Award type：Award from Japanese society, conference, symposium, etc.  Country：Japan

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Award type：Award from Japanese society, conference, symposium, etc.  Country：Japan

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Award type：Award from Japanese society, conference, symposium, etc.  Country：Japan

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

DOI： 10.11470/jsaprev.230414

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

DOI： 10.1103/physrevresearch.5.013222

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevResearch.5.013222/fulltext

Authorship：Lead author,　Corresponding author   Language：English  

DOI： 10.1063/5.0134291

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.tsf.2023.139740

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

Weyl semimetals show unique physical properties exemplified by the colossal anomalous Hall effect, arising from exotic quasiparticles called Weyl fermions emerging around the Weyl nodes. Manipulating these topologically protected Weyl nodes is anticipated to play a leading role towards the on-demand control of quantum properties in Weyl semimetals. We demonstrate non-volatile chirality switching in a ferromagnetic Weyl semimetal Co₃Sn₂S₂ via all-optical magnetization reversal. When excited by circularly polarized mid-infrared light pulses, the sign reversal of the anomalous Hall conductivity stemming from the Berry curvature is observed, manifesting the switching of the chirality of the Weyl nodes accompanying with the magnetization reversal. Magneto-optical imaging measurements reveal that the mechanism of the magnetization/chirality switching is attributed to the helicity-dependent deterministic magnetization associated with the magnetic circular dichroism.

DOI： 10.1038/s42005-022-01106-8

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Other Link： https://www.nature.com/articles/s42005-022-01106-8

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

DOI： 10.1103/physrevmaterials.6.114203

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevMaterials.6.114203/fulltext

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

DOI： 10.1002/adma.202206801

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

Abstract

Unidirectional magnetoresistance (UMR) is an attractive magnetic-field sensing technique as it enables detecting the in-plane direction of the external magnetic field with a single element. However, the UMR amplitude is typically several orders of magnitude smaller than those of other directional magnetoresistances exhibited in ferromagnetic thin films, hindering sensing applications using the UMR. For a directional magnetic sensor using the UMR, the in-depth understanding of mechanisms of the UMR and its enhancement are highly desirable. In this study, the structural dependence of the UMR in Fe-Sn heterostructure devices is investigated. We find a weak dependence of the UMR on interface configuration in the oxide cap/Fe-Sn heterostructures on various oxide substrates. In contrast, the normalized UMR amplitude is enhanced by a factor of 27 with increasing the Fe-Sn layer thickness from 4.0 to 100 nm. These results suggest that the magnetothermal effect dominates the large UMR in the Fe-Sn heterostructure devices.

DOI： 10.35848/1347-4065/ac7bc8

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Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/ac7bc8/pdf

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

We report the effect of annealing on the crystalline ordering and physical properties in thin films of a nodal line semimetal candidate L2₁-type Co₂FeSn. The Co–Fe–Sn films with a composition of Co:Fe:Sn ∼ 2:1:1 were deposited on MgO(001) substrates at a substrate temperature of 150 °C by radio-frequency magnetron sputtering. The as-deposited film showed x-ray diffraction patterns corresponding to the B2 ordering. Annealing at 600 and 700 °C after the deposition resulted in the appearance of the (111) diffraction peak, which is characteristic of the L2₁ ordering. Although anomalous Hall conductivity and transverse thermoelectric conductivity decreased from those of the as-deposited film with the annealing-induced L2₁ ordering, the low anomalous Hall conductivity of the 700 °C-annealed film was consistent with the theoretically estimated low value. These results show the significant influence of crystalline ordering on the electrical and thermoelectric transport properties. The annealing process is beneficial for studying the exotic physics arising from topological band features in the L2₁-ordered Co₂FeSn thin films.

DOI： 10.1063/5.0093195

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

DOI： 10.1063/5.0078021

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

Abstract

A ferromagnetic nanocrystalline Fe-Sn is an excellent platform for magnetic-field sensor based on anomalous Hall effect (AHE) owing to simple fabrication and superior thermal stability. For improvement of the magnetic-field sensitivity, doping impurity and increasing injection current are effective approaches. However, in the light of magnetic-field detectivity, the large current may increase the voltage noise. In this study, a maximum allowable current of was improved by employing the overlayer electrode configuration on a Ta-doped Fe-Sn AHE sensor. In noise measurements, the 1/f noise becomes significant with increasing the current at low frequency, resulting in saturation of the detectivity to 240 nTHz-1/2 at 120 Hz. At high frequency, the detectivity reaches 48 nTHz-1/2 at 3.1 mA showing ten times improvement of the detectivity compared with the non-doped Fe-Sn AHE sensor. Material design and device structure optimization will accelerate further improvement of the sensing properties of the Fe-Sn-based AHE sensor.

DOI： 10.35848/1347-4065/ac465a

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Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/ac465a/pdf

Publishing type：Research paper (scientific journal)   Publisher：American Association for the Advancement of Science (AAAS)  

Ferromagnetic-metal surface state derived from topological polarization and its spin-orbitronics functionalities in FeSi.

DOI： 10.1126/sciadv.abj0498

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

Non-coplanar spin textures with finite scalar spin chirality can be artificially induced at surfaces and interfaces through the interfacial Dzyaloshinskii-Moriya interaction. However, stabilizing a proper magnetic skyrmion crystal via this route remains elusive. Here, using an epitaxial bilayer of platinum and geometrically frustrated kagome-lattice ferromagnet Fe₃Sn, we show the possible formation of a two-dimensional skyrmion crystal under well-regulated Fe₃Sn thickness conditions. Magnetization measurements reveal that the magnetic anisotropy is systematically varied from an inherent in-plane type to a perpendicular type with the thickness reduction. Below approximately 0.5 nm, we clearly detect a topological Hall effect that provides evidence for finite scalar spin chirality. Our topological Hall effect analysis, combined with theoretical simulations, not only establishes its interfacial Dzyaloshinskii-Moriya interaction origin, but also indicates the emergence of a stable skyrmion crystal phase, demonstrating the potential of kagome-lattice ferromagnets in spin chirality engineering using thin-film nanostructures.

DOI： 10.1038/s43246-021-00218-y

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Other Link： https://www.nature.com/articles/s43246-021-00218-y

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

Smart society is forthcoming with a rapid development in the automation of electric appliances requiring abundant sensors. One of the key sensors is a three-dimensional magnetometer for detecting the motion of objects, which is usually driven by cooperative multiple sensors on three orthogonal planes. Here, we demonstrate the fundamental operation of a three-dimensional magnetometer based on a simple Fe-Sn heterostructure Hall device in a planar geometry. Polar coordinates of the magnetic-field vector are uniquely determined by the combination of the sizable anomalous Hall effect, the anisotropic magnetoresistance, and the unidirectional magnetoresistance. Thanks to the ferromagnetic topological features in the Fe-Sn heterostructure, the above-mentioned device overcomes the limitation of conventional semiconductor devices and is highly sensitive even at room temperature. The compact planar geometry will be particularly useful in versatile electrical applications requiring a low-cost three-dimensional magnetometer with space- and energy-saving features.Three-dimensional magnetometers are key for detecting the motion of objects in automated electronics, but typically require multiple sensors on orthogonal planes. Here, a compact planar-type 3D magnetometer with low power consumption is realized using a topological ferromagnetic Fe-Sn heterostructure.

DOI： 10.1038/s43246-021-00206-2

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

DOI： 10.35848/1882-0786/ac114e

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/ac114e/pdf

Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical Engineers of Japan (IEE Japan)  

DOI： 10.1541/ieejeiss.141.767

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>Abstract</title>Two-dimensional (2D) surface of the topological materials is an attractive channel for the electrical conduction reflecting the linearly-dispersive electronic bands. Thickness-dependent sheet conductance measurement is a reliable method to evaluate the 2D and three-dimensional (3D) electrical conducting channel separately but has rarely been applied for Weyl semimetals. By applying this method to thin films of a Weyl semimetal Co₃Sn₂S₂, here we show that the 2D conducting channel clearly emerges under the ferromagnetic phase, indicating a formation of the Fermi arcs projected from Weyl nodes. Comparison between 3D conductivity and 2D conductance provides the effective thickness of the surface conducting region being estimated to be approximately 20 nm, which would reflect the Weyl feature of electronic bands of the Co₃Sn₂S₂. The emergent surface conduction will provide a pathway to activate quantum and spintronic transport features stemming from a Weyl node in thin-film-based devices.

DOI： 10.1038/s42005-021-00627-y

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Other Link： http://www.nature.com/articles/s42005-021-00627-y

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

Co-based shandite Co3Sn2S2 is a representative example of magnetic Weyl semimetals showing rich transport phenomena. We thoroughly investigatemagnetic and transport properties of hole-doped shandites Co3InxSn2-xS2 by first-principles calculations. The calculations reproduce nonlinear reduction of anomalous Hall conductivity with doping In for Co3Sn2S2 as reported in experiments against the linearly decreased ferromagnetic moment within virtual crystal approximation. We show that a drastic change in the band parity character of Fermi surfaces, attributed to the nodal rings lifted energetically with In doping, leads to strong enhancement of anomalous Nernst conductivity with reversing its sign in Co3InxSn2-xS2.

DOI： 10.1103/physrevb.103.205112

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.103.205112/fulltext

Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGERNATURE  

Magnetic Weyl semimetals are quantum phases of matter arising from the interplay of linearly dispersive bands, spin-orbit coupling, and time reversal symmetry breaking. This can be realised, for example, in Co3Sn2S2, based on a cobalt kagome lattice and characterised by intriguing phenomena such as large anomalous Hall effect, Nernst effect, and water oxidation. Here, we attempt to determine the robustness of the twofold necessary conditions for the emergence of the magnetic Weyl semimetal phase in Co3Sn2S2 ultrathin films. Except for two-dimensional layered materials, a reduction of thickness generally makes it difficult to develop topological character and ferromagnetic long-range order. In Co3Sn2S2 films, while ferromagnetic ordering appears robustly even in average thicknesses of one or two unit cells with island-like polycrystalline domains, the anomalous Hall conductivity appears only above a critical thickness of approximately 10 nm. The emergence of surface conduction and large anomalous Hall effect implies the distinct contribution of Weyl nodes and their Berry curvature. These findings reveal an exotic feature of Weyl physics in thin-film based superstructures as well as a potential for future applications in electronic devices.The coexistence of ferromagnetism and topology has recently attracted intense interest. Here, the thickness dependence of magnetisation and anomalous Hall conductivity is investigated in Co3Sn2S2 thin films, revealing a critical thickness of 10 nm for the emergence of the magnetic Weyl semimetal phase.

DOI： 10.1038/s43246-021-00122-5

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

Perpendicular magnetic anisotropy (PMA) in magnetic thin films is a fundamental key feature in the design of spintronic devices. As one of magnetic Weyl semimetals, Co3Sn2S2 has been studied for its large anomalous Hall effect (AHE), uniaxial crystalline magnetic anisotropy, and half metallicity. In this study, we investigated the effect of off-stoichiometric composition on the PMA and AHE of Co3Sn2S2, thin films fabricated by the sputtering technique. The prepared thin films have off-stoichiometric S compositions x of 1.54 (S poor) and 3.27 (S rich) as well as the nearly stoichiometric one of 2.02. In addition to the Co3Sn2S2 phase, the segregated Co metal is found to contribute to the measured magnetization in the S-poor and S-rich films. The coercive field of perpendicular magnetization in all the films is much larger than that in the Co3Sn2S2 bulk crystals despite the fact that effective perpendicular magnetic anisotropy constants (K-u(eff)) between the prepared films are significantly different. In addition, the K-u(eff) values of two samples with x = 2.02 and 2.22 are comparable to those of the bulk crystals. In contrast to the isotropic magnetization behavior in the S-rich film, the S-poor film holds the PMA feature. This result means that the PMA is more robust in the S-poor film than in the S-rich film. For the electrical transport properties, a large tangent of Hall angle of about 0.2 is observed for both the nearly stoichiometric and the S-poor films. This large tangent of Hall angle demonstrates that the Weyl feature of Co3Sn2S2 phase is well maintained even in the S-poor thin films as well as the nearly stoichiometric films although the amount of Co segregation in both S-poor and S-rich films is similar. Our findings on the influence of off-stoichiometry on the PMA and AHE are beneficial to design magnetic devices incorporated with the Weyl features of Co3Sn2S2.

DOI： 10.1103/physrevmaterials.5.024403

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevMaterials.5.024403/fulltext

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>Abstract</title>
The Weyl semimetal (WSM), which hosts pairs of Weyl points and accompanying Berry curvature in momentum space near Fermi level, is expected to exhibit novel electromagnetic phenomena. Although the large optical/electronic responses such as nonlinear optical effects and intrinsic anomalous Hall effect (AHE) have recently been demonstrated indeed, the conclusive evidence for their topological origins has remained elusive. Here, we report the gigantic magneto-optical (MO) response arising from the topological electronic structure with intense Berry curvature in magnetic WSM Co₃Sn₂S₂. The low-energy MO spectroscopy and the first-principles calculation reveal that the interband transitions on the nodal rings connected to the Weyl points show the resonance of the optical Hall conductivity and give rise to the giant intrinsic AHE in dc limit. The terahertz Faraday and infrared Kerr rotations are found to be remarkably enhanced by these resonances with topological electronic structures, demonstrating the novel low-energy optical response inherent to the magnetic WSM.

DOI： 10.1038/s41467-020-18470-0

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Other Link： http://www.nature.com/articles/s41467-020-18470-0

Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>Abstract</title>In quantum spin liquid research, thin films are an attractive arena that enables the control of magnetic interactions via epitaxial strain and two-dimensionality, which are absent in bulk crystals. Here, as a promising candidate for the development of quantum spin liquids in thin films, we propose a robust ilmenite-type oxide with a honeycomb lattice of edge-sharing IrO₆ octahedra artificially stabilised by superlattice formation using the ilmenite-type antiferromagnetic oxide MnTiO₃. Stabilised sub-unit-cell-thick Mn–Ir–O layers are isostructural to MnTiO₃ and have an atomic arrangement corresponding to ilmenite-type MnIrO₃. By performing spin Hall magnetoresistance measurements, we observe that antiferromagnetic ordering in the ilmenite Mn sublattice is suppressed by modified magnetic interactions in the MnO₆ planes via the IrO₆ planes. These findings contribute to the development of two-dimensional Kitaev candidate materials, accelerating the discovery of exotic physics and applications specific to quantum spin liquids.

DOI： 10.1038/s43246-020-00059-1

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Other Link： http://www.nature.com/articles/s43246-020-00059-1

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACS Publications  

DOI： 10.1021/acs.chemmater.0c01497

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

Three-dimensional topological insulators (3D TIs) possess a specific topological order of electronic bands, resulting in gapless surface states via bulk-edge correspondence. Exotic phenomena have been realized in ferromagnetic TIs, such as the quantum anomalous Hall (QAH) effect with a chiral-edge conduction and a quantized value of the Hall resistance R-yx. Here, we report on the emergence of distinct topological phases in paramagnetic Fe-doped (Bi,Sb)(2)Se-3 heterostructures with varying structure architecture, doping, and magnetic and electric fields. Starting from a 3D TI, a two-dimensional insulator appears at layer thicknesses below a critical value, which turns into an Anderson insulator for Fe concentrations sufficiently large to produce localization by magnetic disorder. With applying a magnetic field, a topological transition from the Anderson insulator to the QAH state occurs, which is driven by the formation of an exchange gap owing to a giant Zeeman splitting and reduced magnetic disorder. A topological phase diagram of (Bi,Sb)(2)Se-3 allows exploration of intricate interplay of topological protection, magnetic disorder, and exchange splitting.

DOI： 10.1103/physrevmaterials.4.044202

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Other Link： https://link.aps.org/article/10.1103/PhysRevMaterials.4.044202

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

DOI： 10.1103/physrevresearch.2.013282

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevResearch.2.013282/fulltext

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

Perovskite oxides ABO(3) containing heavy B-site elements are a class of candidate materials to host topological metals with a large spin-orbit interaction. In contrast to the band insulator BaSnO3, the semimetal BaPbO3 is proposed to be a typical example with an inverted band structure, the conduction band of which is composed of mainly the O-2p orbital. In this paper, we exemplify a band-gap modification by systematic structural, optical, and transport measurements in BaSn1-xPbxO3 films. A sudden suppression of the conductivity and an enhancement of the weak antilocalization effect at x = 0.9 indicate the presence of a singular point in the electronic structure as a signature of the band inversion. Our findings provide an intriguing platform for combining topological aspects and electron correlation in perovskite oxides based on band-gap engineering.

DOI： 10.1103/PhysRevB.101.125125

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

We have studied the spin Hall magnetoresistance (SMR) arising at the interface of Pt and antiferromagnetic insulator MnTiO3 thin films. Using c axis oriented single-crystalline ilmenite-type MnTiO3 films on Al2O3 (0001) substrates grown by pulsed-laser deposition, we fabricated L-shaped multi-terminal Pt/MnTiO3 devices in which the longitudinal resistances of two orthogonally arranged channels can be simultaneously measured by the four-probe method. Magnetic field angular dependence of the magnetoresistance was consistent with symmetry relations for the SMR between spin-current polarization in Pt and magnetic moment in MnTiO3, evidencing the predominant SMR contribution. By utilizing distinct SMR responses of the two channels when a magnetic field was applied in parallel to one of the channels, we determined the Neel temperature (TN) to be 63 K in a 20-nm-thick MnTiO3 film, comparable to TN of a 40-nm-thick MnTiO3 film separately measured with a superconducting quantum interference device magnetometer. The electrically determined TN persisted down to the film thickness of 2.9 nm, indicating the robust antiferromagnetic ordering even at the surface of MnTiO3 ultrathin films. Published under license by AIP Publishing.

DOI： 10.1063/1.5142193

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We experimentally investigated the contribution of intrinsic anomalous Hall effect (AHE) in ferromagnetic Fe-Sn nanocrystalline films by means of impurity doping. We found that some heavy transition elements such as Ta, W, and Mo are effective for increasing the anomalous Hall resistivity of Fe-Sn films. The concomitant decrease in magnetization of the Fe-Sn matrix indicated that the increased anomalous Hall resistivity arises from the enhancement of the anomalous Hall coefficient. The increased anomalous Hall resistivity, in combination with the moderately decreased saturation field, substantially increased the derivative of anomalous Hall resistivity with respect to applied magnetic field in the linear Hall response region at low field, which corresponds to the sensitivity in an AHE-type Hall sensor. In particular, optimally Ta-doped Fe-Sn films showed nearly doubled sensitivity in comparison with nondoped Fe-Sn films, while the virtually temperature-independent behavior of the sensitivity was maintained between 400 and 50 K. These improved AHE characteristics enable sensitive detection of magnetic field over a wide temperature range. We discuss that strong spin-orbit coupling inherent to these heavy transition elements contributes to the modification of electronic structure, inducing the large intrinsic AHE. The doping technique demonstrated will be a fundamental strategy for exploiting the performance of Fe-Sn metal-based AHE-type Hall sensors. (C) 2019 Author(s).

DOI： 10.1063/1.5126499

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

Hall devices using anomalous Hall effect of ferromagnetic Fe?Sn nanocrystalline thin films exhibit high sensitivity for detection of magnetic field comparable to that of semiconductor devices. For applying this material to magnetometry, superior detectivity owing to low electrical noise is a critical requirement. We investigate the magnetic-field sensitivity and the low-frequency noise spectrum as a detectivity for Fe?Sn Hall-cross devices. We detected typical noise behavior of 1/f and thermal noise. The noise-equivalent magnetic field reaches 0.3;?T/Hz(1/2), which is comparable to that of semiconductor-based Hall sensors. A sufficiently low-noise spectrum evidences the potential applicability of Fe?Sn to Hall magnetometry.

DOI： 10.7567/1882-0786/ab506a

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

We synthesized the magnetic Weyl semimetal candidate Co3Sn2S2 as a thin film using the co-sputtering technique. By adjusting the film composition using a sulfur-rich SnS(1.5 )target with Co metal chips attached, we obtained highly crystallized, single-phase Co3Sn2S2 films. The films showed a ferromagnetic transition around 180 K with perpendicular remanent magnetization. We observed the anomalous Hall effect with a tangent of Hall angle of 0.20 at 130 K, as previously reported for the bulk single crystals. Angular dependence measurements suggested that the magnetization reversal process governed by the domain wall motion leads to a large coercive field in the films. (C) 2019 The Japan Society of Applied Physics

DOI： 10.7567/1347-4065/ab12ff

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5090407

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE RESEARCH  

The interplay of magnetism and spin-orbit coupling on an Fe kagome lattice in Fe3Sn2 crystal produces a unique band structure leading to an order of magnitude larger anomalous Hall effect than in conventional ferromagnetic metals. In this work, we demonstrate that Fe-Sn nanocrystalline films also exhibit a large anomalous Hall effect, being applicable to magnetic sensors that satisfy both high sensitivity and thermal stability. In the films prepared by a co-sputtering technique at room temperature, the partial development of crystalline lattice order appears as nanocrystals of the Fe-Sn kagome layer. The tangent of Hall angle, the ratio of Hall resistivity to longitudinal resistivity, is maximized in the optimal alloy composition of close to Fe3Sn2, implying the possible contribution of the kagome origin even though the films are composed of nanocrystal and amorphous-like domains. These ferromagnetic Fe-Sn films possess great advantages as a Hall sensor over semiconductors in thermal stability owing to the weak temperature dependence of the anomalous Hall responses. Moreover, the room-temperature fabrication enables us to develop a mechanically flexible Hall sensor on an organic substrate. These demonstrations manifest the potential of ferromagnetic kagome metals as untapped reservoir for designing new functional devices.

DOI： 10.1038/s41598-019-39817-8

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

DOI： 10.1063/1.5079719

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

© 2018 Author(s). In polar crystals, cooperative ionic displacement produces a macroscopic spontaneous polarization. Among such polar materials, LiNbO 3 -type wide bandgap oxides are particularly appealing because they offer useful ferroelectric properties and also potentially lead to multiferroic materials. Using molecular-beam epitaxy, we investigated the thin-film growth of high-pressure phase LiNbO 3 -type ZnSnO 3 and discovered a polar oxide candidate, MgSnO 3 . We found that LiNbO 3 -type substrates play an essential role in the crystallization of these compounds, though corundum-type Al 2 O 3 substrates also have the identical crystallographic arrangement of oxygen sublattice. Optical transmittance and electrical transport measurements revealed their potential as a transparent conducting oxide. Establishment of a thin-film synthetic route would be the basis for exploration of functional polar oxides and research on conduction at ferroelectric interfaces and domain walls.

DOI： 10.1063/1.5054289

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

DOI： 10.7567/1882-0786/aaf285

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

Layer-structured InSe is one of the intensively studied two-dimensional monochalcogenide semiconductors for optical and electrical devices. Significant features of the InSe device are the thickness dependent bandgap modification resulting in a peak shift of photoluminescence and a drastic variation of electron mobility. In this study, by applying the pulsed-laser deposition technique, we investigated the optical and electrical properties of c-axis oriented InSe films with the thickness varying from a few to hundred nanometers. The energy at the absorption edge systematically shifts from about 3.3 to 1.4 eV with the increasing thickness. The InSe films on Al2O3(0001) are highly resistive, while those on InP(111) are conductive, which probably originates from the valence mismatch effect at the interface. The electron mobility of the conducting charge carrier at the interface of InSe/InP is enhanced in thicker samples than the critical thickness of about 10 nm, corresponding to the bandgap modification characterized by the optical measurement. Therefore, the substrate and the film thickness are critically important factors for the materialization of InSe optical and electrical device applications. Published by AIP Publishing.

DOI： 10.1063/1.5064736

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

Formation of a depletion region at the vertically stacked topological insulator p-n heterostructures is one of the effective approaches to minimize residual carrier density in the bulk regions. Here, we report on characterization of field-effect transistor (FET) based on (Bi0.26Sb0.74)(2)Se-3/Bi2Se3 topological insulator heterostructures with a vertical p-n junction configuration. The thicknesses of the top p-(Bi0.26Sb0.74)(2)Se-3 layer vary from d = 8 to 25 nm with 3-nm constant thickness of the bottom n-Bi2Se3. Even at zero gate voltage, increasing thickness of the top layer inverts the dominant conducting carrier from n-type to p-type. With applying gate voltage, the effect of depletion region at the interface is clearly revealed as a voltage shift of charge neutral point in ambipolar FET operation. The systematic shift of charge neutral point in FET operation clearly indicates that the formation of depletion region can be explained with conventional semiconductor p-n junction model. In the topological insulator p-n heterostructures, the thickness tuning of both p-type and n-type layers is quite important to minimize the residual charge density in the whole device region. The band engineering with p-n junction will be further applicable to extract the electrical properties of surface state.

DOI： 10.1103/PhysRevB.98.125415

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Institute of Physics Inc.  

We propose crystalline ZnSnO3 as a new channel material for field-effect transistors. By molecular-beam epitaxy on LiNbO3(0001) substrates, we synthesized films of ZnSnO3, which crystallizes in the LiNbO3-type polar structure. Field-effect transistors on ZnSnO3 exhibit n-type operation with field-effect mobility of as high as 45 cm2V-1s-1 at room temperature. Systematic examination of the transistor operation for channels with different Zn/Sn compositional ratios revealed that the observed high-mobility reflects the nature of stoichiometric ZnSnO3 phase. Moreover, we found an indication of coupling of transistor characteristics with intrinsic spontaneous polarization in ZnSnO3, potentially leading to a distinct type of polarization-induced conduction.

DOI： 10.1063/1.5034403

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Institute of Physics Inc.  

We report on the successful synthesis of highly conductive PdCoO2 ultrathin films on Al2O3 (0001) by pulsed laser deposition. The thin films grow along the c-axis of the layered delafossite structure of PdCoO2, corresponding to the alternating stacking of conductive Pd layers and CoO2 octahedra. The thickness-dependent transport measurement reveals that each Pd layer has a homogeneous sheet conductance as high as 5.5 mS in the samples thicker than the critical thickness of 2.1 nm. Even at the critical thickness, high conductivity exceeding 104 S cm-1 is achieved. Optical transmittance spectra exhibit high optical transparency of PdCoO2 thin films particularly in the near-infrared region. The concomitant high values of electrical conductivity and optical transmittance make PdCoO2 ultrathin films promising transparent electrodes for triangular-lattice-based materials.

DOI： 10.1063/1.5027579

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

We report on an enhancement of the superconducting transition temperature (T-c) of the FeSe-based electric-double-layer transistor (FeSe-EDLT) by applying the multivalent oligomeric ionic liquids (ILs). The IL composed of dimeric cation (divalent IL) enables a large amount of charge accumulation on the surface of the FeSe ultrathin film, resulting in inducing electron-rich conduction even in a rather thick 10 nm FeSe channel. The onset T-c in FeSe-EDLT with the divalent IL is enhanced to be approaching about 50 K at the thin limit, which is about 7 K higher than that in EDLT with conventional monovalent ILs. The enhancement of Tc is a pronounced effect of the application of the divalent IL, in addition to the large capacitance, supposing preferable interface formation of ILs driven by geometric and/or Coulombic effect. The present finding strongly indicates that multivalent ILs are powerful tools for controlling and improving physical properties of materials.

DOI： 10.1103/PhysRevMaterials.2.031801

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics Publishing  

We report on the electronic states and the transport properties of three-dimensional topological insulator (Bi1-xSbx)2Se3 ternary alloy thin films grown on an isostructural Bi2Se3 buffer layer on InP substrates. By angle-resolved photoemission spectroscopy, we clearly detected Dirac surface states with a large bulk band gap of 0.2-0.3 eV in the (Bi1-xSbx)2Se3 film with x = 0.70. In addition, we observed by Hall effect measurements that the dominant charge carrier converts from electron (n-type) to hole (p-type) at around x = 0.7, indicating that the Fermi level can be controlled across the Dirac point. Indeed, the carrier transport was shown to be governed by Dirac surface state in 0.63 x 0.75. These features suggest that Fermi-level tunable (Bi1-xSbx)2Se3-based heterostructures provide a platform for extracting exotic topological phenomena.

DOI： 10.1088/1361-648X/aaa724

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics  

We report the synthesis of tetragonal FeS<inf>x</inf>Se<inf>1−</inf><inf>x</inf>films (x ≤ 0.78) by pulsed-laser deposition. To fabricate tetragonal alloy films with tetragonal FeSe and hexagonal FeS targets, we adopted an alternate deposition technique with an FeSe buffer layer on MgO(001). The overall film composition is controlled by the thickness ratio of FeS/FeSe layers. The out-of-plane lattice parameter of the films follows Vegard’s law, demonstrating homogeneous alloying by interdiffusion. The sulfur solid solubility reaches x = 0.78 in the FeS<inf>x</inf>Se<inf>1−</inf><inf>x</inf>films, which is by far higher than x ∼ 0.40 in bulk governed by the tetragonal phase instability.

DOI： 10.7567/JJAP.56.100308.

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

Wide-bandgap oxides exhibiting high electron mobility hold promise for the development of useful electronic and optoelectronic devices as well as for basic research on two-dimensional electron transport phenomena. A perovskite-type tin oxide, BaSnO3, is currently one of such targets owing to distinctly high mobility at room temperature. The challenge to overcome towards the use of BaSnO3 thin films in applications is suppression of dislocation scattering, which is one of the dominant scattering origins for electron transport. Here, we show that the mobility of the BaSnO3 electric-double-layer transistor reaches 300 cm(2) V-1 s(-1) at 50 K. The improved mobility indicates that charged dislocation scattering is effectively screened by electrostatically doped high-density charge carriers. We also observed metallic conduction persisting down to 2 K, which is attributed to the transition to the degenerate semiconductor. The experimental verification of bulk-level mobility at the densely accumulated surface sheds more light on the importance of suppression of dislocation scattering by interface engineering in doped BaSnO3 thin films for transparent electrode applications. Published by AIP Publishing.

DOI： 10.1063/1.4983611

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

A perovskite oxide, BaSnO3, has been classified as one of transparent conducting materials with high electron mobility, and its application for field-effect transistors has been the focus of recent research. Here we report transistor operation in BaSnO3-based heterostructures with atomically smooth surfaces, fabricated on SrTiO3 substrates by the (Sr,Ba)SnO3 buffer technique. Indeed, modulation of band profiles at the channel interfaces with the insertion of wide bandgap (Sr,Ba)SnO3 as a barrier layer results in a significant improvement of field-effect mobility, implying effective carrier doping at the regulated heterointerface. These results provide an important step towards realization of high-performance BaSnO3-based field-effect transistors. (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： 10.1063/1.4961637

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

We report on the observation of reversible and immediate resistance switching by high-k oxide Ta2O5/organic parylene-C hybrid dielectric-gated VO2 thin films. Resistance change ratios at various temperatures in the insulating regime were demonstrated to occur in the vicinity of phase transition temperature. We also found an asymmetric hole-electron carrier modulation related to the suppression of phase transition temperature. The results in this research provide a possibility for clarifying the origin of metal-insulator transition in VO2 through the electrostatic field-induced transport modulation. (C) 2016 AIP Publishing LLC.

DOI： 10.1063/1.4941233

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

The proposal of a hybrid gate dielectric systematically modulated with low-k material layer has been shown to be a promising strategy in the development of low-consumption field-effect transistors (FETs) with high performance. In this work, by fabricating KTaO3 FETs containing Y-doped Ta2O5/parylene-C hybrid gate dielectrics with different ratios of component thicknesses, we explored the dependence of the transistor electrical properties on the parylene-C layer thickness. Based on the results and analysis, an optimized transistor performance was achieved with an appropriate Y-doped Ta2O5/parylene-C thickness ratio from the point of view on low voltage operation. This study contributes to provide guidance for future device design and applications. (c) 2016 AIP Publishing LLC.

DOI： 10.1063/1.4940387

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

We propose a method of discriminating whether the gating effect is electrostatic or electrochemical on an electric double-layer transistor (EDLT), which can be a good guideline for examining the nature of EDLTs. The electrochemical effect between a channel and an ionic liquid depends on the gate voltage (V-G). Our study on the dependence of the transport properties of a (La0.525Pr0.1Ca0.375)MnO3 (LPCMO) EDLT on V-G revealed that the electrostatic effect is dominant below vertical bar V-G vertical bar = 2 V, whereas the electrochemical effect is dominant at higher voltages. The modulation of the insulator-to-metal transition property of LPCMO was realized through the electrostatic effect. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/APEX.8.073201

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

In the scaling down of electronic devices, functional oxides with strongly correlated electron system provide advantages to conventional semiconductors, namely, huge switching owing to their phase transition and high carrier density, which guarantee their rich functionalities even at the 10 nm scale. However, understanding how their functionalities behave at a scale of 10 nm order is still a challenging issue. Here, we report the construction of the well-defined (La,Pr,Ca)MnO3 epitaxial oxide nanowall wire by combination of nanolithography and subsequent thin-film growth, which allows the direct investigation of its insulator-metal transition (IMT) at the single domain scale. We show that the width of a (La,Pr,Ca)MnO3 nanowall sample can be reduced to 50 nm, which is smaller than the observed 70-200 nm-size electronic domains, and that a single electronic nanodomain in (La,Pr,Ca)MnO3 exhibited an intrinsic first-order IMT with an unusually steep single-step change in its magnetoresistance and temperature-induced resistance due to the domains arrangement in series. A simple model of the first-order transition for single electric domains satisfactorily illustrates the IMT behavior from macroscale down to the nanoscale.

DOI： 10.1021/acs.nanolett.5b00264

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Surface Science Society of Japan  

A novel nanofabrication technique, namely three-dimensional (3D) nanotemplate pulsed laser deposition, that can typically be useful for metal oxides has been developed by combining inclined pulsed laser deposition with a 3D nanotemplate prepared by nanoimprint lithography. This method enables the production of size-adjustable and extremely small functional oxide nanostructures with a high packing density over a large area, with high controllability of their shape, location, and alignment. We report various 3D functional metal oxide nanostructures, such as a luminescent ZnO nanobox structure, a ferromagnetic semiconductor (Fe,Zn)&lt
inf&gt
3&lt
/inf&gt
O&lt
inf&gt
4&lt
/inf&gt
nanobox structure, an (Fe,Mn)&lt
inf&gt
3&lt
/inf&gt
O&lt
inf&gt
4&lt
/inf&gt
epitaxial nanowall wire structure, and an in-plane metal-oxide-semiconductor hetero epitaxial nanowire structure. These nanostructures should be good candidates for optoelectronic, spintronic, and electronic nanoscale device applications.

DOI： 10.1380/ejssnt.2015.279

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

Temperature-dependent conductivities at dc and terahertz (THz) frequency region (sigma(THz)(omega,T)) were obtained for a strongly correlated (La0.275Pr0.35Ca0.375)MnO3 (LPCMO) film using THz time domain spectroscopy. A composite model that describes sigma(THz)(omega,T) for LPCMO through the insulator-metal transition (IMT) was established by incorporating Austin-Mott model characterizing the hopping of localized electrons and Drude model explaining the behavior of free electrons. This model enables us to reliably investigate the dc transport dynamics from THz conductivity measurement, i.e., simultaneously evaluate the dc conductivity and the competing composition of metal and insulator phases through the IMT, reflecting the changes in microscopic conductivity of these phases. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4890109

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Controlling the electronic properties of functional oxide materials via external electric fields has attracted increasing attention as a key technology for next-generation electronics. For transition-metal oxides with metallic carrier densities, the electric-field effect with ionic liquid electrolytes has been widely used because of the enormous carrier doping capabilities. The gate-induced redox reactions revealed by recent investigations have, however, highlighted the complex nature of the electric-field effect. Here, we use the gate-induced conductance modulation of spinel ZnxFe3-xO4 to demonstrate the dual contributions of volatile and non-volatile field effects arising from electronic carrier doping and redox reactions. These two contributions are found to change in opposite senses depending on the Zn content x; virtual electronic and chemical field effects are observed at appropriate Zn compositions. The tuning of field-effect characteristics via composition engineering should be extremely useful for fabricating high-performance oxide field-effect devices.

DOI： 10.1038/srep05818

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

DOI： 10.1002/admi.201300108

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Electrical switching was used to investigate complex phases induced by Cr-substitution in (Pr1/3Sm2/3)(2/3) Sr1/3MnO3. This system was expected to transform from a Type I (Mn4+/Mn3+ approximate to 3/7) to Type II (Mn4+/Mn3+ approximate to 1) manganite at critical Cr content, satisfying a virtual Mn4+/Mn3+ ratio close to unity. The phase diagram of (Pr1/3Sm2/3)(2/3)Sr1/3Mn0.8Cr0.2O3 including charge/spin ordered/disordered phases was probed by electrical switching. The ferromagnetic insulating phase at &lt; similar to 100 K, located next to the charge-ordered antiferromagnetic phase, exhibited a sudden use in conductivity upon electric-held biasing. This resulted from the melting or charge ordering, and demonstrated the presence or a crossover regime or two coexisting magnetic orderings. (C) 2014 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ssc.2014.02.014

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

We report a strategy for controlling nanoscopic electronic domains to produce gigantic Mott metal-insulator transition phenomena in strongly correlated oxides by fabricating oxide micro-nano-wires, nanowalls, nanoboxes. We investigated the dependence of spatial dimensionality on wire width for a disordered configurations of metallic domains in VO2 microwires to nanowires on TiO2(001) and Al2O3(0001) substrates with well-positioned alignment by a nanoimprint (NIL) technique. We observed a temperature-induced steep multistep metal-insulator transition in artificial VO2 micro/nano-wires. With further development, we report a new bottom-up fabrication method for the formation of extremely small transition-metal oxide nanostructures employing a combination of NIL and pulsed laser deposition (PLD) techniques, called the three-dimensional nanotemplate-PLD method, to demonstrate functional oxide nanowall wires, nanoboxes, and hetero-nanowall oxide devices with widths of 20-120nm and excellent size controllability. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.53.05FA10

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

We have established a unique technique to fabricate three-dimensional (3D) well-defined transition-metal oxide epitaxial nanostructures. Fabrication of epitaxial spinel ferrite Fe2.2Zn0.8O4 (FZO) nanowall wires with a tunable width down to 20 nm was achieved. Cross-sectional transmission electron microscopy revealed the existence of an epitaxially matched lateral interface between the FZO nanowall wire and the side surface of 3D-MgO substrate. Magnetoresistance measurements showed ferromagnetic properties of the FZO nanowall wire at 300 K. The role of antiphase boundaries on the functionalities of the FZO nanoconfined wire is discussed. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/APEX.7.045201

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Devices based on pure spin currents have been attracting increasing attention as key ingredients for low-dissipation electronics. To integrate such spintronics devices into charge-based technologies, electric detection of spin currents is essential. The inverse spin Hall effect converts a spin current into an electric voltage through spin-orbit coupling. Noble metals such as Pt and Pd, and also Cu-based alloys, have been regarded as potential materials for a spin-current injector, owing to the large direct spin Hall effect. Their spin Hall resistivity rho(SH), representing the performance as a detector, is not large enough, however, due mainly because of their low charge resistivity. Here we report that a binary 5d transition metal oxide, iridium oxide, overcomes the limitations encountered in noble metals and Cu-based alloys and shows a very large rho(SH)similar to 38 mu Omega cm at room temperature.

DOI： 10.1038/ncomms3893

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

To clarify the mechanism of resistance-switching phenomena, we have investigated the change in the electronic structure of a Ni nanowire device during resistance-switching operations using scanning photoelectron microscopy techniques. We directly observed the disappearance of density of state (DOS) at the Fermi level (E-F) in a high-resistance state and recovery of a finite DOS at E-F in a low-resistance state. These results are direct evidence that the Ni nanowire is fully oxidized after switching to the high-resistance state and that Ni-metal conductive paths in the oxidized nanowire are recovered in the low-resistance state. (C) 2013 AIP Publishing LLC.

DOI： 10.1063/1.4829469

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

A field-effect transistor has been fabricated utilizing an epitaxial film of unstrained zinc-substituted magnetite (Fe3O4) as the active channel. A thin film of Fe2.5Zn0.5O4 was grown on a lattice-matched MgO(001) substrate by pulsed-laser deposition and covered by a parylene gate insulator to dope charge carriers by a field effect. The device showed a field-effect mobility of 1.2 x 10(-2) cm(2) V-1 s(-1) at 300 K, which is higher by a factor of 15 than those of the devices with strained Fe2.5Zn0.5O4 channels on perovskite-type substrates. The enhanced response to the gate electric field is useful in exploring gate-tunable magnetism in magnetite. (C) 2013 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.52.068002

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

We have studied the dielectric breakdown of the insulating charge-ordered state and the associated current switching phenomenon in layered ferrite LuFe2O4. To elucidate the correlation between the ordering pattern and current switching behaviour, we synthesized highly c-axis oriented thin films by pulsed-laser deposition. An enhanced switching effect was achieved in the three-dimensional charge-ordered phase below similar to 310 K, but not in the high-temperature two-dimensional phase. High-field transport measurements revealed that collective depinning of localized charge carriers is essential to induce switching. The lack of collective charge motion is proposed as the origin of the switching suppression in the two-dimensional phase.

DOI： 10.1088/0022-3727/46/15/155108

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

We propose a novel approach to fabricate metal oxide nanostructures with locally modified thicknesses. Utilizing a nanoscale shadowing effect induced by a three-dimensional nanotemplate patterned on a substrate, we demonstrate that pulsed-laser deposition in an in-plane oblique configuration enables us to locally grow an oxide. The analysis on the growth position reveals that it can be precisely defined by adjusting only a few geometrical parameters. A ferromagnetic oxide Fe2.5Zn0.5O4 nanoconstriction with a bottleneck width of 65nm and a length of 53nm is fabricated by controlling the incident azimuthal angle and growth duration in sequential deposition processes. (C) 2013 The Japan Society of Applied Physics

DOI： 10.7567/APEX.6.035201

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

An architecturally designed nanowall-shaped MgO (nanowall MgO) was fabricated by the combination of nanoimprint lithography (NIL) and pulsed-laser deposition (PLD). The sidesurface on the nanowall MgO exhibited (111) facets with edge truncation instead of the most stable (100) face when the aspect ratio between the height and width of the nanowall MgO was lower than 0.7. By optimizing the surface crystallography, typically by designing the nanowall aspect ratio and controlling the postannealing treatment conditions, nanowall MgO with a single-crystal flat (100) sidesurface could be produced. Applying the nanowall MgO to a substrate, we demonstrated the formation of extremely small three-dimensional (3D) epitaxial metal oxide nanostructures with an arbitrarily controlled size. The nanofabrication technique utilizing the nanowall MgO substrate will open a new route to high-quality 3D epitaxial metal oxide nanostructures. (C) 2013 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.52.015001

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

We have developed a new method to fabricate extremely small transition-metal oxide nanowires. Using a combination of nanoimprint template patterning and inclined substrate pulsed laser deposition, we successfully fabricated magnetic oxide Fe2.5Mn0.5O4 nanowall-wires, and controlled the width in a range from 120 nm down to about 20 nm by varying deposition parameters. Magnetoresistance measurements revealed ferromagnetic properties of the Fe2.5Mn0.5O4 nanowall-wire. This method enables the study of mesoscopic transport properties of transition-metal oxides towards the development of oxide-based nanodevices.

DOI： 10.1088/0957-4484/23/48/485308

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

We demonstrate control of spatial dimensionality of disordered configurations of giant electronic domains in systematically size-changed VO2 wires on TiO2 (001) substrates. One-dimensional alignment of the domains appears in wires narrower than 15 mu m width, while two-dimensional configurations were observed for larger ones. The rearrangement of domains from two to one dimension causes modification of electronic properties. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4773371]

DOI： 10.1063/1.4773371

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

Well-ordered ferromagnetic Fe nanodots/LaSrFeO4 nanocomposites have been fabricated by self-assembled crystal growth on La-SrTiO3 substrates having Fe nanoseed array fabricated by nanoimprint lithography (NIL). The Fe nanoseeds with spacing of 200 nm make possible the formation of perfectly arranged Fe/LaSrFeO4 nanocomposites; phase-separated Fe nanodots and the LaSrFeO4 matrix grew only on the nanoseeds and on the area except nanoseeds, respectively. A calculation based on a surface diffusion model has indicated that the nanoseed spacing required for the formation of the perfectly arranged nanocomposite is less than 400 nm. Magnetic force microscopy revealed an arrangement of isolated ferromagnetic domain corresponding to Fe nanodots grown on the Fe nanoseeds. The combination of self-assembled growth and NIL gives a route of the rational formation of high-density ferromagnetic memory devices. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4739719]

DOI： 10.1063/1.4739719

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

The change in the spatial distribution of oxygen ions after an initial voltage application called the forming process was investigated for oxide resistance switching devices by secondary ion mass spectrometry mapping. To track the motion of oxygen ions, tracer (18)O ions were implanted in a planar Pt/CuO/Pt device. We found clear evidence for the oxygen reduction in the conductive bridge structure formed between two electrodes. In addition, the oxygen ions in the bridge structure drift to the anode, implying the oxygen diffusion (migration) induced by high electric field and/or current density. We discuss those results in terms of a filament model. (C) 2010 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.49.060215

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

Highly bulk-sensitive hard x-ray photoemission has been performed for the Magneacuteli phase V(5)O(9) and V(6)O(11), which are composed of V(2)O(3) and VO(2) from chemical point of view. The valence-band spectra near the Fermi level are rather similar between V(5)O(9) and V(6)O(11) in both metallic and insulating phases. Although the core-level spectra in the metallic phase are similar between V(5)O(9) and V(6)O(11), the core-level spectra in the insulating phase are noticeably different. Our results are understood by considering the contribution of the V-V dimerization effects, which are strong in V(6)O(11) and negligible in V(5)O(9). Comparison of the core-level spectra of V(5)O(9) and V(6)O(11) with those of VO(2) has revealed that the mechanism of the metal-insulator transitions for the former two compounds is different from that for VO(2).

DOI： 10.1103/PhysRevB.81.113107

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

The Magneli phase Ti4O7 exhibits two sharp jumps in resistivity with coupled structural transitions as a function of temperature at T-c1 similar to 142 K and T-c2 = 154 K. We have studied electronic structure changes across the two transitions using 7 eV laser, soft x-ray, and hard x-ray (HX) photoemission spectroscopy (PES). Ti 2p - 3d resonant PES and HX PES show a clear metallic Fermi edge and mixed valency above T-c2. The low temperature phase below T-c1 shows a clear insulating gap of similar to 100 meV. The intermediate phase between T-c1 and T-c2 indicates a pseudogap coexisting with remnant coherent states. HX PES and complementary calculations have confirmed the coherent screening in the strongly correlated intermediate phase. The results suggest the existence of a highly anomalous state sandwiched between the mixed-valent Fermi liquid and charge ordered Mott-insulating phase in Ti4O7.

DOI： 10.1103/PhysRevLett.104.106401

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOCIETY APPLIED PHYSICS  

Control of the formation of conductive bridge between the metal electrodes in planar-type resistance switching device was attempted. We demonstrated in Pt/CuO/Pt devices that, using a triangular seed electrode for soft breakdown, the position and the size of the bridge can be controlled. The decrease in the size resulted in the drastic reduction of operation voltage and current to the same level as in capacitor-type stacked device. We argue that the planar-type device might have a certain advantage for future non-volatile memory application. (C) 2009 The Japan Society of Applied Physics

DOI： 10.1143/APEX.2.081401

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

The spatial distribution of chemical states in resistance-switching devices with a planar-type Pt/CuO/Pt structure has been studied by photoemission electron microscopy. It has been found that the change in resistance that occurs with the application of the first voltage is caused by the formation of a reduction path through the CuO channel between the Pt electrodes. A detailed analysis suggests that Joule-heat-assisted reduction induced by the current flowing through the device may play an important role in the formation of the conductive reduction path. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3175720]

DOI： 10.1063/1.3175720

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOCIETY APPLIED PHYSICS  

The resistance switching mechanism of a metal/CuO/metal sandwich with a planar device structure has been studied. We report the direct observation of a conducting bridge within the CuO channel of the device, which is formed upon the initial voltage application (forming process). It is found that the resistance switching phenomenon only occurs when just a single bridge is formed during a soft dielectric breakdown. We argue that the reduction and oxidation of this conducting bridge by the action of an applied field and/or current gives rise to a novel nonvolatile memory effect.

DOI： 10.1143/JJAP.47.6266

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

We present a simple but powerful method to determine the thicknesses of the accumulation and depletion layers and the distribution curve of injected carriers in organic field effect transistors. The conductivity of organic semiconductors in thin film transistors was measured in situ and continuously with a bottom contact configuration, as a function of film thickness at various gate voltages. Using this method, the thicknesses of the accumulation and depletion layers of pentacene were determined to be 0.9 nm (V-G = - 15 V) and 5 nm (V-G = 15 V), respectively.

DOI： 10.1143/JJAP.42.L1408

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Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

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Presentation type：Oral presentation (invited, special)  

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Applicant：東京大学、東北大学

Application no：特願2022-030135  Date applied：28 2 2022

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Applicant：独立行政法人理化学研究所

Application no：特願2010-191414  Date applied：27 8 2010

Announcement no：特開2012-049403  Date announced：8 3 2012

Patent/Registration no：特許第5590488号  Date issued：8 8 2014

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Applicant：株式会社東京大学TLO、エヌイーシーショットコンポーネンツ株式会社

Application no：特願2004-079818  Date applied：19 3 2004

Announcement no：特開2005-268578  Date announced：29 9 2005

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