M. Kawasaki

RIKEN, Вако, Saitama, Japan

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Publications (752)2605.05 Total impact

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    ABSTRACT: The electric field effect on ferromagnetism offers a new dimension in the recent advancement of spintronics. We report on the gate control of transport properties in thin films of oxide-based ferromagnetic metal, SrRuO3. An electric double layer transistor configuration was utilized with an ionic liquid dielectric to apply a strong electric field on a SrRuO3 thin film of 5 monolayers in thickness. The application of gate voltage induced a clear electroresistance effect, despite a considerably-large initial carrier density of the order of 10(22) cm(-3). Furthermore, we found that the gate modulation of the anomalous Hall conductivity sigma(xy), which was as large as similar to +/- 40% at low temperatures, was about three times larger than that of the longitudinal conductivity sigma(xx). The variation of sigma(xy) is characterized by the power-law scaling relation with sigma(xx), which is widely observed in a bad metal regime of the charge transport. (C) 2014 AIP Publishing LLC.
    Applied Physics Letters 01/2016; 105(16). DOI:10.1063/1.4899145 · 3.30 Impact Factor
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    ABSTRACT: Longitudinal spin Seebeck effect has been investigated for an uniaxial antiferromagnetic insulator Cr2O3, characterized by a spin-flop transition under magnetic field along the c-axis. We have found that temperature gradient applied normal to Cr2O3/Pt interface induces inverse spin Hall voltage of spin current origin in Pt, whose magnitude turns out to be always proportional to magnetization in Cr2O3. The observed voltage shows significant enhancement for the lower temperature region, which can be ascribed to the phonon-drag effect on the relevant spin excitations. The above results establish that antiferromagnetic spin waves with high frequency above 100 GHz can be an effective carrier of spin current.
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    ABSTRACT: By breaking the time-reversal-symmetry in three-dimensional topological insulators with introduction of spontaneous magnetization or application of magnetic field, the surface states become gapped, leading to quantum anomalous Hall effect or quantum Hall effect, when the chemical potential locates inside the gap. Further breaking of inversion symmetry is possible by employing magnetic topological insulator heterostructures that host nondegenerate top and bottom surface states. Here, we demonstrate the tailored-material approach for the realization of robust quantum Hall states in the bilayer system, in which the cooperative or cancelling combination of the anomalous and ordinary Hall responses from the respective magnetic and non-magnetic layers is exemplified. The appearance of quantum Hall states at filling factor 0 and +1 can be understood by the relationship of energy band diagrams for the two independent surface states. The designable heterostructures of magnetic topological insulator may explore a new arena for intriguing topological transport and functionality.
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    ABSTRACT: Precise control of magnetic domain walls continues to be a central topic in the field of spintronics to boost infotech, logic, and memory applications. One way is to drive the domain wall by current in metals. In insulators, the incoherent flow of phonons and magnons induced by the temperature gradient can carry the spins, i.e., spin Seebeck effect, but the spatial and time dependence is difficult to control. Here, we report that coherent phonons hybridized with spin waves, magnetoelastic waves, can drive magnetic bubble domains, or curved domain walls, in an iron garnet, which are excited by ultrafast laser pulses at a nonabsorbing photon energy. These magnetoelastic waves were imaged by time-resolved Faraday microscopy, and the resultant spin transfer force was evaluated to be larger for domain walls with steeper curvature. This will pave a path for the rapid spatiotemporal control of magnetic textures in insulating magnets.
    Proceedings of the National Academy of Sciences 07/2015; 112(29). DOI:10.1073/pnas.1504064112 · 9.67 Impact Factor
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    ABSTRACT: In the quest for switching of the charge carrier type in conductive materials, we focus on nonsymmorphic crystals, which are expected to have highly anisotropic folded Fermi surfaces due to the symmetry requirements. Following simple tight-binding model simulation, we prepare nonsymmorphic IrO2 single-crystalline films with various growth orientations by molecular beam epitaxy, and systematically quantify their Hall effect for the corresponding field directions. The results clearly demonstrate that the dominant carrier type can be intrinsically controlled by the magnetic field direction, as also evidenced by first-principles calculations revealing nontrivial momentum dependence of the group velocity and mass tensor on the folded Fermi surfaces and its anisotropic nature for the field direction.
    Physical Review B 07/2015; 91(24). DOI:10.1103/PhysRevB.91.241119 · 3.74 Impact Factor
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    ABSTRACT: To explore a p-type semiconductor lattice-matched with perovskite transition-metal oxides, we have grown α-PbO(001) thin films on (Nb-doped) SrTiO3(001) and GdScO3(110) substrates by pulsed laser deposition. The photovoltaic effect in a Au/α-PbO/Nb:SrTiO3 heterojunction is enhanced compared with that in a Au/Nb:SrTiO3 Schottky junction. The band alignment is deduced from photocurrent action spectra. We conclude that α-PbO facilitates the separation of electron-hole pairs generated at the interface of the SrTiO3 side in the ultraviolet light region and of the α-PbO side in the visible light region. Our results indicate that α-PbO is a promising candidate for photovoltaic heterojunctions involving strongly correlated oxides.
    Applied Physics Express 07/2015; 8(7):074001. DOI:10.7567/APEX.8.074001 · 2.37 Impact Factor
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    ABSTRACT: We investigate skyrmion formation in both a single crystalline bulk and epitaxial thin films of MnSi by measurements of planar Hall effect. A prominent stepwise field profile of planar Hall effect is observed in the well-established skyrmion phase region in the bulk sample, which is assigned to anisotropic magnetoresistance effect with respect to the magnetic modulation direction. We also detect the characteristic planar Hall anomalies in the thin films under the in-plane magnetic field at low temperatures, which indicates the formation of skyrmion strings lying in the film plane. Uniaxial magnetic anisotropy plays an important role in stabilizing the in-plane skyrmions in the MnSi thin film.
    Journal of the Physical Society of Japan 06/2015; 84(10). DOI:10.7566/JPSJ.84.104708 · 1.59 Impact Factor
  • Y. Ohuchi · Y. Kozuka · M. Uchida · K. Ueno · A. Tsukazaki · M. Kawasaki
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    ABSTRACT: We report on the topological Hall effect (THE) in centrosymmetric EuO thin films. This THE signal persists down to the lowest temperature in the metallic region below 50 K for the films thinner than 200 nm. The signal rapidly disappears by tilting the applied magnetic field from surface normal, suggestive of noncoplanar spin configuration such as two-dimensional skyrmions. This observation possibly substantiates the theoretical proposal of magnetic skyrmions in 2D Heisenberg ferromagnets in marked contrast to better established B20-type chiral helimagnets.
    Physical Review B 06/2015; 91(24). DOI:10.1103/PhysRevB.91.245115 · 3.74 Impact Factor
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    ABSTRACT: Topologically stable matters can have a long lifetime, even if thermodynamically costly, when the thermal agitation is sufficiently low. A magnetic skyrmion lattice (SkL) represents a unique form of long-range magnetic order that is topologically stable, and therefore, a long-lived, metastable SkL can form. Experimental observations of the SkL in bulk crystals, however, have mostly been limited to a finite and narrow temperature region in which the SkL is thermodynamically stable; thus, the benefits of the topological stability remain unclear. Here, we report a metastable SkL created by quenching a thermodynamically stable SkL. Hall-resistivity measurements of MnSi reveal that, although the metastable SkL is short-lived at high temperatures, the lifetime becomes prolonged (>> 1 week) at low temperatures. The manipulation of a delicate balance between thermal agitation and the topological stability enables a deterministic creation/annihilation of the metastable SkL by exploiting electric heating and subsequent rapid cooling, thus establishing a facile method to control the formation of a SkL.
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    ABSTRACT: Mechanical control of magnetism is an important and promising approach in spintronics. To date, strain control has mostly been demonstrated in ferromagnetic structures by exploiting a change in magnetocrystalline anisotropy. It would be desirable to achieve large strain effects on magnetic nanostructures. Here, using in situ Lorentz transmission electron microscopy, we demonstrate that anisotropic strain as small as 0.3% in a chiral magnet of FeGe induces very large deformations in magnetic skyrmions, as well as distortions of the skyrmion crystal lattice on the order of 20%. Skyrmions are stabilized by the Dzyaloshinskii-Moriya interaction, originating from a chiral crystal structure. Our results show that the change in the modulation of the strength of this interaction is amplified by two orders of magnitude with respect to changes in the crystal lattice due to an applied strain. Our findings may provide a mechanism to achieve strain control of topological magnetic structures based on the Dzyaloshinskii-Moriya interaction.
    Nature Nanotechnology 06/2015; 10(7). DOI:10.1038/nnano.2015.113 · 34.05 Impact Factor
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    ABSTRACT: To evaluate local atomic structures around Co in high temperature diluted ferromagnetic semiconductor Co-doped TiO2, x-ray fluorescence holography and x-ray absorption fine structure experiments were carried out on rutile paramagnetic Ti0.99Co0.01O2 and ferromagnetic Ti0.95Co0.05O2 films. The Co atoms in the Ti0.99Co0.01O2 simply substituted for Ti sites in the rutile structure, whereas a suboxidic arrangement of CoO2Ti4 formed around Co in the Ti0.95Co0.05O2 films. A theoretical investigation based on a series of first-principles calculations indicated the stability of the aggregated suboxidic clusters in the rutile TiO2, supporting our hypothesis for the formation of suboxidic coordination in the highly Co-doped sample. The suboxidic coordination may be the source of strong exchange interaction, resulting in the high Curie temperature in Co-doped TiO2.
    Applied Physics Letters 06/2015; 106(22):222403. DOI:10.1063/1.4921847 · 3.30 Impact Factor
  • Y. Okamura · F. Kagawa · S. Seki · M. Kubota · M. Kawasaki · Y. Tokura
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    ABSTRACT: Through broadband microwave spectroscopy in Faraday geometry, we observe distinct absorption spectra accompanying magnetoelectric (ME) resonance for oppositely propagating microwaves, i.e., directional dichroism, in the multiferroic chiral-lattice magnet ${\mathrm{Cu}}_{2}{\mathrm{OSeO}}_{3}$. The magnitude of the directional dichroism critically depends on the magnetic-field direction. Such behavior is well accounted for by considering the relative direction of the oscillating electric polarizations induced via the ME effect with respect to microwave electric fields. Directional dichroism in a system with an arbitrary form of ME coupling can be also discussed in the same manner.
    Physical Review Letters 05/2015; 114(19). DOI:10.1103/PhysRevLett.114.197202 · 7.51 Impact Factor
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    ABSTRACT: Spin current, i.e. the flow of spin angular momentum or magnetic moment, has recently attracted much attention as the promising alternative for charge current with better energy efficiency. Genuine spin current is generally carried by the spin wave (propagating spin precession) in insulating ferromagnets, and should hold the chiral symmetry when it propagates along the spin direction. Here, we experimentally demonstrate that such a spin wave spin current (SWSC) shows nonreciprocal propagation characters in a chiral-lattice ferromagnet. This phenomenon originates from the interference of chirality between the SWSC and crystal-lattice, which is mediated by the relativistic spin-orbit interaction. The present finding enables the design of perfect spin current diode, and highlights the importance of the chiral aspect in SWSC.
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    T C Fujita · Y Kozuka · M Uchida · A Tsukazaki · T Arima · M Kawasaki
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    ABSTRACT: A new class of materials termed topological insulators have been intensively investigated due to their unique Dirac surface state carrying dissipationless edge spin currents. Recently, it has been theoretically proposed that the three dimensional analogue of this type of band structure, the Weyl Semimetal phase, is materialized in pyrochlore oxides with strong spin-orbit coupling, accompanied by all-in-all-out spin ordering. Here, we report on the fabrication and magnetotransport of Eu2Ir2O7 single crystalline thin films. We reveal that one of the two degenerate all-in-all-out domain structures, which are connected by time-reversal operation, can be selectively formed by the polarity of the cooling magnetic field. Once formed, the domain is robust against an oppositely polarised magnetic field, as evidenced by an unusual odd field dependent term in the magnetoresistance and an anomalous term in the Hall resistance. Our findings pave the way for exploring the predicted novel quantum transport phenomenon at the surfaces/interfaces or magnetic domain walls of pyrochlore iridates.
    Scientific Reports 05/2015; 5:9711. DOI:10.1038/srep09711 · 5.58 Impact Factor
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    ABSTRACT: Electrolyte gating on correlated VO2 thin films enables electrical control of the “bulk” electronic and structural phases over the electrostatic screening length. Although this unique functionality potentially provides novel electronic and optoelectronic device applications, there are intense discussions on the mechanism of the device operation both from electrostatic and electrochemical viewpoints. Here it is shown that the reversibility of the device operation strongly depends on substrates, suggesting that a governing mechanism might differ depending on substrates. Electrolyte gating on VO2 films grown on lattice-matched TiO2 substrates shows reversible gating effects, whereas that on hexagonal Al2O3 substrates become irreversible, although in both cases metallic states can be induced electrically. X-ray absorption spectroscopy measurements on irreversibly gated VO2/Al2O3 reveal permanent reduction of the valence state of vanadium upon gate-induced metallization, presumably originating from irreversible electrochemical doping under the presence of the extremely large electric field created at an electrolyte/VO2 interface. Our findings suggest essential importance of the film quality for future fundamental researches as well as for practical device applications based on electrolyte-gated devices.
    05/2015; 1(7). DOI:10.1002/aelm.201500093
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    ABSTRACT: The fractional quantum Hall (FQH) effect emerges in high-quality two-dimensional electron systems exposed to a magnetic field when the Landau-level filling factor, νe, takes on a rational value. Although the overwhelming majority of FQH states have odd-denominator fillings, the physical properties of the rare and fragile even-denominator states are most tantalizing in view of their potential relevance for topological quantum computation. For decades, GaAs has been the preferred host for studying these even-denominator states, where they occur at νe = 5/2 and 7/2. Here we report an anomalous series of quantized even-denominator FQH states outside the realm of III–V semiconductors in the MgZnO/ZnO 2DES electron at νe = 3/2 and 7/2, with precursor features at 9/2; all while the 5/2 state is absent. The effect in this material occurs concomitantly with tunability of the orbital character of electrons at the chemical potential, thereby realizing a new experimental means for investigating these exotic ground states.
    Nature Physics 03/2015; 11(4). DOI:10.1038/nphys3259 · 20.15 Impact Factor
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    ABSTRACT: Low-temperature photoluminescence and reflectance measurements were employed to study the optical transitions present in two-dimensional electron systems confined at Mg x Zn 1– x O/ZnO heterojunctions. Transitions involving A- and B-holes and electrons from the two lowest subbands formed within the confinement potential are detected. In the studied density range of 2.0–6.5 × 1011 cm−2, the inter-subband splitting is measured and the first excited electron subband is shown to be empty of electrons.
    Applied Physics Letters 02/2015; 106(8):082102. DOI:10.1063/1.4913313 · 3.30 Impact Factor
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    ABSTRACT: Perovskite-type transition-metal oxides have a wide variety of physical properties and triggered intensive research on functional devices in the form of heteroepitaxial junctions. However, there is a missing component that is a p-type conventional band semiconductor. LaRhO3 (LRO) is one of very few promising candidates having its bandgap between filled t 2g and empty e g of Rh in low-spin state, but there has been no report on the synthesis of large-size single crystals or thin films. Here, we report on the junction properties of single-crystalline thin films of LRO grown on (110) oriented Nb-doped SrTiO3 substrates. The external quantum efficiency of the photo-electron conversion exceeds 1% in the visible-light region due to the wide depletion layer and long diffusion length of minority carriers in LRO. Clear indication of p-type band semiconducting character in a perovskite oxide of LRO will pave a way to explore oxide electronics of perovskite heterostructures.
    Applied Physics Letters 02/2015; 106(7):072103. DOI:10.1063/1.4909512 · 3.30 Impact Factor
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    ABSTRACT: A synchrotron x-ray diffraction study of metal-insulator transitions in W-doped ${\text{VO}}_{2} ({\text{V}}_{1$-${}x}{\text{W}}_{x}{\text{O}}_{2})$ thin films has been carried out. The insulating phase for $x$\le${}0.07$ exhibits cell-doubling with the V dimerization similar to bulk ${\text{VO}}_{2}$, while the insulating phase for $x$\ge${}0.11$ does not. This result suggests that the electronic structure of the $x$\ge${}0.11$ insulators should be different from that of the $x$\le${}0.07$ ones and bulk-insulating phase of ${\text{VO}}_{2}$. The temperature and $x$ dependence of superlattice reflection as observed casts doubt about the direct relationship between the dimerization of V ions and metal-insulator transition. The temperature dependence of the electrical resistivity rather implies the Mott-Anderson localization nature of the insulating phases. X-ray-induced persistent phase transitions are observed at low temperatures in each insulating phase in the vicinity of the boundary to the metallic phase regardless of the difference in the electronic structure. Gradual peak shift suggests that the x-ray irradiation produces nanometer metallic regions.
    Physical Review B 02/2015; 91(6). DOI:10.1103/PhysRevB.91.064101 · 3.74 Impact Factor
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    ABSTRACT: The in-plane Mg doping distribution in molecular beam epitaxy grown MgxZn1−xO/ZnO heterostructures is mapped by low-temperature photoluminescence measurements in an effort to evaluate and control the resultant inhomogeneity formed during the growth process. In an unrotated sample, the independent configuration effects of the O3 and Mg source cells are clearly demonstrated in a composition spread due to flux gradients, while this inhomogeneity is suppressed by sample rotation during the growth. The present mapping results provide an important means for investigating improved doping regimes with the aim of enhancing the quality of quantum transport observable at the MgxZn1−xO/ZnO heterointerface.
    Japanese Journal of Applied Physics 02/2015; 54(2):028004. DOI:10.7567/JJAP.54.028004 · 1.13 Impact Factor

Publication Stats

23k Citations
2,605.05 Total Impact Points


  • 2000–2015
    • RIKEN
      • Strong Correlation Interface Research Group
      Вако, Saitama, Japan
    • Tata Institute of Fundamental Research
      • Department of Astronomy and Astrophysics
      Mumbai, Mahārāshtra, India
  • 1989–2015
    • The University of Tokyo
      • • Department of Applied Physics
      • • Department of Applied Chemistry
      • • Department of Advanced Materials Science
      Tōkyō, Japan
  • 2014
    • University of Hamburg
      • Institute of Applied Physics
      Hamburg, Hamburg, Germany
    • University of Waterloo
      • Department of Physics and Astronomy
      Waterloo, Ontario, Canada
  • 2007–2012
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan
  • 2000–2012
    • Tohoku University
      • • Institute for Materials Research
      • • Department of Physics
  • 2001–2008
    • National Institute of Advanced Industrial Science and Technology
      • Nanoelectronics Research Institute
      Tsukuba, Ibaraki, Japan
  • 2000–2008
    • National Institute for Materials Science
      • Advanced Materials Laboratory
      Tsukuba, Ibaraki, Japan
  • 2005
    • University of Warsaw
      Warszawa, Masovian Voivodeship, Poland
  • 2003
    • Keio University
      • Department of Physics
      Edo, Tōkyō, Japan
  • 1989–2003
    • Tokyo Institute of Technology
      • • Department of Innovative and Engineered Materials
      • • Materials and Structures Laboratory
      Edo, Tōkyō, Japan
  • 2002
    • Waseda University
      • Department of Electrical Engineering and Bioscience
      Edo, Tōkyō, Japan
  • 1996
    • The Hong Kong University of Science and Technology
      • Department of Physics
      Chiu-lung, Kowloon City, Hong Kong