Updated on 2022/08/11

MOCHIZUKI, Masahito

Affiliation
Faculty of Science and Engineering, School of Advanced Science and Engineering
Job title
Professor
Homepage URL
Profile
1998年3月 東京大学理学部物理学科卒業
2000年3月 東京大学大学院理学系研究科物理学専攻修士課程修了
2003年3月 東京大学大学院理学系研究科物理学専攻博士課程修了
2003年4月1日 日本学術振興会特別研究員(PD)
2006年4月1日 (独)理化学研究所基礎科学特別研究員
2007年1月1日 科学技術振興機構戦略的創造研究推進事業研究員
2009年4月16日 東京大学大学院工学系研究科物理工学専攻 特任講師
2013年4月1日 青山学院大学理工学部物理・数理学科 准教授
2013年10月1日 (独)科学技術振興機構 さきがけ（兼任）
2017年4月1日 早稲田大学理工学術院先進理工学部応用物理学科 教授

### Research Institute

• 2020
-
2022

理工学術院総合研究所   兼任研究員

### Degree

• 2003.03   東京大学   博士（理学）

### Professional Memberships

•

THE MAGNETICS SOCIETY OF JAPAN

•

THE JAPAN SOCIETY OF APPLIED PHYSICS

•

THE PHYSICAL SOCIETY OF JAPAN

### Research Areas

• Magnetism, superconductivity and strongly correlated systems

### Papers

• Yuto Uwabo, Masahito Mochizuki

Journal of the Physical Society of Japan   90 ( 10 ) 104712  2021.10  [Refereed]

• Keisuke Kitayama, Yasuhiro Tanaka, Masao Ogata, Masahito Mochizuki

Journal of the Physical Society of Japan   90 ( 10 ) 104705  2021.10  [Refereed]

• Rintaro Eto, Masahito Mochizuki

Physical Review B   104 ( 10 ) 104425  2021.09  [Refereed]

• Keisuke Kitayama, Masahito Mochizuki, Yasuhiro Tanaka, Masao Ogata

Physical Review B   104 ( 7 ) 075127  2021.08  [Refereed]

• Yasuhiro Tanaka, Masahito Mochizuki

Physical Review B   104 ( 8 ) 085123  2021.08  [Refereed]

• Yusuke Miyajima, Yusuke Murata, Yasuhiro Tanaka, Masahito Mochizuki

Physical Review B   104 ( 7 ) 075114  2021.08  [Refereed]

• Takuro Katsufuji, Masayuki Miyake, Makoto Naka, Masahito Mochizuki, Sota Kogo, Tomomasa Kajita, Yasuhiro Shimizu, Masayuki Itoh, Takatoshi Hasegawa, Shunsuke Shimose, Shunta Noguchi, Takuo Saiki, Takuro Sato, Fumitaka Kagawa

Physical Review Research   3 ( 1 ) 013105  2021.02  [Refereed]

• Yasuhiro Tanaka, Takashi Inoue, Masahito Mochizuki

New Journal of Physics   22 ( 8 ) 083054  2020.08  [Refereed]

• Keisuke Kitayama, Masahito Mochizuki

Physical Review Research   2 ( 2 ) 023229  2020.05  [Refereed]

• Masayuki Miyake, Masahito Mochizuki

Physical Review B   101 ( 9 ) 094419  2020.03  [Refereed]

• Kohei Tanaka, Ryosuke Sugawara, Masahito Mochizuki

Physical Review Materials   4 ( 3 )  2020.03  [Refereed]

Authorship：Corresponding author

• Kotaro Shimizu, Masahito Mochizuki

Physical Review B   101 ( 4 )  2020.01  [Refereed]

Authorship：Corresponding author

• Akihito Takeuchi, Shigeyasu Mizushima, Masahito Mochizuki

Scientific Reports   9 ( 1 )  2019.12  [Refereed]

Authorship：Corresponding author

• Tatsuya Koide, Akihito Takeuchi, Masahito Mochizuki

Physical Review B   100 ( 1 )  2019.07  [Refereed]

Authorship：Corresponding author

• Kab-Jin Kim, Masahito Mochizuki, Teruo Ono

Applied Physics Express   12 ( 5 ) 053006 - 053006  2019.05  [Refereed]

View Summary

We investigate the possible emergence of topological Hall effect (THE) in a driven magnetic domain wall (DW). Numerical simulations based on the Landau-Lifshitz-Gilbert-Slonczewski equation shows that the emergent magnetic flux appears when the DW is in a non-equilibrium state. The magnitude of magnetic flux is modulated by the Dzyaloshinskii-Moriya interaction (DMI), providing an experimental test of the predicted THE by a voltage controlled DMI modulation. These results indicate that the THE can be observed even in a topologically trivial magnetic DW, and therefore open up new possibility to electrically detect the dynamical spin structure.

• L. V. Abdurakhimov, S. Khan, N. A. Panjwani, J. D. Breeze, M. Mochizuki, S. Seki, Y. Tokura, J. J. L. Morton, H. Kurebayashi

Physical Review B   99 ( 14 )  2019.04  [Refereed]

View Summary

Anticrossing behavior between magnons in the noncollinear chiral magnet Cu2OSeO3 and a two-mode X-band microwave resonator was studied in the temperature range 5-100 K. In the field-induced ferrimagnetic phase, we observed a strong-coupling regime between magnons and two microwave cavity modes with a cooperativity reaching 3600. In the conical phase, cavity modes are dispersively coupled to a fundamental helimagnon mode, and we demonstrate that the magnetic phase diagram of Cu2OSeO3 can be reconstructed from the measurements of the cavity resonance frequency. In the helical phase, a hybridized state of a higher-order helimagnon mode and a cavity mode-a helimagnon polariton-was found. Our results reveal a class of magnetic systems where strong coupling of microwave photons to nontrivial spin textures can be observed.

• Masaya Kobayashi, Masahito Mochizuki

Physical Review Materials   3 ( 2 )  2019.02  [Refereed]

Authorship：Corresponding author

• K. Kurushima, K. Tanaka, H. Nakajima, M. Mochizuki, S. Mori

Journal of Applied Physics   125 ( 5 ) 053902 - 053902  2019.02  [Refereed]

• Ian Aupiais, Masahito Mochizuki, Hideaki Sakata, Romain Grasset, Yann Gallais, Alain Sacuto, Maximilien Cazayous

npj Quantum Materials   3 ( 1 )  2018.12  [Refereed]

• Masahito Ikka, Akihito Takeuchi, Masahito Mochizuki

Physical Review B   98 ( 18 )  2018.11  [Refereed]

Authorship：Corresponding author

• Selective activation of an isolated magnetic skyrmion in a ferromagnet with microwave electric fields

A. Takeuchi, M. Mochizuki

Appl. Phys. Lett.   113   072404  2018.09  [Refereed]

• Highly efficient induction of spin polarization by circularly polarized electromagnetic waves in the Rashba spin-orbit systems

Masahito Mochizuki, Keisuke Ihara, Jun-Ichiro Ohe, Akihito Takeuchi

Applied Physics Letters   112 ( 12 ) 122401  2018.03  [Refereed]

View Summary

We theoretically demonstrate that a rotating electric-field component of circularly polarized microwaves or terahertz light can induce electron-spin polarization within a few picoseconds in a two-dimensional electron system with the Rashba spin-orbit interaction by taking advantage of magnetoelectric coupling. The efficiency turns out to be several orders of magnitude greater than that of conventional methods, indicating high potential of this technique in future spintronics.

• Spin model for nontrivial types of magnetic order in inverse-perovskite antiferromagnets

Masahito Mochizuki, Masaya Kobayashi, Reoya Okabe, Daisuke Yamamoto

Physical Review B   97 ( 6 ) 060401(R)  2018.02  [Refereed]

View Summary

Nontrivial magnetic orders in the inverse-perovskite manganese nitrides are theoretically studied by constructing a classical spin model describing the magnetic anisotropy and frustrated exchange interactions inherent in specific crystal and electronic structures of these materials. With a replica-exchange Monte Carlo technique, a theoretical analysis of this model reproduces the experimentally observed triangular Γ5g and Γ4g spin-ordered patterns and the systematic evolution of magnetic orders. Our Rapid Communication solves a 40-year-old problem of nontrivial magnetism for the inverse-perovskite manganese nitrides and provides a firm basis for clarifying the magnetism-driven negative thermal expansion phenomenon discovered in this class of materials.

• Formation process of skyrmion lattice domain boundaries: The role of grain boundaries

H. Nakajima, A. Kotani, M. Mochizuki, K. Harada, S. Mori

APPLIED PHYSICS LETTERS   111 ( 19 ) 192401  2017.11  [Refereed]

View Summary

We report on the formation process of skyrmion lattice (SkL) domain boundaries in FeGe using Lorentz transmission electron microscopy and small-angle electron diffraction. We observed that grain boundaries and edges play an important role in the formation of SkL domain boundaries; The SkL domain boundary is stabilized at the intersection of two grains. A micromagnetic simulation using the Landau-Lifshitz-Gilbert equation revealed that the SkL domains separated by a boundary represent the lowest energy configuration. Conversely, in a wide area, SkL domain boundaries were not formed and SkL domains with different orientations rotated to form a single SkL domain. Published by AIP Publishing.

• Controlled creation of nanometric skyrmions using external magnetic fields

Masahito Mochizuki

APPLIED PHYSICS LETTERS   111 ( 9 ) 92403  2017.08  [Refereed]

View Summary

To exploit nanometric magnetic skyrmions as information carriers in high-density storage devices, a method is needed that creates an intended number of skyrmions at specified places in the device preferably at a low energy cost. We theoretically propose that using a system with a fabricated hole or notch, the controlled creation of individual skyrmions can be achieved even when using an external magnetic field applied to the entire specimen. The fabricated defect turns out to work like a catalyst to reduce the energy barrier for the skyrmion creation. Published by AIP Publishing.

• 21pBC-3 Theory of electrical creation of skyrmions on multiferroic chiral -magnet thin films

Mochizuki Masahito, Watanabe Yoshio

Meeting Abstracts of the Physical Society of Japan   71   1258 - 1258  2016

• Creation of Skyrmions by Electric Field on Chiral-Lattice Magnetic Insulators

Masahito Mochizuki

ADVANCED ELECTRONIC MATERIALS   2 ( 1 ) 1500180  2016.01  [Refereed]

View Summary

It is theoretically proposed that magnetic skyrmions, nanometric spin vortices characterized by a quantized topological number, can be electrically created on a thin-film specimen of chiral-lattice magnetic insulator within a few nanoseconds by applying an electric field via an electrode tip taking advantage of coupling between noncollinear skyrmion spins and electric polarizations. This finding paves the way for utilizing multiferroic skyrmions as information carriers for low-energy-consuming magnetic storage devices without Joule-heating energy losses.

• Theory of magnetic-field-induced polarization flop in spin-spiral multiferroics

Masahito Mochizuki

PHYSICAL REVIEW B   92 ( 22 ) 224412  2015.12  [Refereed]

View Summary

The magnetic-field-induced 90 degrees. flop of ferroelectric polarization P in a spin-spiral multiferroic material TbMnO3 is theoretically studied based on a microscopic spin model. I find that the direction of the P flop or the choice of +P-a or -P-a after the flop is governed by magnetic torques produced by the applied magnetic field H acting on the Mn spins and thus is selected in a deterministic way, in contradistinction to the naively anticipated probabilistic flop. This mechanism resolves a puzzle of the previously reported memory effect in the P direction depending on the history of the magnetic-field sweep, and enables controlled switching of multiferroic domains by externally applied magnetic fields. My Monte-Carlo analysis also uncovers that the magnetic structure in the P parallel to a phase under H parallel to b is not a previously anticipated simple ab-plane spin cycloid but a conical spin structure.

• Neel-type skyrmion lattice with confined orientation in the polar magnetic semiconductor GaV4S8

I. Kezsmarki, S. Bordacs, P. Milde, E. Neuber, L. M. Eng, J. S. White, H. M. Ronnow, C. D. Dewhurst, M. Mochizuki, K. Yanai, H. Nakamura, D. Ehlers, V. Tsurkan, A. Loidl

NATURE MATERIALS   14 ( 11 ) 1116 - +  2015.11  [Refereed]

View Summary

Following the early prediction of the skyrmion lattice (SkL)-a periodic array of spin vortices-it has been observed recently in various magnetic crystals mostly with chiral structure. Although non-chiral but polar crystals with C-nv symmetry were identified as ideal SkL hosts in pioneering theoretical studies, this archetype of SkL has remained experimentally unexplored. Here, we report the discovery of a SkL in the polar magnetic semiconductor GaV4S8 with rhombohedral (C-3v) symmetry and easy axis anisotropy. The SkL exists over an unusually broad temperature range compared with other bulk crystals and the orientation of the vortices is not controlled by the external magnetic field, but instead confined to the magnetic easy axis. Supporting theory attributes these unique features to a new Neel-type of SkL describable as a superposition of spin cycloids in contrast to the Bloch-type SkL in chiral magnets described in terms of spin helices.

• Writing a skyrmion on multiferroic materials

Masahito Mochizuki, Yoshio Watanabe

APPLIED PHYSICS LETTERS   107 ( 8 ) 82409  2015.08  [Refereed]

View Summary

This paper reports on a theoretical proposal for electrical creation of magnetic skyrmions on a thin-film specimen of a multiferroic chiral magnet by local application of an electric field, instead of an electric current, via an electrode tip. This method can be traced back to the mutual coupling between skyrmion spins and the electric polarizations in multiferroics and represents a unique technique for use in potential skyrmion-based memory devices without Joule-heating losses. (C) 2015 AIP Publishing LLC.

• Magnetoelectric domain control in multiferroic TbMnO3

Masakazu Matsubara, Sebastian Manz, Masahito Mochizuki, Teresa Kubacka, Ayato Iyama, Nadir Aliouane, Tsuyoshi Kimura, Steven L. Johnson, Dennis Meier, Manfred Fiebig

SCIENCE   348 ( 6239 ) 1112 - 1115  2015.06  [Refereed]

View Summary

The manipulation of domains by external fields in ferroic materials is of major interest for applications. In multiferroics with strongly coupled magnetic and electric order, however, the magnetoelectric coupling on the level of the domains is largely unexplored. We investigated the field-induced domain dynamics of TbMnO3 in the multiferroic ground state and across a first-order spin-flop transition. In spite of the discontinuous nature of this transition, the reorientation of the order parameters is deterministic and preserves the multiferroic domain pattern. Landau-Lifshitz-Gilbert simulations reveal that this behavior is intrinsic. Such magnetoelectric correlations in spin-driven ferroelectrics may lead to domain wall-based nanoelectronics devices.

• Microwave Magnetochiral Effect in Cu2OSeO3

Masahito Mochizuki

PHYSICAL REVIEW LETTERS   114 ( 19 ) 197203  2015.05  [Refereed]

View Summary

We theoretically find that in the multiferroic chiral magnet Cu2OSeO3 resonant magnetic excitations are coupled to the collective oscillation of the electric polarization, and thereby attain simultaneous activity to the ac magnetic field and ac electric field. Because of the interference between these magnetic and electric activation processes, this material hosts a gigantic magnetochiral dichroism for microwaves, that is, a directional dichroism at gigahertz frequencies in the Faraday geometry. The absorption intensity of a microwave differs by as much as similar to 30% depending on whether its propagation direction is parallel or antiparallel to the external magnetic field.

• 19aDB-5 Real space observation of magnetoelectric domain control in spin-spiral TbMnO_3

Matsubara M., Manz S., Mochizuki M., Iyama A., Kimura T., Fiebig M.

Meeting Abstracts of the Physical Society of Japan   70   2090 - 2090  2015

• 19aDB-9 Magneto-chiral Effect of Chiral-Lattice Magnetic Insulator Cu_2OSeO_3

Mochizuki Masahito

Meeting Abstracts of the Physical Society of Japan   70   2093 - 2093  2015

• 19aDB-6 Magnetic-Field-Induced Polarization Flop in Spin-Spiral Multiferroics TbMnO_3

Mochizuki Masahito

Meeting Abstracts of the Physical Society of Japan   70   2091 - 2091  2015

• Thermally driven ratchet motion of a skyrmion microcrystal and topological magnon Hall effect

M. Mochizuki, X. Z. Yu, S. Seki, N. Kanazawa, W. Koshibae, J. Zang, M. Mostovoy, Y. Tokura, N. Nagaosa

NATURE MATERIALS   13 ( 3 ) 241 - 246  2014.03  [Refereed]

View Summary

Spontaneously emergent chirality is an issue of fundamental importance across the natural sciences(1). It has been argued that a unidirectional (chiral) rotation of a mechanical ratchet is forbidden in thermal equilibrium, but becomes possible in systems out of equilibrium(2). Here we report our finding that a topologically nontrivial spin texture known as a skyrmion-a particle-like object in which spins point in all directions to wrap a sphere(3)-constitutes such a ratchet. By means of Lorentz transmission electron microscopy we show that micrometre-sized crystals of skyrmions in thin films of Cu2OSeO3 and MnSi exhibit a unidirectional rotation motion. Our numerical simulations based on a stochastic Landau-Lifshitz-Gilbert equation suggest that this rotation is driven solely by thermal fluctuations in the presence of a temperature gradient, whereas in thermal equilibrium it is forbidden by the Bohr-van Leeuwen theorem(4,5). We show that the rotational flow of magnons driven by the effective magnetic field of skyrmions gives rise to the skyrmion rotation, therefore suggesting that magnons can be used to control the motion of these spin textures.

• Thermally driven ratchet motion of a skyrmion microcrystal and topological magnon Hall effect (vol 13, pg 241, 2014)

M. Mochizuki, X. Z. Yu, S. Seki, N. Kanazawa, W. Koshibae, J. Zang, M. Mostovoy, Y. Tokura, N. Nagaosa

NATURE MATERIALS   13 ( 3 ) 247 - 247  2014.03  [Refereed]

• Current-induced skyrmion dynamics in constricted geometries

Junichi Iwasaki, Masahito Mochizuki, Naoto Nagaosa

NATURE NANOTECHNOLOGY   8 ( 10 ) 742 - 747  2013.10  [Refereed]

View Summary

Magnetic skyrmions-vortex-like swirling spin structures with a quantized topological number that are observed in chiral magnets-are appealing for potential applications in spintronics because it is possible to control their motion with ultralow current density. To realize skyrmion-based spintronic devices, it is essential to understand skyrmion motions in confined geometries. Here we show by micromagnetic simulations that the current-induced motion of slcyrmions in the presence of geometrical boundaries is very different from that in an infinite plane. In a channel of finite width, transverse confinement results in steady-state characteristics of the skyrmion velocity as a function of current that are similar to those of domain walls in ferromagnets, whereas the transient behaviour depends on the initial distance of the skyrmion from the boundary. Furthermore, we show that a single slcyrmion can be created by an electric current in a simple constricted geometry comprising a plate-shaped specimen of suitable size and geometry. These findings could guide the design of skyrmion-based devices in which skyrmions are used as information carriers.

• Magnetoelectric resonances and predicted microwave diode effect of the skyrmion crystal in a multiferroic chiral-lattice magnet

Masahito Mochizuki, Shinichiro Seki

PHYSICAL REVIEW B   87 ( 13 ) 134403  2013.04  [Refereed]

View Summary

We theoretically discover that unique eigenmodes of skyrmion crystal (SkX) are not only magnetically active to an ac magnetic field (H-omega) but also electrically active to an ac electric field (E-omega) in a multiferroic chiral-lattice magnet Cu2OSeO3, which amplifies the dynamical magnetoelectric coupling between E-omega and the spin texture. The resulting intense interference between their electric and their magnetic activation processes can lead to an unprecedentedly large diode effect on the microwave, i.e., its absorption by SkX changes up to similar to 20% when the incident direction is reversed. Our results demonstrate that the skyrmion could be a promising building block for microwave devices. DOI:10.1103/PhysRevB.87.134403

• Universal current-velocity relation of skyrmion motion in chiral magnets

Junichi Iwasaki, Masahito Mochizuki, Naoto Nagaosa

NATURE COMMUNICATIONS   4   1463  2013.02  [Refereed]

View Summary

Current-driven motion of the magnetic domain wall in ferromagnets is attracting intense attention because of potential applications such as racetrack memory. There, the critical current density to drive the motion is similar to 10(9)-10(12) A m(-2). The skyrmions recently discovered in chiral magnets have much smaller critical current density of similar to 10(5)-10(6) A m(-2), but the microscopic mechanism is not yet explored. Here we present a numerical simulation of Landau-Lifshitz-Gilbert equation, which reveals a remarkably robust and universal current-velocity relation of the skyrmion motion driven by the spin-transfer-torque unaffected by either impurities or nonadiabatic effect in sharp contrast to the case of domain wall or spin helix. Simulation results are analysed using a theory based on Thiele's equation, and it is concluded that this behaviour is due to the Magnus force and flexible shape-deformation of individual skyrmions and skyrmion crystal, which enable them to avoid pinning centres.

• Microwave magnetoelectric effect via skyrmion resonance modes in a helimagnetic multiferroic

Y. Okamura, F. Kagawa, M. Mochizuki, M. Kubota, S. Seki, S. Ishiwata, M. Kawasaki, Y. Onose, Y. Tokura

Nature Communications   4   3391  2013  [Refereed]

View Summary

Magnetic skyrmion, a topologically stable spin-swirling object, can host emergent electromagnetism, as exemplified by the topological Hall effect and electric-current-driven skyrmion motion. To achieve efficient manipulation of nano-sized functional spin textures, it is imperative to exploit the resonant motion of skyrmions, analogously to the role of the ferromagnetic resonance in spintronics. The magnetic resonance of skyrmions has recently been detected with oscillating magnetic fields at 1-2 GHz, launching a search for new skyrmion functionality operating at microwave frequencies. Here we show a microwave magnetoelectric effect in resonant skyrmion dynamics. Through microwave transmittance spectroscopy on the skyrmion-hosting multiferroic crystal Cu 2 OSeO 3 combined with theoretical simulations, we reveal nonreciprocal directional dichroism (NDD) at the resonant mode, that is, oppositely propagating microwaves exhibit different absorption. The microscopic mechanism of the present NDD is not associated with the conventional Faraday effect but with the skyrmion magnetoelectric resonance instead, suggesting a conceptually new microwave functionality. © 2013 Macmillan Publishers Limited.

• Spin-Wave Modes and Their Intense Excitation Effects in Skyrmion Crystals

Masahito Mochizuki

PHYSICAL REVIEW LETTERS   108 ( 1 ) 17601  2012.01  [Refereed]

View Summary

We theoretically study spin-wave modes and their intense excitations activated by microwave magnetic fields in the Skyrmion-crystal phase of insulating magnets by numerically analyzing a two-dimensional spin model using the Landau-Lifshitz-Gilbert equation. Two peaks of spin-wave resonances with frequencies of similar to 1 GHz are found for in-plane ac magnetic field where distribution of the out-of-plane spin components circulates around each Skyrmion core. Directions of the circulations are opposite between these two modes, and hence the spectra exhibit a salient dependence on the circular polarization of irradiating microwave. A breathing-type mode is also found for an out-of-plane ac magnetic field. By intensively exciting these collective modes, melting of the Skyrmion crystal accompanied by a redshift of the resonant frequency is achieved within nanoseconds.

• Origin of the Large Polarization in Multiferroic YMnO3 Thin Films Revealed by Soft- and Hard-X-Ray Diffraction

H. Wadati, J. Okamoto, M. Garganourakis, V. Scagnoli, U. Staub, Y. Yamasaki, H. Nakao, Y. Murakami, M. Mochizuki, M. Nakamura, M. Kawasaki, Y. Tokura

PHYSICAL REVIEW LETTERS   108 ( 4 ) 47203  2012.01  [Refereed]

View Summary

We investigated the magnetic structure of an orthorhombic YMnO3 thin film by resonant soft x-ray and hard x-ray diffraction. We observed a temperature-dependent incommensurate magnetic reflection below 45 K and a commensurate lattice-distortion reflection below 35 K. These results demonstrate that the ground state is composed of coexisting E-type and cycloidal states. Their different ordering temperatures clarify the origin of the large polarization to be caused by the E-type antiferromagnetic states in the orthorhombic YMnO3 thin film.

• Hall Effect of Spin-Chirality Origin in a Triangular-Lattice Helimagnet Fe1.3Sb

Y. Shiomi, M. Mochizuki, Y. Kaneko, Y. Tokura

PHYSICAL REVIEW LETTERS   108 ( 5 ) 56601  2012.01  [Refereed]

View Summary

We report on a topological Hall effect possibly induced by scalar spin chirality in a quasi-two-dimensional helimagnet Fe1+delta Sb. In the low-temperature region where the spins on interstitial-Fe (concentration delta similar to 0.3) intervening the 120 degrees spin-ordered triangular planes tend to freeze, a nontrivial component of Hall resistivity with opposite sign of the conventional anomalous Hall term is observed under magnetic field applied perpendicular to the triangular-lattice plane. The observed unconventional Hall effect is ascribed to the scalar spin chirality arising from the heptamer spin clusters around the interstitial-Fe sites, which can be induced by the spin modulation by the Dzyaloshinsky-Moriya interaction.

• Theory of spin-phonon coupling in multiferroic manganese perovskites RMnO3

Masahito Mochizuki, Nobuo Furukawa, Naoto Nagaosa

PHYSICAL REVIEW B   84 ( 14 ) 144409  2011.10  [Refereed]

View Summary

Magnetoelectric phase diagrams of the rare-earth (R) Mn perovskites RMnO3 are theoretically studied by focusing on crucial roles of the symmetric magnetostriction or the Peierls-type spin-phonon coupling through extending our previous work [M. Mochizuki et al., Phys. Rev. Lett. 105, 037205 (2010)]. We first construct a microscopic classical Heisenberg model for RMnO3 including the frustrated spin exchanges, single-ion anisotropy, and Dzyaloshinskii-Moriya interaction. We also incorporate the lattice degree of freedom coupled to the Mn spins via the Peierls-type magnetostriction. By analyzing this model using the replica-exchange Monte Carlo technique, we reproduce the entire phase diagram of RMnO3 in the plane of temperature and magnitude of the orthorhombic lattice distortion. Surprisingly it is found that in the ab-plane spiral spin phase, the (S . S)-type magnetostriction plays an important role for the ferroelectric order with polarization P parallel to a whose contribution is comparable to or larger than the contribution from the (S x S)-type magnetostriction, whereas in the bc-plane spiral phase, the ferroelectric order with P parallel to c is purely of (S x S) origin. This explains much larger P in the ab-plane spiral phase than the bc-plane spiral phase as observed experimentally and gives a clue how to enhance the magnetoelectric coupling in the spin-spiral-based multiferroics. We also predict a noncollinear deformation of the E-type spin structure resulting in the finite (S x S) contribution to the ferroelectric order with P parallel to a, and a wide coexisting regime of the commensurate E and incommensurate spiral states, which resolve several experimental puzzles.

• Magnetic Field Induced Dehybridization of the Electromagnons in Multiferroic TbMnO3

P. Rovillain, M. Cazayous, Y. Gallais, M-A. Measson, A. Sacuto, H. Sakata, M. Mochizuki

PHYSICAL REVIEW LETTERS   107 ( 1 ) 27202  2011.07  [Refereed]

View Summary

We have studied the impact of the magnetic field on the electromagnon excitations in TbMnO3 crystal. Applying a magnetic field along the c axis, we show that the electromagnons transform into pure antiferromagnetic modes, losing their polar character. Entering in the paraelectric phase, we are able to track the spectral weight transfer from the electromagnons to the magnon excitations and we discuss the magnetic excitations underlying the electromagnons. We also point out the phonons involved in the phase transition process. This reveals that the Mn-O distance plays a key role in understanding the ferroelectricity and the polar character of the electromagnons. Magnetic field measurements along the b axis allow us to detect a new electromagnon resonance in agreement with a Heisenberg model.

• Theory of Magnetic Switching of Ferroelectricity in Spiral Magnets

Masahito Mochizuki, Nobuo Furukawa

PHYSICAL REVIEW LETTERS   105 ( 18 ) 187601  2010.10  [Refereed]

View Summary

We propose a microscopic theory for magnetic switching of electric polarization (P) in the spin-spiral multiferroics by taking TbMnO(3) and DyMnO(3) as examples. We reproduce their phase diagrams under a magnetic field H(ex) by Monte Carlo simulation of an accurate spin model and reveal that competition among the Dzyaloshinskii-Moriya interaction, spin anisotropy, and spin exchange is controlled by the applied H(ex), resulting in magnetic transitions accompanied by reorientation or vanishing of P. We also discuss the relevance of the proposed mechanisms to many other multiferroics such as LiCu(2)O(2), MnWO(4), and Ni(3)V(2)O(4).

• Theoretically Predicted Picosecond Optical Switching of Spin Chirality in Multiferroics

Masahito Mochizuki, Naoto Nagaosa

PHYSICAL REVIEW LETTERS   105 ( 14 ) 147202  2010.09  [Refereed]

View Summary

We show theoretically with an accurate spin Hamiltonian describing the multiferroic Mn perovskites that the application of the picosecond optical pulse with a terahertz frequency can switch the spin chirality through intensely exciting the electromagnons. There are four states with different spin chiralities, i.e., clockwise and counterclockwise ab/bc-plane spin spirals, and by tuning the strength, shape and length of the pulse, the switching among these states can be controlled at will. Dynamical pattern formation during the switching is also discussed.

• Spin Model of Magnetostrictions in Multiferroic Mn Perovskites

Masahito Mochizuki, Nobuo Furukawa, Naoto Nagaosa

PHYSICAL REVIEW LETTERS   105 ( 3 ) 37205  2010.07  [Refereed]

View Summary

We theoretically study origins of the ferroelectricity in the multiferroic phases of the rare-earth (R) Mn perovskites, RMnO(3), by constructing a realistic spin model including the spin-phonon coupling, which reproduces the entire experimental phase diagram in the plane of temperature and Mn-O-Mn bond angle for the first time. Surprisingly we reveal a significant contribution of the symmetric (S . S)-type magneto-striction to the ferroelectricity even in a spin-spiral-based multiferroic phase, which can be larger than the usually expected antisymmetric (S X S)-type contribution. This explains well the nontrivial behavior of the electric polarization. We also predict the noncollinear deformation of the E-type spin structure and a wide coexisting regime of the E and spiral states, which resolve several experimental puzzles.

• Theory of Electromagnons in the Multiferroic Mn Perovskites: The Vital Role of Higher Harmonic Components of the Spiral Spin Order

Masahito Mochizuki, Nobuo Furukawa, Naoto Nagaosa

PHYSICAL REVIEW LETTERS   104 ( 17 ) 177206  2010.04  [Refereed]

View Summary

We study theoretically the electromagnon and its optical spectrum ( OS) of the terahertz-frequency regime in the magnetic-spiral-induced multiferroic phases of the rare-earth-metal (R) Mn perovskites, RMnO(3), taking into account the spin-angle modulation or the higher harmonics of the spiral spin configuration, which has been missed so far. A realistic spin Hamiltonian, which gives phase diagrams in agreement with experiments, resolves a puzzle, i.e., the double-peak structure of the OS with a larger low-energy peak originating from magnon modes hybridized with the zone-edge state. We also predict the magnon branches associated with the electromagnon, which can be tested by neutron-scattering experiment.

• Roles of Bond Alternation in Magnetic Phase Diagram of RMnO3

Nobuo Furukawa, Masahito Mochizuki

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   79 ( 3 ) 33708  2010.03  [Refereed]

View Summary

In order to investigate the nature of the antiferromagnetic structures in perovskite RMnO3, we study a Heisenberg J(1)-J(2) model with bond alternation using analytical and numerical approaches. The magnetic phase diagram which includes incommensurate spiral states and commensurate collinear states is reproduced. We discuss that the magnetic structure with spin up arrow up arrow down arrow down arrow configuration (E-type structure) and the ferroelectricity emerge cooperatively to stabilize this phase. Magnetoelastic couplings are crucial to understand the magnetic and electric phase diagram of RMnO3.

• Microscopic model and phase diagrams of the multiferroic perovskite manganites

Masahito Mochizuki, Nobuo Furukawa

PHYSICAL REVIEW B   80 ( 13 ) 134416  2009.10  [Refereed]

View Summary

Orthorhombically distorted perovskite manganites, RMnO3 with R being a trivalent rare-earth ion, exhibit a variety of magnetic and electric phases including multiferroic (i.e., concurrently magnetic and ferroelectric) phases and fascinating magnetoelectric phenomena. We theoretically study the phase diagram of RMnO3 by constructing a microscopic spin model, which includes not only the superexchange interaction but also the single-ion anisotropy (SIA) and the Dzyaloshinsky-Moriya interaction (DMI). Analysis of this model using the Monte Carlo method reproduces the experimental phase diagrams as functions of the R-ion radius, which contain two different multiferroic states, i.e., the ab-plane spin cycloid with ferroelectric polarization P parallel to a and the bc-plane spin cycloid with P parallel to c. The orthorhombic lattice distortion or the second-neighbor spin exchanges enhanced by this distortion exquisitely controls the keen competition between these two phases through tuning the SIA and DMI energies. This leads to a lattice-distortion-induced reorientation of P from a to c in agreement with the experiments. We also discuss spin structures in the A-type antiferromagnetic state, those in the cycloidal spin states, origin and nature of the sinusoidal collinear spin state, and many other issues.

• Novel Multiferroic State of Eu1-xYxMnO3 in High Magnetic Fields

M. Tokunaga, Y. Yamasaki, Y. Onose, M. Mochizuki, N. Furukawa, Y. Tokura

PHYSICAL REVIEW LETTERS   103 ( 18 ) 187202  2009.10  [Refereed]

View Summary

Magnetic and dielectric properties of Eu1-xYxMnO3 (x = 0 and 0.4) are studied in pulsed magnetic fields up to 55 T. For x = 0, application of magnetic fields higher than 20 T along the b axis causes magnetic transitions accompanied by generation of electric polarization (P) along the a axis. Similar first-order transitions are also observed in crystals of x = 0: 4, in which the ground state at zero magnetic field is already a ferroelectric P parallel to a phase of different origin. Realistic model calculation indicates the presence of a novel multiferroic state induced by the spin exchange striction mechanism in high magnetic fields as an essential nature of the frustrated Mn spin system in this class of manganites.

• Mechanism of Lattice-Distortion-Induced Electric-Polarization Flop in the Multiferroic Perovskite Manganites

Masahito Mochizuki, Nobuo Furukawa

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   78 ( 5 ) 53704  2009.05  [Refereed]

View Summary

We theoretically study a competition between two types of spin cycloids (ab-plane and bc-plane ones) in the multiferroic perovskite manganites, which is an origin of intriguing tnagnetoelectric phenomena in these compounds. Analysis of a microscopic model using the Monte-Carlo method reveals that their competition originates from a conflict between the single-ion anisotropy and the Dzyaloshinsky-Moriya interaction. It is demonstrated that the conflict can be controlled by tuning the second-neighbor spin exchanges through the GdFeO3-type distortion, which leads to a cycloidal-plane flop from ab to be observed in the solid-solution systems like Eu1-xYxMnO3 and Gd1-xTbxMnO3 with increasing x.

• Dynamics of Multiferroic Domain Wall in Spin-Cycloidal Ferroelectric DyMnO3

F. Kagawa, M. Mochizuki, Y. Onose, H. Murakawa, Y. Kaneko, N. Furukawa, Y. Tokura

PHYSICAL REVIEW LETTERS   102 ( 5 ) 57604  2009.02  [Refereed]

View Summary

We report the dielectric dispersion of the giant magnetocapacitance (GMC) in multiferroic DyMnO3 over a wide frequency range. The GMC is found to be attributable not to the softened electromagnon but to the electric-field-driven motion of multiferroic domain wall (DW). In contrast to conventional ferroelectric DWs, the present multiferroic DW motion holds an extremely high relaxation rate of similar to 10(7) s(-1) even at low temperatures. This mobile nature as well as the model simulation suggests that the multiferroic DW is not atomically thin as in ferroelectrics but thick, reflecting its magnetic origin.

• Evidence for an Electric-Dipole Active Continuum Band of Spin Excitations in Multiferroic TbMnO(3)

Y. Takahashi, N. Kida, Y. Yamasaki, J. Fujioka, T. Arima, R. Shimano, S. Miyahara, M. Mochizuki, N. Furukawa, Y. Tokura

PHYSICAL REVIEW LETTERS   101 ( 18 ) 187201  2008.10  [Refereed]

View Summary

The wide range optical spectra on a multiferroic prototype TbMnO(3) have been investigated to clarify the origin of spin excitations observed in the far-infrared region. We elucidate the full band structure, whose high energy edge (133 cm(-1)) exactly corresponds to twice of the highest-lying magnon energy. Thus the origin of this absorption band is clearly assigned to two-magnon excitation driven by the electric field of light. There is an overlap between the two-magnon and phonon energy ranges, where the strong coupling between them is manifested by the frequency shift and transfer of oscillator strength of the phonon mode.

• Precise control of band filling in NaxCoO2

Daisuke Yoshizumi, Yuji Muraoka, Yoshihiko Okamoto, Yoko Kiuchi, Jun-Ichi Yamaura, Masahito Mochizuki, Masao Ogata, Zenji Hiroi

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   76 ( 6 ) 63705  2007.06  [Refereed]

View Summary

Electronic properties of the sodium cobaltate NaxCoO2 are systematically studied through a precise control of band filling. Resistivity, magnetic susceptibility and specific heat measurements are carried out on a series of high-quality polycrystalline samples prepared at 200 degrees C with Na content in a wide range of 0.35 &lt;= x &lt;= 0.70. It is found that dramatic changes in electronic properties take place at a critical Na concentration x* that lies between 0.58 and 0.59, which separates a Pauli paramagnetic and a Curie-Weiss metals. It is suggested that at x* the Fermi level touches the bottom of the a(1g) band at the Gamma point, leading to a crucial change in the density of states across x* and the emergence of a small electron pocket around the Gamma point for x &gt; x*.

• Specific heat and superfluid density for possible two different superconducting states in NaxCoO2 center dot gamma H2O

Masahito Mochizuki, H. Q. Yuan, Masao Ogata

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   76 ( 2 ) 23702  2007.02  [Refereed]

View Summary

Several thermodynamic measurements for the cobaltate superconductor, NaxCoO2 center dot gamma H2O, have so far provided results inconsistent with each other. In order to solve the discrepancies, we microscopically calculate the temperature dependences of specific heat and superfluid density for this superconductor. We show that two distinct specific-heat data from Oeschler et al. and Jin et al. are reproduced, respectively, for the extended s-wave state and the p-wave state. Two different superfluid-density data are also reproduced for each case. These support our recent proposal of possible two different pairing states,in this material. In addition, we discuss the experimentally proposed large residual Sommerfeld coefficient and extremely huge effective carrier mass.

• CoO2-layer-thickness dependence of magnetic properties and possible two different superconducting states in NaxCoO2 center dot yH(2)O

Masahito Mochizuki, Masao Ogata

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   76 ( 1 ) 13704  2007.01  [Refereed]

View Summary

In order to understand the experimentally proposed phase diagrams of NaxCoO2 center dot yH(2)O, we theoretically study the CoO2-layer-thickness dependence of its magnetic and superconducting (SC) properties by analyzing a multiorbital Hubbard model using the random phase approximation. When the Co valence s is +3.4, we show that the magnetic fluctuation exhibits a strong layer-thickness dependence, where it is enhanced at finite (zero) momentum in the thicker (thinner) layer system. A magnetic order phase appears sandwiched by two SC phases, consistent with the experiments. These two SC phases have different pairing states, where one is the singlet extended s-wave state and the other is the triplet p-wave state. On the other hand, only a triplet p-wave SC phase with a dome-shaped behavior of T-c is predicted when s = +3.5, which is also consistent with the experiments. Controversial experimental results on the magnetic properties are also discussed.

• Deformation of electronic structures due to CoO6 distortion and phase diagrams of NaxCoO2 center dot yH(2)O

Masahito Mochizuki, Masao Ogatai

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   75 ( 11 ) 113703  2006.11  [Refereed]

View Summary

Motivated by recently reported experimental phase diagrams, we study the effects Of CoO6 distortion on the electronic structure in NaxCoO2.yH(2)O. We construct the multiband tight-binding model by employing the LDA result. Analyzing this model, we show the deformation of band dispersions and Fermi-surface topology as functions Of CoO2-layer thickness. Considering these results together with previous theoretical ones, we propose a possible schematic phase diagram with three successive phases: the extended s-wave superconductivity (SC), the magnetic order, and the spin-triplet SC phases when the Co valence number s is +3.4. A phase diagram with only one phase of spin-triplet SC is also proposed for the s = +3.5 case.

• Magnetic fluctuation and pairing instability controlled by the CoO6 distortion in the sodium cobalt oxide

M Mochizuki, Y Yanase, M Ogata

PHYSICA B-CONDENSED MATTER   378-80   875 - 877  2006.05  [Refereed]

View Summary

We study how the CoO6 distortion along the c-axis affects the magnetic properties and superconductivity in the novel Co-oxide superconductor NaxCoO2.yH(2)O by analyzing the multiorbital Hubbard model using the fluctuation-exchange approximation. It is shown that through generating the trigonal crystal field, the distortion pushes the Co e'(g) bands up and consequently gives rise to the hole-pocket Fermi surfaces, which have been predicted in the band calculations. As the distortion increases, the hole pockets are enlarged and the ferromagnetic fluctuation as well as the pairing instability increases, which is in good aggreement with recent NQR results. (c) 2006 Elsevier B.V. All rights reserved.

• Role of spin-orbit interaction on the superconductivity in NaxCoO2 center dot yH(2)O: Phase diagram under the magnetic field

Y Yanase, M Mochizuki, M Ogata

PHYSICA B-CONDENSED MATTER   378-80   873 - 874  2006.05  [Refereed]

View Summary

The d-vector in possible spin triplet superconductor Na2CoO2 (.) gamma H2O is microscopically analyzed on the basis of the multi-orbital Hubbard model including the atomic spin-orbit coupling. As a result of the perturbation theory, it is shown that the d-vector is parallel to the plane when the pairing symmetry is p-wave, while both d parallel to xy and d parallel to z are possible in the f-wave state. We point out that the multiple phase transitions are expected tinder the in-plane magnetic field when the pairing symmetry is p-wave. (c) 2006 Elsevier B.V. All rights reserved.

• Role of spin-orbit coupling on the spin triplet pairing in Na(x)oO(2)center dot yH(2)O II: Multiple phase diagram under the magnetic field

Y Yanase, M Mochizuki, M Ogata

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   74 ( 12 ) 3351 - 3364  2005.12  [Refereed]

View Summary

The possibility of multiple phase diagram in the novel superconductor Na(x)CoO(2)(.)yH(2)O is analyzed on the basis of the multi-orbital Hubbard model including the atomic spin-orbit coupling. We have shown that the spin triplet pairing state is stable in this model. The p-wave (f-wave) state is stabilized when the Hund's rule coupling is large (small). In the precedent paper, we have determined the direction of d-vector at T = T-c and H = 0 within the linearized Dyson-Gorkov equation. In this paper, the pairing state below T, and under the magnetic field is determined within the weak coupling approximation including the paramagnetic effect. We find that the p + f coexistent state is stabilized at low temperatures in a part of parameter range. We point out that the phase diagram in the H-T plane is quite different between the p-wave, f-wave and p + f-wave superconductivities. The characteristics of each phase are clarified by showing the magnetic susceptibility and specific heat. We discuss the comparison with experimental results and suggest some future experiments to detect the multiple phase transition.

• Role of spin-orbit coupling on the spin triplet pairing in Na xCoO2·yH2O: I. d-vector under zero magnetic field

Youichi Yanase, Masahito Mochizuki, Masao Ogata

Journal of the Physical Society of Japan   74 ( 9 ) 2568 - 2578  2005.09  [Refereed]

View Summary

The d-vector in possibile spin triplet superconductor Na xCoO2·yH2O is microscopically investigated on the basis of the multi-orbital Hubbard model including the atomic spin-orbit coupling. As a result of the perturbation theory, we obtain the stable spin triplet superconductivity where the p-wave and f-wave states can be stabilized. If we neglect the spin-orbit coupling, superconducting state has 6-fold (3-fold) degeneracy in the p-wave (f-wave) state. This degeneracy is lifted by the spin-orbit coupling. We determine the d-vector within the linearlized Dyson-Gorkov equation. It is shown that the d-vector is always along the plane when the pairing symmetry is p-wave, while it depends on the parameters in case of the f-wave state. The lifting of degeneracy is significant in the p-wave state while it is very small in the f-wave state. This is because the first order term with respect to the spin-orbit coupling is effective in the former case, while it is ineffective in the latter case. The consistency of these results with NMR and μSR measurements are discussed. ©2005 The Physical Society of Japan.

• "Ferromagnetic and triplet-pairing instabilities controlled by trigonal distortion of CoO6 octahedra in NaCoO2 center dot yH(2)O" (vol 74, pg 1670, 2005)

M Mochizuki, Y Yanase, M Ogata

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   74 ( 8 ) 2381 - 2381  2005.08  [Refereed]

• Ferromagnetic and triplet-pairing instabilities controlled by trigonal distortion of CoO6 octahedra in NaxCoO2 center dot yH(2)O

M Mochizuki, Y Yanase, M Ogata

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   74 ( 6 ) 1670 - 1673  2005.06  [Refereed]

View Summary

In the recently discovered Co-oxide superconductor NaxCoO2 center dot yH(2)O, the edge-shared CoO6 octahedra are trigonally contracted along the c-axis in the CoO2-plane. We study how this CoO6 distortion affects the magnetic properties and superconductivity in this compound by analyzing the multiorbital Hubbard model using the fluctuation-exchange approximation. It is shown that through generating the trigonal crystal field, the distortion pushes the Co e'(g) bands up and consequently gives rise to the hole-pocket Fermi surfaces, which have been predicted in the band calculations. As the distortion increases, the hole pockets are enlarged and the ferromagnetic fluctuation as well as the pairing instability increases, which is in good agreement with recent NQR results.

• Theoretical analysis on the multi-orbital model for the superconductivity in NaxCoO2

Y Yanase, M Mochizuki, M Ogata

PHYSICA B-CONDENSED MATTER   359   1351 - 1353  2005.04  [Refereed]

View Summary

We analyze the multi-orbital model which describes the electronic structure of recently discovered superconductor NaxCoO2 center dot yH(2)O. The instability to the unconventional superconductivity is investigated by means of the perturbation theory. It is shown that the spin triplet superconductivity is stabilized in the wide parameter region, basically owing to the ferromagnetic spin fluctuation. In a small part of the parameter region, the d-wave superconductivity is also stabilized. We point out that the orbital degree of freedom plays an essential role for the superconductivity. (c) 2005 Elsevier B.V. All rights reserved.

• Ferromagnetic fluctuation and superconductivity in Na0.35CoO2 center dot 1.3H(2)O: FLEX study of multi-orbital Hubbard model

M Mochizuki, Y Yanase, M Ogata

PHYSICA B-CONDENSED MATTER   359   594 - 596  2005.04  [Refereed]

View Summary

Spin and charge fluctuations and superconductivity in Na0.35CoO2 center dot 1.3H(2)O are studied based on a multi-orbital Hubbard model. By applying the fluctuation exchange (FLEX) approximation, we show that the Hund's-rule coupling between the Co t(2g) orbitals causes ferromagnetic spin fluctuation. Triplet pairing is favored by this ferromagnetic fluctuation on the hole-pocket band. We propose that, in Na0.35CoO2 center dot 1.3H(2)O, Co t(2g) orbitals and inter-orbital Hund's-rule coupling play important roles on the triplet pairing, and this compound can be a first example of the triplet superconductor mediated by inter-orbital interaction-induced ferromagnetic fluctuation. (c) 2005 Elsevier B.V. All rights reserved.

• Ferromagnetic fluctuation and possible triplet superconductivity in NaxCoO2 center dot yH(2)O: Fluctuation-exchange study of the multiorbital Hubbard model

M Mochizuki, Y Yanase, M Ogata

PHYSICAL REVIEW LETTERS   94 ( 14 ) 147005  2005.04  [Refereed]

View Summary

Spin and charge fluctuations and superconductivity in Na(x)CoO(2)center dot yH(2)O are studied based on a multiorbital Hubbard model. Tight-binding parameters are determined to reproduce the results of band calculations. By applying the fluctuation-exchange approximation, we show that the Hund's-rule coupling between the Co t(2g) orbitals causes ferromagnetic (FM) spin fluctuation. Triplet f(y(y)(2)-3x(2))-wave and p-wave pairings are favored by this FM fluctuation on the hole-pocket band. We propose that, in Na(x)CoO(2)center dot yH(2)O, the Co t(2g) orbitals and interorbital Hund's-rule coupling play important roles on the triplet pairing, and this compound can be a first example of the triplet superconductor in which the orbital degrees of freedom play substantial roles.

• Multi-orbital analysis on the superconductivity in NaxCoO2 center dot yH(2)O

Y Yanase, M Mochizuki, M Ogata

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   74 ( 1 ) 430 - 444  2005.01  [Refereed]

View Summary

We preform a multi-orbital analysis on the novel superconductivity in Na(x)CoO(2)(.)yH(2)O. We construct a three-orbital model which reproduces the band structure expected from the LDA calculation. The effective interaction leading to the pairing is estimated by means of the perturbation theory. It is shown that the spin triplet superconductivity is stabilized in the wide parameter region. This is basically owing to the ferromagnetic character of spin fluctuation. The p-wave and f-wave superconductivity are nearly degenerate. The former is realized when the Hund's rule coupling is large, and vice versa. In a small part of the parameter space, the d-wave superconductivity is also stabilized. We point out that the orbital degeneracy plays an essential role for these results. In particular, the momentum dependence of wave function of quasi-particles is quite important. The nearly degeneracy of p-wave and f-wave superconductivity is explained by analysing the microscopic character of each Fermi surface. We discuss the validity of some reduced models. While the single-orbital Hubbard model reproducing the Fermi surface is qualitatively inappropriate, we find an effective two-orbital model appropriate for studying the superconductivity. We investigate the vertex corrections higher than the third order on the basis of the two-orbital model. It is shown that the vertex correction induces the screening effect but does not affect qualitative results.

• Orbital physics in the perovskite Ti oxides

New Journal of Physics   6   1 - 42  2004.11  [Refereed]

View Summary

Titanate compounds have been recognized as key materials for understanding the coupling of magnetism and orbitals in strongly correlated electron systems. In the perovskite Ti oxide RTiO3 (where R represents the trivalent rare-earth ions), which is a typical Mott-Hubbard insulator, the Ti t 2g orbitals and spins in the 3d1 state couple each other through the strong electron correlations, resulting in a rich variety of orbital-spin phases. One way of controlling the coupling is to change the tiltings of the TiO6 octahedra (namely the GdFeO3-type distortion) by varying the R ions, through which the relative ratio of the electron bandwidth to the Coulomb interaction is controlled. With this control, these Mott insulators exhibit an antiferromagnetic-to-ferromagnetic (AFM-FM) phase transition, which has turned out to be a consequence of rich orbital physics in these materials. The origin and nature of orbital-spin structures of these Mott insulators have been intensively studied both experimentally and theoretically. When the Mott insulators are doped with carriers, the titanates show touchstone properties of the filling controlled Mott transition. In this paper, we first review the state of the art on the studies for understanding physics contained in the properties of the perovskite titanates. On the properties of the insulators, we focus on the following three topics: (1) the origin and nature of the ferromagnetism as well as the orbital ordering in the compounds with relatively small R ions such as GdTiO3 and YTiO 3, (2) the origin of the G-type antiferromagnetism and the orbital state in LaTiO3 and (3) the orbital-spin structures in other AFM(G) compounds with relatively large R ions (R = Ce, Pr, Nd and Sm). On the basis of these discussions, we discuss the whole phase diagram together with mechanisms of the magnetic phase transition. On the basis of the microscopic (understanding of the orbital-spin states, we show that the Ti t2g degeneracy is inherently lifted in the titanates, which allows the single-band descriptions of the ground-state and the low-energy electronic structures as a good starting point. Our analyses indicate that these compounds offer good touchstone materials described by the single-band Hubbard model on the cubic lattice. From this insight, we also re-analyse the hole-doped titanates R1-xA xTiO3 (where A represents the divalent alkaline-earth ions). Experimentally revealed filling- and bandwidth-dependent properties and the critical behaviour of the metal-insulator transitions are discussed in the light of theories based on the single-band Hubbard models.

• G-type antiferromagnetism and orbital ordering due to the crystal field from the rare-earth ions induced by the GdFeO_3-type distortion in RTiO_3 with R=La, Pr, Nd and Sm

J. Phys. Soc. Jpn.   73   1833 - 1850  2003.12  [Refereed]

View Summary

The origin of the antiferromagnetic order and puzzling properties of LaTiO_3
as well as the magnetic phase diagram of the perovskite titanates are studied
theoretically. We show that in LaTiO_3, the t_{2g} degeneracy is eventually
lifted by the La cations in the GdFeO_3-type structure, which generates a
crystal field with nearly trigonal symmetry. This allows the description of the
low-energy structure of LaTiO_3 by a single-band Hubbard model as a good
starting point. The lowest-orbital occupation in this crystal field stabilizes
the AFM(G) state, and well explains the spin-wave spectrum of LaTiO_3 obtained
by the neutron scattering experiment. The orbital-spin structures for RTiO_3
with R=Pr, Nd and Sm are also accounted for by the same mechanism. We point out
that through generating the R crystal field, the GdFeO_3-type distortion has a
universal relevance in determining the orbital-spin structure of the perovskite
compounds in competition with the Jahn-Teller mechanism, which has been
overlooked in the literature. Since the GdFeO_3-type distortion is a universal
phenomenon as is seen in a large number of perovskite compounds, this mechanism
may also play important roles in other compounds of this type.

• Orbital-Spin Structure and Lattice Coupling in RTiO<inf>3</inf> where R = La, Pr, Nd, and Sm

Physical Review Letters   91 ( 16 ) 1672031 - 1672034  2003.10  [Refereed]

View Summary

The origin of the G-type antiferromagnetism of RTiO3 with R = La was studied. It was clarified that the crystal field from La lifts the t 2g degeneracy at Ti 3d orbitals. It was suggested that the GdFeO 3-type distortion has a universal mechanism of controlling orbital-spin structure.

• Orbital-spin structure and lattice coupling in [formula presented] where [formula presented], [formula presented], [formula presented], and [formula presented]

Physical Review Letters   91 ( 16 )  2003  [Refereed]

View Summary

The origin of the G-type antiferromagnetism [AFM(G)] and puzzling properties of [Formula presented] with [Formula presented] are studied. We clarify that the crystal field from La caused by the [Formula presented]-type distortion lifts the [Formula presented] degeneracy at Ti [Formula presented] orbitals. The lift stabilizes the AFM(G) with spin-exchange constant in agreement with neutron-scattering results. The orbital-spin structures for [Formula presented], Nd, and Sm are also consistent with experiments. We propose that the [Formula presented]-type distortion has a universal mechanism of controlling orbital-spin structure competing with the Jahn-Teller (JT) mechanism. © 2003 The American Physical Society.

• Spin and orbital states and their phase transitions of the perovskite-type Ti oxides: Weak coupling approach

M Mochizuki

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   71 ( 8 ) 2039 - 2047  2002.08  [Refereed]

View Summary

The magnetic phase diagram of the perovskite-type Ti oxides as a function of the GdFeO3-type distortion is examined by using the Hartree-Fock analysis of a multiband d-p Hamiltonian from a viewpoint of competitions of the spin-orbit interaction, the Jahn-Teller (JT) level-splitting and spin-orbital superexchange interactions. Near the anti ferro magnetic (AFM)-to-ferromagnetic (FM) phase boundary, A-type AFM [AFM(A)] and FM states accompanied by a certain type of orbital ordering are lowered in energy at large JT distortion, which is in agreement with the previous strong coupling study. With increasing the GdFeO3-type distortion, their phase transition occurs. Through this magnetic phase transition, the orbital state hardly changes, which induces nearly continuous change in the spin coupling along the c-axis from negative to positive. The strong suppression of T-N and T-C, which is experimentally observed is attributed to the resulting strong two-dimensionality in the spin coupling near the phase boundary. On the other hand, at small GdFeO3-type without JT distortions, which correspond to LaTiO3, the most stable solution is not G-type AFM [AFM(G)] but FM. Although the spin-orbit interaction has been considered to be relevant at the small or no JT distortion of LaTiO3 in the literature, our analysis indicates that the spin-orbit interaction is irrelevant to the AFM(G) state in LaTiO3 and superexchange-type interaction dominates. On the basis of further investigations on the nature of this FM state and other solutions, this discrepancy is discussed in detail.

• Origin of G-type Antiferromagnetism and Orbital-Spin Structures in ${\rm LaTiO}_3$

J. Phys. Soc. Jpn.   70   2872 - 2875  2001.05  [Refereed]

View Summary

The possibility of the $D_{3d}$ distortion of ${\rm TiO}_6$ octahedra is
examined theoretically in order to understand the origin of the G-type
antiferromagnetism (AFM(G)) and experimentally observed puzzling properties of
${\rm LaTiO}_3$. By utilizing an effective spin and pseudospin Hamiltonian with
the strong Coulomb repulsion, it is shown that AFM(G) state is stabilized
through the lift of the $t_{2g}$-orbital degeneracy accompanied by a tiny
$D_{3d}$-distortion . The estimated spin-exchange interaction is in agreement
with that obtained by the neutron scattering. Moreover, the level-splitting
energy due to the distortion can be considerably larger than the spin-orbit
interaction even when the distortion becomes smaller than the detectable limit
under the available experimental resolution. This suggests that the orbital
momentum is fully quenched and the relativistic spin-orbit interaction is not
effective in this system, in agreement with recent neutron-scattering
experiment.

• Magnetic and Orbital States and Their Phase Transition of the Perovskite-Type Ti Oxides: Strong Coupling Approach

J. Phys. Soc. Jpn.   70   1777 - 1789  2001.02  [Refereed]

View Summary

The properties and mechanism of the magnetic phase transition of the
perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle
distortion, are studied theoretically by using the effective spin and
pseudospin Hamiltonian with strong Coulomb repulsion. It is shown that the
A-type antiferromagnetic (AFM(A)) to ferromagnetic (FM) phase transition occurs
as the Ti-O-Ti bond angle is decreased. Through this phase transition, the
orbital state changes only little whereas the spin-exchange coupling along the
c-axis is expected to change from positive to negative nearly continuously and
approaches zero at the phase boundary. The resultant strong two-dimensionality
in the spin coupling causes rapid suppression of the critical temperature, as
observed experimentally. It may induce large quantum fluctuations in this
region.

• Magnetic Phase Transition of the Perovskite-type Ti Oxides

J. Phys. Soc. Jpn.   69   1982 - 1985  2000.04  [Refereed]

View Summary

Properties and mechanism of the magnetic phase transition of the
perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle
distortion, are studied theoretically by using the effective spin and
pseudo-spin Hamiltonian with strong Coulomb repulsion. It is shown that the
A-type antiferromagnetic(AFM(A)) to ferromagnetic(FM) phase transition occurs
as the Ti-O-Ti bond angle is decreased. Through this phase transition, the
orbital state is hardly changed so that the spin-exchange coupling along the
c-axis changes nearly continuously from positive to negative and takes
approximately zero at the phase boundary. The resultant strong
two-dimensionality in the spin coupling causes a rapid suppression of the
critical temperature as is observed experimentally.

### Books and Other Publications

• Multiferroics: Fundamentals and Applications

M. Mochizuki, edited by D. Meier, A. Cano, M. Trassin( Part： Joint author, Chapter contribution to a book, 15pages)

De Gruyter  2021.06 ISBN: 3110580977

• Chirality, magnetism and magnetoelectricity : separate phenomena and Joint effects in metamaterial structures

M. Mochizuki, edited by E. Kamenetskii( Part： Joint author, Chapter contribution to a book, 24pages)

Springer  2021 ISBN: 9783030628437

• Skyrmions in Magnetic Materials

S. Seki, M. Mochizuki( Part： Joint author)

Springer  2016 ISBN: 9783319246512

• Topological Structures in Ferroic Materials: Domain Walls, Skyrmions and Vortices

M. Mochizuki, edited by J. Seidel( Part： Joint author, Chapter contribution to a book, 27 pages)

Springer  2016 ISBN: 9783319253015

### Misc

• 温めると縮む磁石の謎がついに解けた！

望月維人

日本物理学会誌   75 ( 3 ) 130 - 138  2020.03

• 局所電場印加による磁気スキルミオン書き込み技術の理論設計

望月維人

日本磁気学会報「まぐね」   15 ( 1 ) 192 - 198  2020.02

• 冷やすと膨らむ不思議な磁石 ―半世紀の謎がついに解けた―

望月 維人

現代化学   ( 582 ) 26 - 29  2019.09  [Invited]

• 磁気スキルミオンのマイクロ波誘起現象と素子機能

望月 維人

応用物理   88 ( 7 ) 460 - 464  2019.07  [Invited]

• 磁気スキルミオンのメモリ素子応用を目指した研究の潮流

望月 維人

IDEMA Japan News    2016.11  [Invited]

•   40 ( 16 ) 39 - 44  2016.06

• 磁気スキルミオンのデバイス機能とダイナミクス

望月 維人

電子通信学会技報   116 ( 83 ) 39 - 44  2016.06  [Invited]

• スパイラル磁性強誘電体TbMnO3におけるマルチフェロイックドメインの外場応答

松原正和, 望月維人, 木村剛

固体物理   51 ( 3 ) 25(173) - 37(185)  2016.03  [Invited]

• マルチフェロイックTbMnO3における電気磁気ドメイン制御

松原正和, 望月維人, 木村剛

Japanese Scientists in Science 2015     41 - 41  2016  [Refereed]  [Invited]

Article, review, commentary, editorial, etc. (scientific journal)

• マルチフェロイック物質の磁気強誘電ドメインを直接観察

松原正和, 望月維人, 木村剛

Scienceに載った日本人 2016年号     41  2016  [Invited]

• スキルミオンを作る・消す・動かす

望月維人

日本磁気学会報「まぐね」   10 ( 4 ) 192 - 198  2015.08  [Invited]

• 磁気スキルミオンのダイナミクスとデバイス機能

望月維人

(公社)日本磁気学会第205回研究会資料     39 - 44  2015  [Invited]

• 27aBE-5 Theoretical Study on the Magnetoelectric Phenomena in Multiferroic Mn Perovskites

Mochizuki Masahito

Meeting abstracts of the Physical Society of Japan   69 ( 1 ) 542 - 542  2014.03

• Mochizuki Masahito, Seki Shinichiro

69 ( 3 ) 132 - 139  2014.03  [Invited]

View Summary

Magnetic skyrmions, vortex-like swirling spin textures characterized by a quantized topological invariant in chiral-lattice magnets, are currently attracting intensive research interest. In particular, their dynamics under external fields is an issue of vital importance because its understanding is crucial both for fundamental science and for technical applications. It has turned out that the skyrmions in an insulating magnet Cu_2OSeO_3 exhibit a multiferroic nature with magnetism-induced ferroelectricity. In this article, we discuss that this multiferroic skyrmion phase shows unique coupled oscillation modes of magnetizations and polarizations, which are both magnetically and electrically active, and the intense interference between their electric and magnetic activation processes leads to an unprecedentedly large diode effect on microwaves. The microwave absorption by this skyrmion phase changes up to 〜20% when the incident direction is reversed. Our results demonstrate that the multiferroic skyrmions can be a promising building block for microwave devices.

• 磁気スキルミオンが示す特異な熱励起・電流誘起ダイナミクス

望月維人, 永長直人

固体物理   49 ( 3 ) 25(125) - 35(135)  2014.03  [Invited]

• Electric-current-induced skyrmion dynamics in constricted geometries

Iwasaki Junichi, Mochizuki Masahito, Nagaosa Naoto

Meeting abstracts of the Physical Society of Japan   68 ( 2 ) 417 - 417  2013.08

• 20aCC-7 Theory of current-driven motion of Skyrmions and spirals in helical magnet

Iwasaki Junichi, Mochizuki Masahito, Nagaosa Naoto

Meeting abstracts of the Physical Society of Japan   67 ( 2 ) 426 - 426  2012.08

• マルチフェロイックスにおけるエレクトロマグノン励起

望月維人, 古川信夫, 永長直人

日本物理学会誌   66 ( 4 ) 280 - 284  2011.04  [Invited]

• マルチフェロイック希土類Mnペロフスカイトにおける電気磁気現象の理論

望月維人, 古川信夫, 永長直人

固体物理   46 ( 2 ) 1(55) - 19(73)  2011.02  [Invited]

• 24pWJ-7 Theory of Optical Switching of spin chirality in multiferroics

Mochizuki Masahito, Nagaosa Naoto

Meeting abstracts of the Physical Society of Japan   65 ( 2 ) 574 - 574  2010.08

• 26aWY-7 Theory of Multiferroics

Mochizuki Masahito

Meeting abstracts of the Physical Society of Japan   65 ( 2 ) 925 - 925  2010.08

• 21aGF-8 Theory of Electromagnon in the Multiferroic Mn Perovskites

Mochizuki Masahito, Furukawa Nobuo, Nagaosa Naoto

Meeting abstracts of the Physical Society of Japan   65 ( 1 ) 598 - 598  2010.03

• 27aTA-1 Theoretical Study on the T-H phase diagrams of multiferroic RMnO_3

Mochizuki Masahito, Furukawa Nobuo

Meeting abstracts of the Physical Society of Japan   64 ( 1 ) 518 - 518  2009.03

• 21aQA-7 Control of spin helicity and electric polarization vector by rotating magnetic field in cycloidal magnet Eu_<0.55>Y_<0.45>MnO_3

Murakawa H., Onose Y., Kagawa F., Ishiwata S., Mochizuki M., Kaneko Y., Tokura Y.

Meeting abstracts of the Physical Society of Japan   63 ( 2 ) 482 - 482  2008.08

• 21aQA-4 Mechanism of magnetic-field-induced polarization flop in multiferroic RMnO3

Mochizuki Masahito, Furukawa Nobuo

Meeting abstracts of the Physical Society of Japan   63 ( 2 ) 481 - 481  2008.08

• 23aYB-6 Frustration and Magnetic Phase Diagrams of Mn Perovskites

Mochizuki Masahito, Furukawa Nobuo

Meeting abstracts of the Physical Society of Japan   63 ( 1 ) 517 - 517  2008.02

• 25pZL-9 Electronic Structures of Co-oxide Superconductor : Effetcs of CoO_6 Distortion

Mochizuki Masahito, Ogata Masao

Meeting abstracts of the Physical Society of Japan   61 ( 2 ) 505 - 505  2006.08

• 25pZL-10 Phase Diagrams of Co-oxide Superconductor

Mochizuki Masahito, Ogata Masao

Meeting abstracts of the Physical Society of Japan   61 ( 2 ) 505 - 505  2006.08

• 25pZL-11 Specific Heat, Magnetic-Field Penetration Depth and Other Physical Quantities of Co-oxide Superconductor

Mochizuki Masahito, Ogata Masao

Meeting abstracts of the Physical Society of Japan   61 ( 2 ) 505 - 505  2006.08

• 21aXD-3 Superconductivity in Na_xCoO_2 : FLEX study of multi-orbital Hubbard model

Mochizuki Masahito, Yanase Youichi, Ogata Masao

Meeting abstracts of the Physical Society of Japan   60 ( 2 ) 466 - 466  2005.08

• 21aXD-1 Spin Triplet Superconductivity and Phase Diagram under Magnetic Field in Na_xCoO_2(H_2O)_y : Orbital Effect, Paramagnetic Effect and Spin-Orbit Interaction

Yanase Y., Udagawa M., Mochizuki M., Ogata M.

Meeting abstracts of the Physical Society of Japan   60 ( 2 ) 466 - 466  2005.08

• 27aWM-4 Spin Triplet Superconductivity and D-veclor in Na_xCoO_2(H_2O)_y : Microscopic Theory on the Spin-Orbit Interaction and Phase Diagram in Magnetic Field

Yanase Y., Mochizuki M., Ogata M.

Meeting abstracts of the Physical Society of Japan   60 ( 1 ) 607 - 607  2005.03

• 12aWD-1 Multi-orbital analysis on the Superconductivity in Na_xCoO_2・yH_2O

Yanase Y, Mochizuki M, Ogata M

Meeting abstracts of the Physical Society of Japan   59 ( 2 ) 426 - 426  2004.08

• 12aWD-2 Superconductivity in Na_xCoO_2 : FLEX study of multi-orbital Hubbard model

Mochizuki Masahito, Yanase Youichi, Ogata Masao

Meeting abstracts of the Physical Society of Japan   59 ( 2 ) 426 - 426  2004.08

• 29pXE-3 Orbital States in the Perovskite Titanates

Mochizuki Masahito

Meeting abstracts of the Physical Society of Japan   59 ( 1 ) 631 - 631  2004.03

• Orbital-Spin Structures in the Perovskite-Titanates : Effects of Crystal Fields Due to R Cations

Meeting abstracts of the Physical Society of Japan   58 ( 1 ) 540 - 540  2003.03

• ペロフスカイト型チタン酸化物における磁気-軌道状態 : 希土類イオンの作る結晶場の効果(2002年度基研研究会「軌道自由度を持つ強相関電子系の理論の進展」,研究会報告)

望月 維人, 今田 正俊

物性研究   79 ( 6 ) 995 - 996  2003

View Summary

この論文は国立情報学研究所の電子図書館事業により電子化されました。

• 27aYH-5 Spin and Orbital States in the Perovskite-Titanates

Meeting abstracts of the Physical Society of Japan   57 ( 1 ) 581 - 581  2002.03

• ぺロフスカイト型チタン酸化物における磁気-軌道状態と相転移

望月維人, 今田正俊

36 ( 11 ) 793 - 802  2001.11  [Invited]

Article, review, commentary, editorial, etc. (scientific journal)

• Magnetic and Orbital States in the Perovskite-Type Ti Oxide LaTiO_3

Meeting abstracts of the Physical Society of Japan   56 ( 1 ) 516 - 516  2001.03

• Magnetic phase transition of the perovskite-type Ti oxides.

Meeting abstracts of the Physical Society of Japan   55 ( 2 ) 472 - 472  2000.09

### Industrial Property Rights

• 電磁波の透過率制御方法、電磁波の透過率制御デバイス

岡村嘉大, 賀川史敬, 関真一郎, 久保田将司, 石渡晋太郎, 小野瀬佳文, 望月維人, 十倉好紀, 川崎雅司

Patent

• スキルミオン生成装置、スキルミオン生成方法、および磁気記憶装置

望月維人

Patent

• スキルミオンの生成、消去方法および磁気素子

永長直人, 望月維人, 岩崎惇一

Patent

### Awards

• JPSJ Outstanding Referee 2022

2022.03   The Physical Society of Japan

Winner： Masahito Mochizuki

• 早稲田大学リサーチアワード（国際研究発信力）

2019.02   早稲田大学

Winner： 望月 維人

• 科学技術分野の文部科学大臣表彰若手科学者賞

2015.04   文部科学省   マグノン励起による磁性制御の理論研究

Winner： 望月 維人

• 日本物理学会若手奨励賞(領域8)

2014.03   （一社）日本物理学会   マルチフェロイックMn ペロブスカイトの電気磁気現象に関する理論研究

Winner： 望月 維人

### Research Projects

• スキルミオニクス創成に向けた基盤技術と材料の開拓

Project Year :

2020.04
-
2024.03

View Summary

磁性体中に発現するナノサイズの磁気渦「磁気スキルミオン」は (1)ナノスケールの極小サイズと，(2)トポロジカルに保護された安定性，(3)室温を超える動作温度，（4） 高い省電力性，といった磁気デバイスへの応用に最適な性質をいくつも持っています。これらの性質を生かし、近い将来の高度情報化社会を担う「磁気メモリ」や「磁気センサ」、「脳型情報処理素子」の実現を目指し、これらの素子の基盤となる「高性能な磁気材料」と「生成・消去・検出・伝送といった要素技術」を「理論設計」と「実験実証」により開発します。これにより、スキルミオンを使った次世代エレクトロニクスである「スキルミオニクス」の創出を目指します

• Metal-oxide spin-orbit electronics

Project Year :

2019.04
-
2022.03

• 磁気スキルミオンを使った脳型コンピューティング素子の理論設計

三菱財団  自然科学研究助成

Project Year :

2021.04
-
2022.03

望月維人

• Theoretical study and materials search on gigantic negative thermal expansion phenomena in correlated electron systems

Project Year :

2019.06
-
2021.03

• Materials Design and Exploration of Functions for Strongly Correlated Materials - Challenges to Non-equilibrium and Non-Periodic Systems

Project Year :

2016.05
-
2021.03

• トポロジカル磁気テクスチャの非平衡ダイナミクスと量子輸送現象の理論研究

Project Year :

2017.04
-
2020.03

View Summary

トポロジカル磁気テクスチャである「磁気スキルミオン」が示すスピン波励起やマイクロ波応答、外場応答に関する非平衡ダイナミクスやデバイス機能の理論的研究および、実験研究者との連携研究で数多くの成果を上げた。まず、キャビティ共振器内に配置した絶縁性キラル磁性体に発現する磁気スキルミオンとマイクロ波のキャビティモードとの結合の強度や性質を理論計算により明らかにし、実験結果を再現・説明することに成功した(PRB誌に発表)。また、強磁性磁気細線中の強磁性磁壁を外部磁場の印加により駆動した時に、駆動された強磁性磁壁中にダイナミカルにスキルミオン様の渦状トポロジカル磁気構造が発現することや、それがトロジカルホール抵抗の測定で検出可能であることを理論的に予言した(APEX誌に発表)。また、強磁性多層膜において交流電場によりラシュバ型スピ軌道相互作用に由来する界面DM相互作用を誘起できることや、それによりスキルミオンのスピン波励起モード励起できることをを非平衡グリーン関数を用いた場の量子論に基づく理論計算により明らかにした（Sci.Rep.誌に発表）。また、スキルミオンが発現しているキラル磁性体の薄膜試料や磁性多層膜に面直方向から傾いた外部磁場を印加し、そこにマイクロ波を照射することで、スキルミオンのスピン波励起により直流起電力成分を発生させることができることを理論的に明らかにした（PRB誌に発表）。このスキルミオンを利用した「マイクロ波-直流電圧変換」は将来のスキルミオンを利用したスピントロニクス素子への電力供給に利用できる可能性を秘めている。また、スキルミオンを発現するキラル磁性体に一軸張力を加えることで、スキルミオン相が劇的に安定化すること（PRMat誌に発表）や、円偏光マイクロ波照射によるスキルミオン生成の技術について理論的に予言し（PRB誌に発表）、その基礎物理を解明した。令和元年度が最終年度であるため、記入しない。令和元年度が最終年度であるため、記入しない

• Theoretical study of dynamical magnetoelectric phenomena in strongly correlated models

Project Year :

2013.04
-
2016.03

View Summary

We have studied dynamical magnetic and electric responces for microscopic models of strongly correlated electron systems, in particular Mott insulators. We have investigated magnetoelectric interactions which appear in these models. We have also tried to predict and design novel dynamical magnetoelectric phenomena through these interactions.From the interference of the dynamical electric and magnetic response caused by incident electromagnetic waves, we investigated characteristic dynamic response , such as non-reciprocal direction dichroisms. From a microscopic model , we derived novel magnetoelectric interactions in non-collinear spin systems. We focused on the skyrmion phases as a novel spin structure which causes a dynamic electric magnetic phenomena, to examine their dynamical natures and to discuss the control of spin textures

• Theoretical study on intense-excitation effects and optical functions of electromagnons in multiferroic materials

Project Year :

2013.04
-
2016.03

View Summary

I have theoretically predicted that interference of two activation channels inherent to electromagnon excitations in insulating chiral magnet Cu2OSeO3 leads to gigantic microwave directional dichroism, and have confirmed this prediction by a collaboration with experimental groups. I have also theoretically demonstrated that skyrmions can be created by applying electric fields on a thin-film sample of Cu2OSeO3 via an electrode tip, which is achieved by electric-field-driven local magnetization flop through magnetoelectric coupling in this multiferroic system. In addition, I have discovered a new insulating skyrmionic material GaV4S8, and have uncovered that skyrmions in this material are Neel-type one, which has not been discovered in any real material so far. We also theoretically revealed a microscopic mechanism of magnetic-field induced 90-degree flop of ferroelectric polarization in spin-spiral multiferroics TbMnO3, and have shown that this phenomenon occurs in a deterministic way

• 磁性体におけるトポロジカル現象の理論設計学

Project Year :

2012.04
-
2013.03

View Summary

固体中電子の「トポロジカル現象」を、磁性体を中心とする実際の物質で実現するための理論設計学を構築することを目指した。3つのテーマそれぞれに対して得られた成果を以下に示す。1.1電子状態のトポロジー設計:パイロクロア格子を持つ反強磁性体を念頭に、バルクでは通常の反強磁性体で、表面が量子ホール系であるような系の電子状態を解析した。この場合、2次元表面電子系と3次元のバルク状態が混成するので、量子ホール効果やエッジチャンネルが生き残るかどうかは自明な問題ではなくなる。タイトバインディングハミルトニアンを用いてこの問題を数値的に調べ、2次元状態がそのトポロジーに保護されて表面近傍に局在し、エッジチャンネルも生き残ることを示した。さらに進んで解析的な理論を構築し、トポロジカル数が局在をもたらす理由を、自己エネルギーと巻つき数の関係から理解することができた。2.トポロジカル磁性体の設計:1.の成果を元に、パイロクロア反強磁性絶縁体の第一原理計算を行うための準備を行った。具体的には非共線構造のスピン配置とスピン軌道相互作用の両者を扱えるコードの開発を行った。3.スピンテクスチャーの設計:ヘリカル磁性体におけるスキルミオンのダイナミックスをLLG方程式を数値的に解くことで調べた。電流下、不純物によるピン止め効果を取り入れた模型でその運動を追跡し、スピントランスファートルクから予想される速度での運動が普遍的に成り立ち、ピン止め効果は極めて小さいことを見出した。この結果は、従来調べられてきた強磁性体の磁壁の運動と著しい対照をなしており、スキルミオンのメモリー等への応用に道を開くものである。以上の成果に基づき、1.と3.に関しては論文をまとめ投稿中である

• Thoretical Study on Quantum Domain Walls in Ferroelectrics

View Summary

I have completely clarified the magnetoelectric phase diagram of the multiferroic Mn perovskites through constructing their microscopic spin model and analyzing it using Monte-Carlo method for the first time in the world. I have also elucidated their phase diagrams in magnetic fields, a mechanism of the field-induced electric transition, and a mechanism of the electromagnon excitations activated by oscillating electric-field component of light in the study on magnetoelectric cross-correlation responses of this class of materials. Also I have predicted the optical switching of spin chirality via the intense electromagnon excitations, and have clarified the chirality reversal and flop processes accompanied by characteristic spatial-pattern formations with dynamical chirality-domain stripes

### Presentations

• 3次元近藤格子模型における四重波数磁気ヘッジホッグ格子

衛藤倫太郎, 望月維人

日本物理学会第77回年次大会(2022年) 17pT11-1

Presentation date： 2022.03

• 機械学習によるXXZ模型のBKT転移および2次相転移の検出

宮島悠輔, 望月維人

日本物理学会第77回年次大会(2022年) 16aB19-10

Presentation date： 2022.03

• 三角格子上の近藤格子モデルにおける光誘起120度スピン秩序の理論的研究

井上隆, 望月維人

日本物理学会第77回年次大会(2022年) 16aB26-2

Presentation date： 2022.03

• Spin and Charge Excitations of Skyrmion Crystals in Itinerant Magnet

Rintaro Eto, Masahito Mochizuki

International Conference on Frustration, Topology, and Spin Textures (ICFTS2021)

Presentation date： 2021.12

Event date：
2021.12

• Machine learning detection of multiple phase transitions including BKT transition in the q-state clock models

Yusuke Miyajima, Masahito Mochizuki

International Conference on Frustration, Topology, and Spin Textures (ICFTS2021)

Presentation date： 2021.12

Event date：
2021.12

• Theory of optical spin-polarization induction in Rashba spin-orbit electron systems

Yasuhiro Tanaka, Takashi Inoue, Masahito Mochizuki

International Conference on Frustration, Topology, and Spin Textures (ICFTS2021)

Presentation date： 2021.12

Event date：
2021.12

• Photoinduced 120-degree spin order and Z2 vortices in the Kondo-lattice model on a triangular lattice

Takashi Inoue, Masahito Mochizuki

International Conference on Frustration, Topology, and Spin Textures (ICFTS2021)

Presentation date： 2021.12

Event date：
2021.12

• Dynamical manipulations of topological magnetism with light and microwave electromagnetic fields

Masahito Mochizuki  [Invited]

International Conference on Frustration, Topology, and Spin Textures (ICFTS2021)

Presentation date： 2021.12

Event date：
2021.12

• Reservoir computing with spin waves in magnetic skyrmion crystal

Masahito Mochizuki  [Invited]

RIKEN CEMS Seminar

Presentation date： 2021.12

Event date：
2021.12
-

• 機械学習によるKosterlitz-Thouless転移の検出

宮島悠輔, 村田優介, 田中康寛, 望月維人

日本物理学会2021年秋季大会 22pL4-3

Presentation date： 2021.09

• 遍歴ヘリカル磁性体中における磁気構造のダイナミクスの理論研究

衛藤倫太郎, Rico Pohle, 望月維人

日本物理学会2021年秋季大会 20aC1-4

Presentation date： 2021.09

• Theory of Photoinduced Spin Polarization in Spin-Orbit-Coupled Systems

Masahito Mochizuki  [Invited]

The 11th International Conference on Metamaterials, Photonic Crystals and Plasmonics (META 2021) Chirality, magnetism, and magnetoelectricity: Separate phenomena and joint effects in metamaterial structures"

Presentation date： 2021.07

Event date：
2021.07

• 機械学習を用いたクロック模型のKT転移を含む多段磁気転移の検出

宮島悠輔, 村田優介, 田中康寛, 望月維人

日本物理学会2020年秋季大会 PSL-22

Presentation date： 2020.09

Event date：
2020.09

• 円偏光レーザー誘起スピン偏極のフロケ理論：振動磁場と振動電場の効果

井上隆, 田中康寛, 望月維人

日本物理学会2020年秋季大会 9aC2-5

Presentation date： 2020.09

• 有機導体α-(BEDT-TTF)2I3における円偏光誘起トポロジカル状態：時間依存シュレディンガー方程式によるアプローチ

田中康寛, 望月維人

日本物理学会2020年秋季大会 9aH2-1

Presentation date： 2020.09

• 張力ひずみ由来の異方性ジャロシンスキー守谷相互作用による磁気スキルミオンの安定化・不安定化の理論研究

田中康平, 朝日透, 望月維人

日本物理学会2020年秋季大会 9aH2-4

Presentation date： 2020.09

• 有機導体α-(BEDT-TTF)2I3の光誘起トポロジカル相転移の理論研究

北山圭亮, 望月維人

日本物理学会2020年秋季大会 9aH2-7

Presentation date： 2020.09

• Floquet理論を用いたα-(BEDT-TTF)2I3の光誘起トポロジカル相転移の理論研究

北山圭亮, 望月維人

日本物理学会第75年次大会(2020年) 19aB21-8

Presentation date： 2020.03

• 有機導体α-(BEDT-TTF)2I3における円偏光誘起トポロジカル状態の性質

田中康寛, 望月維人

日本物理学会第75年次大会(2020年) 19aB21-9

Presentation date： 2020.03

• ニューラルネットワークによるn状態クロック模型のKT転移の検出

宮島悠輔, 村田優介, 田中康寛, 望月維人

日本物理学会第75年次大会(2020年) 19pK36-2

Presentation date： 2020.03

• La5Mo4O16の電子状態と伝導性ドメイン壁に関する理論研究

三宅将之, 中惇, 勝藤拓郎, 望月維人

日本物理学会第75年次大会(2020年) 18pB21-1

Presentation date： 2020.03

• 一軸張力ひずみによるスキルミオン相安定化・不安定化の理論研究：基底状態相図と有限温度相図

田中康平, 朝日透, 望月維人

日本物理学会第75年次大会(2020年) 18pB21-9

Presentation date： 2020.03

• 円偏光レーザーが誘起するスピン偏極におけるスピン軌道相互作用の効果の理論研究

井上隆, 田中康寛, 望月維人

日本物理学会第75年次大会(2020年) 17aPS-155

Presentation date： 2020.03

• スピン軌道相互作用のある電子系における円偏光誘起スピン偏極の理論：数値計算とフロケ解析

田中康寛, 望月維人

Presentation date： 2019.09

• Rashba電子系におけるスリット型電子干渉実験の理論研究

清水宏太郎, 望月維人

Presentation date： 2019.09

• Microwave-Active Dynamics of Magnetic Skyrmions under Application of a Tilted Magnetic field

望月維人  [Invited]

The 10th International Conference on Metamaterials, Photonic Crystals and Plasmonics (META 2019) Chirality, magnetism, and magnetoelectricity: Separate phenomena and joint effects in metamaterial structures"

Presentation date： 2019.07

• Microwave-Induced Dynamical Phenomena of Magnetic Skyrmions

望月維人  [Invited]

The 4th International Conference Nanomagnetism and spintronics" (Sol-SkyMag 2019)

Presentation date： 2019.06

• Microwave-driven dynamics of magnetic skyrmions

望月維人  [Invited]

The 5th International Worskhop on Topological Structures in Ferroic Materials (TOPO2019)

Presentation date： 2019.06

• Theoretical Study on the Magnetism-Driven Negative Thermal Expansion Phenomenon in the Inverse-Perovskite Antiferromagnets

望月維人

International Conference on Frontiers of Correlated Electron Sciences (FCES2019)

Presentation date： 2019.05

• 一軸張力歪みによるスキルミオン相安定化の数値的研究

田中康平, 菅原諒介, 朝日透, 望月維人

日本物理学会第74回年次大会 17aF201-8

Presentation date： 2019.03

• 空間反転対称性の破れた磁性体における磁気スキルミオン

望月維人  [Invited]

新学術領域「J-Physics：多極子伝導系の物理」Tropical Topical Meeting「カイラル磁性体EuPtSiの電子物性--研究の現状と今後の展開--」

Presentation date： 2018.12

• 逆ペロブスカイト型マンガン窒化物における負の熱膨張と磁気相転移の理論研究

小林賢也, 望月維人

日本物理学会2018年秋季大会 11aC210-4

Presentation date： 2018.09

• マイクロ波によるスキルミオン伝送の理論研究

一花幹人, 竹内祥人, 望月維人

日本物理学会2018年秋季大会 10aC210-2

Presentation date： 2018.09

• スキルミオン型トポロジカル磁気構造のスピン軌道トルク駆動

大木祐人, 竹内祥人, 望月維人

日本物理学会2018年秋季大会 10aC210-3

Presentation date： 2018.09

• スキルミオン結晶を用いたマイクロ波による直流スピン起電力生成の理論研究

小出竜也, 竹内祥人, 望月維人

日本物理学会2018年秋季大会 9pC117-12

Presentation date： 2018.09

• 磁気構造の存在するラシュバ電子系における電気伝導の理論

竹内祥人, 柴田絢也, 河野浩, 多々良源

日本物理学会2018年秋季大会 9pC117-13

Presentation date： 2018.09

• 円偏光マイクロ波による磁化反転とスキルミオン生成の理論研究

三宅将之, 望月維人

日本物理学会2018年秋季大会 9aPS-28

Presentation date： 2018.09

• エレクトロマグノン励起を用いた孤立スキルミオンの駆動

竹内祥人, 望月維人

日本物理学会第73回年次大会 23pK304-12

Presentation date： 2018.03

• 負の熱膨張を示す逆ペロブスカイト型Mn窒化物の磁気秩序と相転移の理論研究

小林賢也, 岡部玲央哉, 山本大輔, 望月維人

日本物理学会第73回年次大会 22pPSA-24

Presentation date： 2018.03

• スキルミオン結晶相においてマイクロ波が誘起する直流スピン起電力成分の理論研究

小出竜也, 竹内祥人, 望月維人

日本物理学会第73回年次大会 22pPSB-12

Presentation date： 2018.03

• スキルミオン型トポロジカル磁気構造のスピン軌道トルク駆動の理論研究

大木祐人, 竹内祥人, 望月維人

日本物理学会第73回年次大会 22pPSB-21

Presentation date： 2018.03

• 傾斜磁場中のスキルミオン結晶相が示すマイクロ波磁気共鳴モードの理論研究

一花幹人, 竹内祥人, 望月維人

日本物理学会第73回年次大会 22pPSB-22

Presentation date： 2018.03

• Dynamical Phenomena and Device Functions of Magnetic Skyrmions

望月維人  [Invited]

EDTM (Electron Devices Technology and Manufacturing Conference)2018「Symposium on Frontier Researches of Functional Oxide Devices and Materials」

Presentation date： 2018.03

• Dynamical Phenomena and Device Functions of Magnetic Skyrmions

望月維人  [Invited]

The 19th Japan-Korea-Taiwan Symposium on Strongly Correlated Electron Systems (JKT19)

Presentation date： 2018.01

• 磁性強誘電体が示す電場励起マグノンと光・マイクロ波現象

望月 維人  [Invited]

第５回豊田理研ワークショップ「スピン秩序の動的光制御」  公益財団法人豊田理化学研究所

Presentation date： 2017.10

• トポロジカル磁気構造が拓く新しいスピントロニクス

望月 維人  [Invited]

第78回応用物理学会秋季学術講演会特別シンポジウム「物質中のトポロジー：応用にどのように結びつくのか？」  応用物理学会

Presentation date： 2017.09

• Dynamical Phenomena of Magnetic Skyrmions

望月 維人  [Invited]

International Workshop on Topological Structures in Ferroic Material  Leeds University

Presentation date： 2017.08

• 交流ラシュバ場が誘起するスピン流とスピントルク

竹内祥人, 水島成康, 望月維人

日本物理学会第72回年次大会 20aC24-4  日本物理学会

Presentation date： 2017.03

• ネール型スキルミオン相を発現する極性絶縁磁性体GaV4S8の相図と磁気構造の理論

望月維人

日本物理学会第72回年次大会 18pH11-13  日本物理学会

Presentation date： 2017.03

• 高いデバイス機能を有するナノスケールトポロジカル磁気テクスチャの理論設計

望月維人

科学技術振興機構 さきがけ研究 [平成25～28年度]研究終了報告会  科学技術振興機構

Presentation date： 2017.01

• 磁気スキルミオンが示す電気磁気現象

望月 維人  [Invited]

豊田理研ワークショップ「キラル対称系の電磁応答」  公益財団法人豊田理化学研究所

Presentation date： 2016.11

• 高いデバイス機能を有するナノスケールトポロジカル磁気テクスチャの理論設計

望月維人

科学技術振興機構 さきがけ研究 [平成25～28年度]第７回領域会議  科学技術振興機構

Presentation date： 2016.11

• 磁気スキルミオンのデバイス機能とダイナミクス

望月 維人  [Invited]

日本HDD協会(IDEMA)ヘッドディスク部会ワークショップ  日本HDD協会(IDEMA)

Presentation date： 2016.11

• 磁気スキルミオンの特異なダイナミクスとデバイス機能

望月 維人  [Invited]

豊田理化学研究所・特定課題研究研究会「スピンと熱・体積機能」  公益財団法人豊田理化学研究所

Presentation date： 2016.08

• Invited as a chair of the multiferroic session (discussion leader)

望月 維人  [Invited]

Gordon Research Conference: Multiferroic & Magnetoelectric Materials  Gordon Research Conference

Presentation date： 2016.08

• 磁気スキルミオンのダイナミクスとデバイス機能

望月 維人  [Invited]

電子情報通信学会磁気記録・情報ストレージ研究会  電子情報通信学会

Presentation date： 2016.06

• 高いデバイス機能を有するナノスケールトポロジカル磁気テクスチャの理論設計

望月維人

科学技術振興機構 さきがけ研究 [平成25～28年度]第６回領域会議  科学技術振興機構

Presentation date： 2016.05

• Dynamical magnetoelectric phenomena in multiferroic Mn perovskites

望月 維人  [Invited]

ICSM (International Conference on Superconductivity and Magnetism) 2016  ICSM

Presentation date： 2016.04

• 絶縁性キラル磁性体薄膜への電場によるスキルミオン書き込みの理論

望月維人

日本物理学会第71回年次大会 21pBC-3  日本物理学会

Presentation date： 2016.03

• 磁気スキルミオンが示す特異なダイナミクスの理論

望月 維人  [Invited]

第55回化合物新磁性材料専門研究会「トポロジカルなスピンテクスチャの可視化とダイナミクス」  日本磁気学会

Presentation date： 2016.03

• Dynamical phenomena of magnetic skyrmions

望月 維人  [Invited]

RIKEN CEMS Discussion Meeting Topological Phenomena in Noncentrosymmetric Magnets"  理化学研究所

Presentation date： 2016.01

• 磁気スキルミオンのダイナミクスとデバイス機能

望月 維人  [Invited]

日本磁気学会第205回研究会/第56回スピンエレクトロニクス専門研究会（共催） 将来の磁気デバイスを担う新しいナノ磁気構造とスピン操作の可能性 ～カイラル磁性とスピンオービトロニクスが拓く新現象～  日本磁気学会

Presentation date： 2015.12

• Dyamics of magnetic skyrmions in multiferroics

望月 維人  [Invited]

EMN (Energy Materials and Nanotechnology) Hong Kong Meeting  EMN

Presentation date： 2015.12

• 高いデバイス機能を有するナノスケールトポロジカル磁気テクスチャの理論設計

望月維人

科学技術振興機構 さきがけ研究 [平成25～28年度]第５回領域会議  科学技術振興機構

Presentation date： 2015.11

• スパイラル磁性強誘電体TbMnO3における磁場誘起分極フロップの理論

望月維人

Presentation date： 2015.09

• スパイラル磁性強誘電体TbMnO3における電場磁場ドメイン制御の実空間観測

松原正和, Sebastian Manz, 望月維人, 井山彩人, 木村剛, Manfred Fiebig

Presentation date： 2015.09

• 絶縁キラル磁性体Cu2OSeO3におけるマイクロ波磁気カイラル効果の理論

望月維人

Presentation date： 2015.09

• 熱勾配によるスキルミオンのラチェット回転とトポロジカルマグノンホール効果

望月維人, 于秀珍, 関真一郎, 金澤直也, 小椎八重航, J. Zang, M. Mostovo, 十倉好紀, 永長直人

日本物理学会2015年秋季大会 18aCM-3  日本物理学会

Presentation date： 2015.09

• マルチフェロイックスにおける電荷・スピン・格子結合と熱揺らぎの物理

望月 維人  [Invited]

豊田理化学研究所特定課題研究「巨大負熱膨張材料を用いた革新的熱膨張制御技術の開発」第3回研究会  公益財団法人豊田理化学研究所

Presentation date： 2015.08

• 高いデバイス機能を有するナノスケールトポロジカル磁気テクスチャの理論設計

望月維人

科学技術振興機構 さきがけ研究 [平成25～28年度]第４回領域会議  科学技術振興機構

Presentation date： 2015.07

• Dynamics of Skyrmions in Chiral-Lattice Magnets

望月 維人  [Invited]

International Workshop on "Topological structures in ferroic materials"  IOP

Presentation date： 2015.05

• 高いデバイス機能を有するナノスケールトポロジカル磁気テクスチャの理論設計

望月維人

科学技術振興機構 さきがけ研究 [平成25～28年度]第３回領域会議  科学技術振興機構

Presentation date： 2015.01

• Thermally Driven Ratchet Motion of a Skyrmion Microcrystal and Topological Magnon Hall Effect

望月 維人  [Invited]

59th Annual Conference on Magnetism and Magnetic Materials (MMM 2014)  MMM

Presentation date： 2014.11

• Current-Driven Dynamics of Magnetic Skyrmions in Chiral-Lattice Magnets

望月 維人  [Invited]

Japan-Swiss Joint Workshop Trends in Theory of Correlated Materials"  日本学術振興会

Presentation date： 2014.10

• 高いデバイス機能を有するナノスケールトポロジカル磁気テクスチャの理論設計

望月維人

科学技術振興機構 さきがけ研究 [平成25～28年度]第２回領域会議  科学技術振興機構

Presentation date： 2014.09

• Magnon Current-Driven Dynamics of Magnetic Skyrmions in Chiral-Lattice Magnets

望月 維人  [Invited]

International Conference on Strongly Correlated Electron Systems (SCES 2014)  SCES

Presentation date： 2014.07

• カイラル絶縁磁性体における磁気スキルミオンの電場駆動・制御の理論

望月 維人  [Invited]

日本学術振興会「産業応用をめざした新物質機能の設計と実証」に関する研究開発専門委員会  日本学術振興会

Presentation date： 2014.05

• マルチフェロイックMnペロブスカイトの電気磁気現象に関する理論研究

望月 維人  [Invited]

日本物理学会第69回年次大会 第8回日本物理学会若手奨励賞受賞記念講演（領域８）  日本物理学会

Presentation date： 2014.03

• Current-induced skyrmion dynamics in constricted geometries

J. Iwasaki, M. Mochizuki, N. Nagaosa

2014 APS March Meeting  APS

Presentation date： 2014.03

• Thermally Driven Ratchet Motion of Skyrmion Microcrystal and Topological Magnon Hall Effect

M. Mochizuki, X. Z. Yu, S. Seki, N. Kanazawa, W. Koshibae, J. Zang, M. Mostovoy, Y. Tokura, N. Nagaosa

2014 APS March Meeting  APS

Presentation date： 2014.03

• Theoretical Studies on Dynamical Phenomena of Magnetic Skyrmions

望月 維人  [Invited]

2014 American Physical Society (APS) March Meeting  APS

Presentation date： 2014.03

• 高いデバイス機能を有するナノスケールトポロジカル磁気テクスチャの理論設計

望月維人

科学技術振興機構 さきがけ研究 [平成25～28年度]第１回領域会議  科学技術振興機構

Presentation date： 2014.02

• Current-driven dynamics of skyrmions

望月 維人  [Invited]

Workshop on Current-Driven Magnetisation Dynamics  Leeds University

Presentation date： 2014.01

• 最近のスキルミオン研究

望月 維人  [Invited]

第3回強相関電子系理論の最前線‒ 若手によるオープン・イノベーション ‒

Presentation date： 2013.12

• 制限されたサンプル形状におけるスキルミオンの電流駆動ダイナミクス

岩崎惇一, 望月維人, 永長直人

日本物理学会2013年秋季大会 27aKF-9  日本物理学会

Presentation date： 2013.09

• スキルミオン結晶の電磁場活性スピン波モードと干渉効果

望月維人, 関真一郎

日本物理学会2013年秋季大会 25aKQ-10  日本物理学会

Presentation date： 2013.09

• Theory of dynamical magnetoelectric phenomena in multiferroics with noncollinear spin structures

望月 維人  [Invited]

5th Asia Pacific Center for Theoretical Physics (APCTP) Workshop on Multiferroics  APCTP

Presentation date： 2013.05

• スキルミオンの特異なダイナミクス

望月 維人  [Invited]

Workshop: CROSSroads of Users and J-PARC 第3回「カイラル磁性体－起源と機能－」  中性子科学センター

Presentation date： 2012.12

• カイラル磁性体のスキルミオンの電気磁気・熱励起ダイナミクス

望月 維人  [Invited]

第2回強相関電子系理論の最前線--若手によるオープン・イノベーション--

Presentation date： 2012.12

• 電流駆動スキルミオンダイナミクスの理論的研究

岩崎惇一, 望月維人, 永長直人

日本物理学会2012年秋季大会 20aCC-7  日本物理学会

Presentation date： 2012.09

• カイラル磁性体におけるスキルミオンクラスタの熱揺らぎ誘起自発ラチェット回転の理論

望月維人, 永長直人

日本物理学会2012年秋季大会 20aCC-5  日本物理学会

Presentation date： 2012.09

• スキルミオン結晶相のマイクロ波吸収に見られる巨大非相反方向二色性の理論

望月維人, 日本物理学会

日本物理学会2012年秋季大会 19pGA-7  日本物理学会

Presentation date： 2012.09

• スカーミオン結晶相のマグノンモード、円偏光依存性と強励起効果

望月維人

日本物理学会第67回年次大会 25aYF-13  日本物理学会

Presentation date： 2012.03

• マルチフェロイックスにおけるエレクトロマグノン励起とスピンカイラリティの光スイッチ現象

望月 維人  [Invited]

テラヘルツテクノロジーフォーラム平成23年度第2回技術検討会  日本分光学会

Presentation date： 2012.03

• Proposed Optical/Microwave Manipulation of Nanoscale Spin Textures: Multiferroics and Skyrmions

望月維人

FIRST-QS2C Workshop on 「Emergent Phenomena of Correlated Materials」

Presentation date： 2011.12

• Theories of Dynamical Magnetoelectric Phenomena in Multiferroic RMnO3

望月維人, 古川信夫, 永長直人

International Workshop on 「Novel Phenomena in Frustrated Systems」

Presentation date： 2011.05

• Dynamical Magnetoelectric Phenomena in the Multiferroic Mn Perovskites

望月 維人  [Invited]

2011 American Physical Society (APS) March Meeting  APS

Presentation date： 2011.03

• スパイラル磁性体におけるスピンカイラリティの光スイッチの理論

望月 維人  [Invited]

「固体光誘起状態の非平衡ダイナミックス理論とその今後の展望」  KEK研究会

Presentation date： 2011.02

• マルチフェロイックスにおけるスピンカイラリティの光スイッチの理論

望月維人, 永長直人

日本物理学2010年秋季大会 24pWJ-7  日本物理学会

Presentation date： 2010.09

• マルチフェロイックスの理論

望月 維人  [Invited]

日本物理学会2010年秋季大会 領域10,8,7,3シンポジウム「誘電体に潜む新しい秩序:最近の話題」  日本物理学会

Presentation date： 2010.09

• マルチフェロイックMnペロフスカイトのエレクトロマグノン励起の理論:二つのピークを持つスペクトルの起源

望月維人, 古川信夫, 永長直人

日本物理学会第65回年次大会 21aGF-8  日本物理学会

Presentation date： 2010.03

• Microscopic Theory on the Electromagnetic Phase Diagram for Multiferroic RMnO3: A-type, Incommensurate Spiral, and E-type Spin Phases

古川信夫, 望月維人

2010 APS March Meeting  APS

Presentation date： 2010.03

• Vital roles of the biquadratic interaction on electromagnon excitations

望月維人, 古川信夫, 永長直人

2010 American Physical Society (APS) March Meeting  APS

Presentation date： 2010.03

• マルチフェロイック物質におけるスピンダイナミクスの数値シミュレーション

望月 維人  [Invited]

次世代スパコン／ナノ統合プロジェクト(情報機能・材料分野)研究会「新物質とエネルギー」  次世代スパコン／ナノ統合プロジェクト

Presentation date： 2010.03

• 微視的理論が解き明かすRMnO3の相図の全貌:A型反強磁性，非整合螺旋磁性，そしてE型反強磁性

望月維人, 古川信夫

日本物理学会秋季大会 27aRL-10  日本物理学会

Presentation date： 2009.09

• Competing Two Multiferroic Phases and Their Control in Perovskite Manganites

望月 維人  [Invited]

Joint European Japanese Conference: Frustration in Condensed Matter  日欧産業協力センター

Presentation date： 2009.05

• マルチフェロイックRMnO3の磁場中電気磁気相図の理論

望月維人, 古川信夫

日本物理学会第64回年次大会 27aTA-1  日本物理学会

Presentation date： 2009.03

• マルチフェロイックRMnO3の磁場誘起分極フロップの機構

望月維人, 古川信夫

日本物理学会秋季大会 21aQA-4  日本物理学会

Presentation date： 2008.09

• マルチフェロイックMnペロフスカイトにおけるフラストレーションと電気-磁気相図の理論

望月 維人  [Invited]

科研費特定領域研究「フラストレーションが創る新しい物性」第2回トピカルミーティング「フラストレーションとマルチフェロイックス」  JSPS

Presentation date： 2008.06

• マンガンペロフスカイトにおけるフラストレーションと磁気相図

望月維人, 古川信夫

日本物理学会第63回年次大会 23aYB-6  日本物理学会

Presentation date： 2008.03

• 強相関超伝導体における軌道自由度:Co酸化物超伝導体の例

望月 維人  [Invited]

科研費特定領域「異常量子物質の創製」若手の会第三回会議「新しい物理を生む新物質」  JSPS

Presentation date： 2007.12

• Phase Diagrams, Thermodynamic Quantities and Possible Two Different Superconducting States of Multiorbital Superconductor NaxCoO2・yH2O

望月維人, 小形正男

2007 American Physical Society (APS) March Meeting  APS

Presentation date： 2007.03

• 多軌道型超伝導体NaxCoO2・yH2Oにおける超伝導と磁性相関

望月 維人  [Invited]

日本応用磁気学会第24回化合物新磁性材料研究会「強磁性相関を内在する超伝導体の最近の話題」  日本応用磁気学会

Presentation date： 2007.03

• Exotic Superconductivity on the Triangular Lattice in NaxCoO2・yH2O: Theories and Experiments

望月 維人  [Invited]

International Focus Workshop on Mobile Fermions and Bosons on Frustrated Lattice  Max-Planck Institute

Presentation date： 2007.01

• Co酸化物超伝導体の比熱、磁場侵入長、その他の物理量

望月維人, 小形正男

日本物理学会第61回秋季大会 25pZL-11  日本物理学会

Presentation date： 2006.09

• Co酸化物超伝導体の相図:二つの超伝導相と磁気秩序相

望月維人, 小形正男

日本物理学会第61回秋季大会 25pZL-10  日本物理学会

Presentation date： 2006.09

• Co酸化物超伝導体の電子構造:CoO2面の厚みとの関係

望月維人, 小形正男

日本物理学会第61回秋季大会 25pZL-9  日本物理学会

Presentation date： 2006.09

• Band Structures, Fermi Surface Topology and Superconductivity in NaxCoO2・yH2O: Effects of the Trigonal CoO6 Distortion

望月維人, 柳瀬陽一, 小形正男

2006 American Physical Society (APS) March Meeting  APS

Presentation date： 2006.03

• Ferromagnetic Fluctuation and Possible Triplet Superconductivity Due to Interorbital Hund's-Rule Coupling in Na0.35CoO2・1.3H2O

望月 維人  [Invited]

KINKEN International Workshop t2g軌道縮退系の新展開"  東北大学金属材料研究所

Presentation date： 2005.10

• NaxCoO2・yH2Oにおけるスピン三重項超伝導と磁場中相図: 軌道効果、常磁性効果とスピン-軌道相互作用

柳瀬陽一, 宇田川将文, 望月維人, 小形正男

日本物理学会第60回秋季大会 21aXD-1  日本物理学会

Presentation date： 2005.09

• 多軌道型超伝導体NaxCoO2・yH2Oにおける軌道自由度に由来する物性現象

望月維人, 柳瀬陽一, 小形正男

日本物理学会第60回秋季大会 21aXD-3  日本物理学会

Presentation date： 2005.09

• NaxCoO2・yH2Oにおけるスピン三重項超伝導とD-vector: スピン-軌道相互作用と磁場中相図の微視的理論

柳瀬陽一, 望月維人, 小形正男

日本物理学会第60回年次大会 27aWM-4  日本物理学会

Presentation date： 2005.03

• Possible Triplet Superconductivity Due to the Interorbital Hund's-Rule Coupling in NaxCoO2・yH2O

望月維人, 柳瀬陽一, 小形正男

2005 American Physical Society (APS) March Meeting  APS

Presentation date： 2005.03

• NaxCoO2・yH2Oの超伝導: 多軌道ハバード模型に基づく研究

柳瀬陽一, 望月維人, 小形正男

日本物理学会第59回秋季大会 12aWD-2  日本物理学会

Presentation date： 2004.09

• NaxCoO2・yH2Oの超伝導に対する多軌道解析

望月維人, 柳瀬陽一, 小形正男

日本物理学会第59回秋季大会 12aWD-1  日本物理学会

Presentation date： 2004.09

• NaxCoO2・yH2Oの超伝導: 多軌道ハバード模型のFLEX近似による解析

望月維人, 小形正男

日本物理学会第59回年会 29pXA-14  日本物理学会

Presentation date： 2004.03

• Ti酸化物系の軌道状態

望月 維人  [Invited]

日本物理学会第59回年会 領域8シンポジウム「t2g電子系の軌道ダイナミクッスと絶縁体-金属転移」  日本物理学会

Presentation date： 2004.03

• ペロフスカイト型Ti酸化物における磁気-軌道構造: 希土類イオンが 作る結晶場の効果

望月維人, 今田正俊

日本物理学会第58回年会 29pWA-8  日本物理学会

Presentation date： 2003.03

• Orbital-Spin Structures and Their Phase Transitions of Perovskite Titanates

望月 維人  [Invited]

京都大学基研研究会:軌道自由度を持つ強相関電子系の理論の進展  京都大学

Presentation date： 2002.11

• ペロフスカイト型Ti酸化物におけるスピン-軌道状態の理論

望月維人, 今田正俊

日本物理学会第57回年会 27aYH-5  日本物理学会

Presentation date： 2002.03

• Origin of G-Type Antiferromagnetism and Orbital-Spin Structures in LaTiO3

望月 維人  [Invited]

KINKEN International Workshop Novel Quantum Orbital State in Strongly Correlated Electron Systems  東北大学金属材料研究所

Presentation date： 2001.05

• ペロフスカイト型Ti酸化物LaTiO3における磁気、軌道状態の理論

望月維人, 今田正俊

日本物理学会第56回年会 28pZA-3  日本物理学会

Presentation date： 2001.03

• ペロフスカイト型Ti酸化物RTiO3におけるRサイト置換による磁気相転移の理論

望月維人, 今田正俊

日本物理学会第55回年会 23aSD-6  日本物理学会

Presentation date： 2000.09

### Specific Research

• 2021

View Summary

フェルミ準位にディラックコーン分散を持つα型有機化合物にレーザーを照射することで引き起こされる「光誘起トポロジカル相転移現象」や「光誘起トポロジカル非平衡定常状態」を理論的に探索・研究した。非平衡定常励起状態を有効的に「静的定常状態の問題」にマップするフロケ理論を用いた解析により，この物質に円偏光や楕円偏光のレーザーを照射することで発現する光誘起トポロジカル絶縁体相の性質や相図を明らかにした。また，結晶軸に対してある偏光角を持つ直線偏光レーザーをこの物質に照射することで，2次元版モノポール・アンチモノポール対消滅ともいうべき現象が起こることや，同じくある偏光角を持つ楕円偏光レーザーを照射することで，ディラック点の衝突・崩壊を伴う新しいタイプの光誘起トポロジカル-非トポロジカル相転移が起こることを明らかにした。

• 2020   北山圭亮, 田中康寛, 小形正男, 井上隆, 衛藤倫太郎

View Summary

空間反転対称性の破れにより活性化したスピン軌道相互作用が顕著な効果を示す磁性体や有機導体などの物質系を対象に，円偏光をや直線偏光の電磁波（レーザー光やマイクロ波）を定常的に照射した時に発現する非平衡定常状態や非平衡相転移について理論的に研究し，欧米の著名な論文誌に2編の論文を掲載する成果を上げた。

• 2019

View Summary

温度を下げる（上げる）と膨張（収縮）する「負熱膨張現象」を示す「逆ペロフスカイト型マンガン窒化物」の負熱膨張のメカニズムを、量子力学的な数理モデルを構築・解析することで解明した。この成果を論文に発表し、「日本物理学会誌」と「現代化学」に解説記事を発表した。また、「スキルミオン」と呼ばれるナノ磁気渦構造が示す現象や機能を理論的に数多く解明した。例として、「スキルミオンのマグノン励起（国際共同研究・PRB誌に発表）」、「スキルミオンによるマイクロ波-直流電圧変換現象（PRB誌に発表）」などがある。また成果の一部について、解説記事や書籍（一章分）を依頼執筆したり、国内外の学会等で発表を行った。

• 2018   Maximilien Cazayous, 森茂生, 竹内祥人

View Summary

キラル磁性体や磁気ヘテロ細線に発現する「スキルミオン」と呼ばれるトポロジカルナノ磁気構造をメモリ素子やマイクロ波素子に応用することを目指し、その特異なマイクロ波誘起ダイナミクスやスピントロニクス機能について理論研究を進めた。磁気ヘテロ細線にマイクロ波AC電場を印加しDzyaloshinskii-守谷相互作用を時間変調することでスキルミオンの並進運動が駆動できることを発見しApplied Physics Letters誌に発表した。また、傾斜定常磁場中の磁性体試料にマイクロ波磁場を印加し、スキルミオンのスピン波モードを励起することでスキルミオンの並進運動が駆動できることを発見しPhysical Review B誌に発表した。この成果は、効率的なマイクロ波－電圧変換の研究へと発展している。

• 2017

View Summary

本研究課題では，近年，次世代磁気メモリ素子の情報担体として期待が集まっている「磁気スキルミオン」の動的応答現象を理論的に解明し，スキルミオン磁気メモリ素子実現のための基盤技術を理論的に設計することを目的とする。磁気スキルミオンが発現するキラル磁性体の微視的な数理モデルを構築し，有限温度における磁化の外場応答をシミュレーションするプログラムを完成させた。さらに，開発したコードを使い，従来は技術的に困難であると考えられていた数ナノメートルの極小スキルミオン構造を「磁場」によって書き込む技術を理論的に設計した。また，磁気スキルミオンと，そのドメイン構造の形成過程をシミュレーションにより明らかにした。

### Syllabus

• School of Fundamental Science and Engineering

2022   fall semester

• School of Fundamental Science and Engineering

2022   spring semester

• School of Creative Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   full year

• School of Advanced Science and Engineering

2022   full year

• School of Advanced Science and Engineering

2022   full year

• School of Advanced Science and Engineering

2022   full year

• School of Advanced Science and Engineering

2022   full year

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   full year

• School of Advanced Science and Engineering

2022   full year

• School of Advanced Science and Engineering

2022   full year

• School of Advanced Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   fall semester

• School of Advanced Science and Engineering

2022   fall semester

2022   full year

2022   full year

2022   fall semester

2022   spring semester

2022   fall semester

2022   spring semester

2022   full year

2022   full year

2022   full year

2022   full year

2022   full year

2022   full year

2022   full year

### Committee Memberships

• 2019.04
-
2023.03

日本物理学会  JPSJ編集委員(Associate Editor)

• 2021.11
-
2022.03

日本物理学会  日本物理学会英文誌SNS関連ワーキンググループ委員

• 2020.06
-
2021.03

日本物理学会  JPS Hot Topicsウェブサイトアドバイザー

• 2019.04
-
2021.03

日本物理学会誌新著紹介委員

• 2014.04
-
2017.03

日本物理学会誌編集委員

• 2014.04
-
2015.03

日本物理学会領域8運営委員