   Publisher：一般社団法人 日本物理学会  

DOI： 10.11316/jpsgaiyo.70.2.0_959

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.70.1.0_1088

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.2.0_842

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

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Language：Japanese   Publisher：The Physical Society of Japan  

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Language：Japanese  

CiNii Books

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：English   Publisher：関西大学先端科学技術推進機構  

CiNii Books

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Application no：特願2022-007392  Date applied：2022.1

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Application no：特願2021-063692  Date applied：2021.4

Patent/Registration no：特許第7555120号  Date registered：2024.9 

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Patent/Registration no：特許第7555119号  Date registered：2024.9 

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Patent/Registration no：特許第7539700号  Date registered：2024.8 

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Application no：特願2019-196646  Date applied：2019.10

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Application no：特願2019-100670  Date applied：2019.5

Patent/Registration no：特許7360121 

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Applicant：学校法人 関西大学, 国立大学法人 名古屋工業大学

Application no：特願2016-049981  Date applied：2016.3

Announcement no：特開2017-168514  Date announced：2017.9

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Applicant：国立大学法人 筑波大学

Application no：特願2013-209545  Date applied：2013.10

Announcement no：特開2015-076419  Date announced：2015.4

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Applicant：国立大学法人九州大学

Application no：JP2013065217  Date applied：2013.5

Announcement no：WO2013-180277  Date announced：2013.12

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Applicant：国立大学法人九州大学, 国立大学法人 筑波大学, 学校法人 関西大学

Application no：特願2013-082098  Date applied：2013.4

Announcement no：特開2014-204106  Date announced：2014.10

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