M. Kawasaki

RIKEN, Вако, Saitama, Japan

Are you M. Kawasaki?

Claim your profile

Publications (762)2636.55 Total impact

  • [Show abstract] [Hide abstract]
    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
  • T. Murata · Y. Kozuka · M. Uchida · M. Kawasaki ·

    Journal of Applied Physics 11/2015; 118(19):193901. DOI:10.1063/1.4935818 · 2.18 Impact Factor

  • Physical Review B 11/2015; 92(19). DOI:10.1103/PhysRevB.92.195115 · 3.74 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report magnetotransport measurements on a high-mobility two-dimensional electron system at the nonmagnetic MgZnO/ZnO heterointerface showing distinct behavior for electrons with spin-up and spin-down orientations. The low-field Shubnikov-de Haas oscillations manifest alternating resistance peak heights which can be attributed to distinct scattering rates for different spin orientations. The tilt-field measurements at a half-integer filling factor reveal that the majority spins show usual diffusive behavior, i.e., peaks with the magnitude proportional to the index of the Landau level at the Fermi energy. By contrast, the minority spins develop "plateaus" with the magnitude of dissipative resistivity that is fairly independent of the Landau level index and is of the order of the zero-field resistivity.
    Physical Review Letters 11/2015; 115(19). DOI:10.1103/PhysRevLett.115.197601 · 7.51 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Quantum anomalous Hall effect (QAHE), which generates dissipation-less edge current without external magnetic field, is observed in magnetic-ion doped topological insulators (TIs), such as Cr- and V-doped (Bi,Sb)2Te3. The QAHE emerges when the Fermi level is inside the magnetically induced gap around the original Dirac point of the TI surface state. Although the size of gap is reported to be about 50 meV, the observable temperature of QAHE has been limited below 300 mK. We attempt magnetic-Cr modulation doping into topological insulator (Bi,Sb)2Te3 films to increase the observable temperature of QAHE. By introducing the rich-Cr-doped thin (1 nm) layers at the vicinity of the both surfaces based on non-Cr-doped (Bi,Sb)2Te3 films, we have succeeded in observing the QAHE up to 2 K. The improvement in the observable temperature achieved by this modulation-doping appears to be originating from the suppression of the disorder in the surface state interacting with the rich magnetic moments. Such a superlattice designing of the stabilized QAHE may pave a way to dissipation-less electronics based on the highertemperature and zero magnetic-field quantum conduction.
    Applied Physics Letters 11/2015; 107(18):182401. DOI:10.1063/1.4935075 · 3.30 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The spin-momentum locking at the Dirac surface state of a topological insulator (TI) offers a distinct possibility of a highly efficient charge-to-spin current (C-S) conversion compared with spin Hall effects in conventional paramagnetic metals. For the development of TI-based spin current devices, it is essential to evaluate its conversion efficiency quantitatively as a function of the Fermi level EF position. Here we exemplify a coefficient of qICS to characterize the interface C-S conversion effect by using spin torque ferromagnetic resonance (ST-FMR) for (Bi1-xSbx)2Te3 thin films whose EF is tuned across the band gap. In bulk insulating conditions, interface C-S conversion effect via Dirac surface state is evaluated as nearly constant large values of qICS, reflecting that the qICS is inversely proportional to the Fermi velocity vF that is almost constant. However, when EF traverses through the Dirac point, the qICS is remarkably suppressed possibly due to the degeneracy of surface spins or instability of helical spin structure. These results demonstrate that the fine tuning of the EF in TI based heterostructures is critical to maximizing the efficiency using the spin-momentum locking mechanism.
  • J. Falson · Y. Kozuka · J.H. Smet · T. Arima · A. Tsukazaki · M. Kawasaki ·
    [Show abstract] [Hide abstract]
    ABSTRACT: The remarkable historic advances experienced in condensed matter physics have been enabled through the continued exploration and proliferation of increasingly richer and cleaner material systems. In this work, we report on the scattering times of charge carriers confined in state-of-the-art MgZnO/ZnO heterostructures displaying electron mobilities in excess of 106 cm2/V s. Through an examination of low field quantum oscillations, we obtain the effective mass of charge carriers, along with the transport and quantum scattering times. These times compare favorably with high mobility AlGaAs/GaAs heterostructures, suggesting the quality of MgZnO/ZnO heterostructures now rivals that of traditional semiconductors.
    Applied Physics Letters 08/2015; 107(8). DOI:10.1063/1.4929381 · 3.30 Impact Factor
  • Source
    S. Seki · T. Ideue · M. Kubota · Y. Kozuka · R. Takagi · M. Nakamura · Y. Kaneko · M. Kawasaki · Y. Tokura ·
    [Show abstract] [Hide abstract]
    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.
  • Source
    [Show abstract] [Hide abstract]
    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.
    Nature Communications 07/2015; 6. DOI:10.1038/ncomms9530 · 11.47 Impact Factor
  • [Show abstract] [Hide abstract]
    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
  • Source
    [Show abstract] [Hide abstract]
    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
  • Eishi Majima · Yusuke Kozuka · Masaki Uchida · Masao Nakamura · Masashi Kawasaki ·
    [Show abstract] [Hide abstract]
    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
  • Source
    [Show abstract] [Hide abstract]
    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 ·
    [Show abstract] [Hide abstract]
    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
  • Source
    H. Oike · A. Kikkawa · N. Kanazawa · Y. Taguchi · M. Kawasaki · Y. Tokura · F. Kagawa ·
    [Show abstract] [Hide abstract]
    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.
  • [Show abstract] [Hide abstract]
    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
  • Source
    [Show abstract] [Hide abstract]
    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 ·
    [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    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.
  • Source
    T C Fujita · Y Kozuka · M Uchida · A Tsukazaki · T Arima · M Kawasaki ·
    [Show abstract] [Hide abstract]
    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

Publication Stats

26k Citations
2,636.55 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 Chemistry
      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
      Sendai-shi, Miyagi, Japan
  • 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
  • 1989-2004
    • Tokyo Institute of Technology
      • • Materials and Structures Laboratory
      • • Department of Innovative and Engineered Materials
      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