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Papers - Nii, Yoichi

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Current-Induced Sliding Motion in a Helimagnet MnAu2

Yuta Kimoto, Hidetoshi Masuda, Takeshi Seki, Yoichi Nii, Jun-ichiro Ohe, Yusuke Nambu, Yoshinori Onose

Physical Review Letters (American Physical Society (APS)) 134 ( 5 ) 2025.02

ISSN 0031-9007

We found signatures of current-induced sliding motion in helimagnetic MnAu2 thin films. An abrupt change in differential resistivity occurred at a threshold bias current in the helimagnetic state, whereas it was absent in the induced ferromagnetic state. Broadband voltage noise also emerged above the threshold current in the helimagnetic state. Based on the similarity to canonical charge and spin density wave systems, we ascribed the origin of these phenomena to the sliding motion of the helimagnetic structure.

Published by the American Physical Society 2025

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Band asymmetry-driven nonreciprocal electronic transport in a helimagnetic semimetal a-EuP3

Alex Hiro Mayo, Darius-Alexandru Deaconu, Hidetoshi Masuda, Yoichi Nii, Hidefumi Takahashi, Rodion Vladimirovich Belosludov, Shintaro Ishiwata, Mohammad Saeed Bahramy, Yoshinori Onose

Proceedings of the National Academy of Sciences ( 122(4) ) e2405839122 2025.01

Research paper (scientific journal), Joint Work, Accepted

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Band asymmetry–driven nonreciprocal electronic transport in a helimagnetic semimetal α-EuP 3

Alex Hiro Mayo, Darius-Alexandru Deaconu, Hidetoshi Masuda, Yoichi Nii, Hidefumi Takahashi, Rodion Vladimirovich Belosludov, Shintaro Ishiwata, Mohammad Saeed Bahramy, Yoshinori Onose

Proceedings of the National Academy of Sciences (Proceedings of the National Academy of Sciences) 122 ( 4 ) 2025.01

ISSN 0027-8424

Chiral magnetic textures give rise to unconventional magnetotransport phenomena such as the topological Hall effect and nonreciprocal electronic transport. While the correspondence between topology or symmetry of chiral magnetic structures and such transport phenomena has been well established, a microscopic understanding based on the spin-dependent band structure in momentum space remains elusive. Here, we demonstrate how a chiral magnetic superstructure introduces an asymmetry in the electronic band structure and triggers a nonreciprocal electronic transport in a centrosymmetric helimagnet α-EuP 3 . The magnetic structure of α-EuP 3 is highly tunable by a magnetic field and closely coupled to its semimetallic electronic band structure, enabling a systematic study across chiral and achiral magnetic phases on the correspondence between nonreciprocal transport and electronic band asymmetry. Our findings reveal how a microscopic change in the magnetic configuration of charge carriers can lead to nonreciprocal electronic transport, paving the way for designing chiral magnets with desirable properties.

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Observation of Nonreciprocal Diffraction of Surface Acoustic Wave

Y. Nii, K. Yamamoto, M. Kanno, S. Maekawa, Y. Onose

Physical Review Letters (American Physical Society (APS)) 134 ( 2 ) 2025.01

Lead author, Corresponding author, Accepted, ISSN 0031-9007

Rectification phenomenon caused by the simultaneous breaking of time reversal
and spatial inversion symmetries has been extended to a wide range of
(quasi)particles and waves; however, the nonreciprocal diffraction, which is
the imbalance of upward and downward deflections, was previously observed only
for photons and remained to be extended to other (quasi)particles. In this
study, we present evidence of the nonreciprocal diffraction of surface acoustic
wave (SAW) utilizing a magnetoelastic grating on a SAW device. Asymmetric
diffraction intensities were observed when the ferromagnetic resonance was
acoustically excited. Based on a theoretical model, we attribute the
microscopic origin of this phenomenon to the resonant scattering involving
ferromagnetic resonance excitations. The novel property may pave an avenue to
further development of SAW devices for various purposes, including microwave
communications and quantum engineering applications.

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A high-temperature multiferroic Tb2(MoO4)3

Shimon Tajima, Hidetoshi Masuda, Yoichi Nii, Shojiro Kimura, Yoshinori Onose

Communications Materials (Springer Science and Business Media LLC) 5 ( 1 ) 2024.12

Abstract

Magnetoelectric mutual control in multiferroics, which is the electric control of magnetization, or reciprocally the magnetic control of polarization has attracted much attention because of its possible applications to spintronic devices, multi-bit memories, and so on. While the required working temperature for the practical application is much higher than room temperature, which ensures stable functionality at room temperature, the reported working temperatures were at most around room temperature. Here, we demonstrated magnetic control of ferroelectric polarization at 432 K in ferroelectric and ferroelastic Tb2(MoO4)3, in which the polarity of ferroelectric polarization is coupled to the orthorhombic strain below the transition temperature 432 K. The paramagnetic but strongly magnetoelastic Tb3+ magnetic moments enable the magnetic control of ferroelectric and ferroelastic domains; the ferroelectric polarization is controlled depending on whether the magnetic field is applied along [110] or [1$$\bar{1}$$0]. This result may pave a new avenue for designing high-temperature multiferroics.

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Microscopic evaluation of spin and orbital moment in ferromagnetic resonance

Yuta Ishii, Yuichi Yamasaki, Yusuke Kozuka, Jana Lustikova, Yoichi Nii, Yoshinori Onose, Yuichi Yokoyama, Masaichiro Mizumaki, Jun-ichi Adachi, Hironori Nakao, Taka-hisa Arima, Yusuke Wakabayashi

Scientific Reports (Springer Science and Business Media LLC) 14 ( 1 ) 2024.07

Abstract

Time-resolved X-ray magnetic circular dichroism under the effects of ferromagnetic resonance (FMR), known as X-ray ferromagnetic resonance (XFMR) measurements, enables direct detection of precession dynamics of magnetic moment. Here we demonstrated XFMR measurements and Bayesian analyses as a quantitative probe for the precession of spin and orbital magnetic moments under the FMR effect. Magnetization precessions in two different Pt/Ni-Fe thin film samples were directly detected. Furthermore, the ratio of dynamical spin and orbital magnetic moments was evaluated quantitatively by Bayesian analyses for XFMR energy spectra around the Ni $$L_{2,3}$$ absorption edges. Our study paves the way for a microscopic investigation of the contribution of the orbital magnetic moment to magnetization dynamics.

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A high-temperature multiferroic Tb$_2$(MoO$_4$)$_3$

Shimon Tajima, Hidetoshi Masuda, Yoichi Nii, Shojiro Kimura, Yoshinori Onose

2024.05

Magnetoelectric mutual control in multiferroics, which is the electric
control of magnetization, or reciprocally the magnetic control of polarization
has attracted much attention because of its possible applications to spintronic
devices, multi-bit memories, and so on. While the required working temperature
for the practical application is much higher than room temperature, which
ensures stable functionality at room temperature, the reported working
temperatures were at most around room temperature. Here, we demonstrated
magnetic control of ferroelectric polarization at 432 K in ferroelectric and
ferroelastic Tb$_2$(MoO$_4$)$_3$, in which the polarity of ferroelectric
polarization is coupled to the orthorhombic strain below the transition
temperature 432 K. The paramagnetic but strongly magnetoelastic Tb$^{3+}$
magnetic moments enable the magnetic control of ferroelectric and ferroelastic
domains; the ferroelectric polarization is controlled depending on whether the
magnetic field is applied along [110] or [$\bar{1}$10]. This result may pave a
new avenue for designing high-temperature multiferroics.

Room temperature chirality switching and detection in a helimagnetic MnAu2 thin film

Hidetoshi Masuda, Takeshi Seki, Jun-ichiro Ohe, Yoichi Nii, Hiroto Masuda, Koki Takanashi, Yoshinori Onose

Nature Communications (Springer Science and Business Media LLC) 15 ( 1 ) 2024.03

Abstract

Helimagnetic structures, in which the magnetic moments are spirally ordered, host an internal degree of freedom called chirality corresponding to the handedness of the helix. The chirality seems quite robust against disturbances and is therefore promising for next-generation magnetic memory. While the chirality control was recently achieved by the magnetic field sweep with the application of an electric current at low temperature in a conducting helimagnet, problems such as low working temperature and cumbersome control and detection methods have to be solved in practical applications. Here we show chirality switching by electric current pulses at room temperature in a thin-film MnAu2 helimagnetic conductor. Moreover, we have succeeded in detecting the chirality at zero magnetic fields by means of simple transverse resistance measurement utilizing the spin Berry phase in a bilayer device composed of MnAu2 and a spin Hall material Pt. These results may pave the way to helimagnet-based spintronics.

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A New Pathway of Solid-State Cooling with a Multiferroic

Yoichi Nii

JPSJ News and Comments (Physical Society of Japan) 21 2024.01

Lead author, Corresponding author, ISSN 2188-076X

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Imaging an Acoustic Topological Edge Mode on a Patterned Substrate with Microwave Impedance Microscopy

Y. Nii, Y. Onose

Physical Review Applied (American Physical Society (APS)) 19 ( 1 ) 2023.01

Lead author, Corresponding author, Accepted

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Nonreciprocal microwave response at room temperature in multiferroic Y-type hexaferrite BaSrCo2Fe11AlO22

Sakyo Hirose, Yusuke Iguchi, Yoichi Nii, Tsuyoshi Kimura, Yoshinori Onose

Applied Physics Letters 121 ( 22 ) 2022.11

Accepted, ISSN 0003-6951

We investigated the microwave response in room-temperature multiferroic BaSrCo2Fe11AlO22. Microwave absorption ascribed to magnetic resonances was observed in the frequency range from 6 to 20 GHz. When the ferroelectric polarization is aligned by the electric and magnetic fields, the magnitude depends on the sign of the microwave propagation vector that indicates the nonreciprocal directional dichroism (NDD). The phenomenon can be observed even at room temperature, the sign can be controlled by the external electric and magnetic fields, and the magnitude of NDD attained is approximately 11% at 300 K and 28% at 200 K. Such microwave properties will open the avenue of practical applications for future wireless communications.

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Room-temperature chirality switching in a helimagnetic thin film

Hidetoshi Masuda, Takeshi Seki, Yoichi Nii, Hiroto Masuda, Koki Takanashi, Yoshinori Onose

2022.05

Helimagnetic structures, in which the magnetic moments are spirally ordered,
host an internal degree of freedom called chirality (or helicity) corresponding
to the handedness of the helix. The chirality seems quite robust against
disturbances and is therefore promising for next-generation magnetic memory.
While the chirality control was recently achieved by the magnetic field sweep
with the application of an electric current at low temperature in a conducting
helimagnet, problems such as low working temperature and cumbersome control
sequence have to be solved in practical applications. Another issue is the thin
film fabrication that enables the development of spintronic devices based on
helimagnets. Here we show chirality switching by electric current pulses at
room temperature in a thin-film MnAu$_2$ helimagnetic conductor. The result
demonstrates the feasibility of helimagnet-based spintronics that can overcome
all the above problems.

Elastic study of electric quadrupolar correlation in the paramagnetic state of the frustrated quantum magnet Tb2Ti2O7

Y. Nii, Y. Hirokane, S. Nakamura, N. Kabeya, S. Kimura, Y. Tomioka, T. Nojima, Y. Onose

Physical Review B (American Physical Society (APS)) 105 ( 9 ) 2022.03

Lead author, Corresponding author, Accepted

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Do electron distributions with orbital degree of freedom exhibit anisotropy?

Taishun Manjo, Shunsuke Kitou, Naoyuki Katayama, Shin Nakamura, Takuro Katsufuji, Yoichi Nii, Taka-hisa Arima, Joji Nasu, Takumi Hasegawa, Kunihisa Sugimoto, Daisuke Ishikawa, Alfred Q. R. Baron, Hiroshi Sawa

MATERIALS ADVANCES (ROYAL SOC CHEMISTRY) 3 ( 7 ) 3192 - 3198 2022.02

Accepted, ISSN 2633-5409

Orbital degrees of freedom appear in partially filled degenerate orbitals and are responsible for a variety of physical properties. Although many methods for directly observing orbital states in ordered states have been proposed, it is difficult to determine degenerate orbital states directly. We found that high-precision diffraction data obtained by synchrotron radiation X-rays can be used to observe only the valence electron density distribution with minimal analytical bias. The results obtained using this method clearly showed the anisotropy of the partially filled degenerate 3d orbital states in spinel vanadium oxide FeV2O4. The 3d orbital quantum state was successfully determined from this anisotropy.

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Magnetization control by angular momentum transfer from surface acoustic wave to ferromagnetic spin moments

R. Sasaki, Y. Nii, Y. Onose

Nature Communications (Springer Science and Business Media LLC) 12 ( 1 ) 2021.12

Accepted, ISSN 2041-1723

<title>Abstract</title>Interconversion between electron spin and other forms of angular momentum is useful for spin-based information processing. Well-studied examples of this are the conversion of photon angular momentum and rotation into ferromagnetic moment. Recently, several theoretical studies have suggested that the circular vibration of atoms work as phonon angular momentum; however, conversion between phonon angular momentum and spin-moment has yet to be demonstrated. Here, we demonstrate that the phonon angular momentum of surface acoustic wave can control the magnetization of a ferromagnetic Ni film by means of the phononic-to-electronic conversion of angular momentum in a Ni/LiNbO3 hybrid device. The result clearly shows that the phonon angular momentum is useful for increasing the functionality of spintronic devices.

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Effect of symmetry breaking on short-wavelength acoustic phonons in the chiral magnet MnSi

Y. Nii, Y. Hirokane, T. Koretsune, D. Ishikawa, A. Q. R. Baron, Y. Onose

PHYSICAL REVIEW B (AMER PHYSICAL SOC) 104 ( 8 ) 2021.08

Lead author, Corresponding author, Accepted, ISSN 2469-9950

We have investigated the effects of spatial-inversion and time-reversal symmetry breaking on the acoustic phonon branches in chiral MnSi using high-resolution inelastic x-ray scattering and first-principles calculations. We find a momentum-transfer-dependent (q-dependent) splitting between transverse phonon bands having angular momentum parallel or antiparallel to q. This is understood by a phenomenological theory using a gyrotropic tensor. We observed no significant impact from the time-reversal symmetry breaking induced by a magnetic field (energy shifts < 0.3 meV). This suggests the effect of time-reversal symmetry breaking is small or is restricted to a very-low-energy regime in this material, possibly due to small spin-orbit interaction.

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Nonreciprocal electronic transport in PdCrO2: Implication of spatial inversion symmetry breaking

M. Akaike, Y. Nii, H. Masuda, Y. Onose

Physical Review B (American Physical Society (APS)) 103 ( 18 ) 2021.05

Accepted, ISSN 2469-9950

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Chirality Memory Stored in Magnetic Domain Walls in the Ferromagnetic State of MnP

N. Jiang, Y. Nii, H. Arisawa, E. Saitoh, J. Ohe, Y. Onose

PHYSICAL REVIEW LETTERS (AMER PHYSICAL SOC) 126 ( 17 ) 2021.04

Accepted, ISSN 0031-9007

Chirality in a helimagnetic structure is determined by the sense of magnetic moment rotation. We found that the chiral information did not disappear even after the phase transition to the high-temperature ferromagnetic phase in a helimagnet MnP. The 2nd harmonic resistivity rho(2f), which reflects the breaking down of mirror symmetry, was found to be almost unchanged after heating the sample above the ferromagnetic transition temperature and cooling it back to the helimagnetic state. The application of a magnetic field along the easy axis in the ferromagnetic state quenched the chirality-induced rho(2f). This indicates that the chirality memory effect originated from the ferromagnetic domain walls.

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Nonreciprocal thermal transport in a multiferroic helimagnet

Yuji Hirokane, Yoichi Nii, Hidetoshi Masuda, Yoshinori Onose

Science Advances (American Association for the Advancement of Science (AAAS)) 6 ( 40 ) eabd3703 - eabd3703 2020.09

Accepted

Breaking of spatial inversion symmetry induces unique phenomena in condensed matter. In particular, by combining this symmetry with magnetic fields or another type of time-reversal symmetry breaking, noncentrosymmetric materials can be made to exhibit nonreciprocal responses, which are responses that differ for rightward and leftward stimuli. However, the effect of spatial inversion symmetry breaking on thermal transport in uniform media remains to be elucidated. Here, we show nonreciprocal thermal transport in the multiferroic helimagnet TbMnO3. The longitudinal thermal conductivity depends on whether the thermal current is parallel or antiparallel to the vector product of the electric polarization and magnetization. This phenomenon is thermal rectification that is controllable with external fields in a uniform crystal. This discovery may pave the way to thermal diodes with controllability and scalability.

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Electric current control of spin helicity in an itinerant helimagnet

N. Jiang, Y. Nii, H. Arisawa, E. Saitoh, Y. Onose

Nature communications ( 11 ) 1601 2020.03

Accepted

Phononic thermal Hall effect in diluted terbium oxides

Yuji Hirokane, Yoichi Nii, Yasuhide Tomioka, Yoshinori Onose

PHYSICAL REVIEW B ( 99 ) 134419 2019.04

Accepted

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Surface acoustic wave coupled to magnetic resonance on multiferroic CuB2O4

Sasaki R, Nii Y, Onose Y

PHYSICAL REVIEW B 99 ( 1 ) 2019.01

Accepted, ISSN 2469-9950

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Microwave nonreciprocity of magnon excitations in the noncentrosymmetric antiferromagnet Ba2MnGe2O7

Iguchi Y, Nii Y, Kawano M, Murakawa H, Hanasaki N, Onose Y

PHYSICAL REVIEW B 98 ( 6 ) 2018.08

Accepted, ISSN 2469-9950

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Magnetic excitations in the orbital disordered phase of MnV2O4

Keisuke Matsuura, Hajime Sagayama, Amane Uehara, Yoichi Nii, Ryoichi Kajimoto, Kazuya Kamazawa, Kazuhiko Ikeuchi, Sungdae Ji, Nobuyuki Abe, Taka-hisa Arima

PHYSICA B-CONDENSED MATTER (ELSEVIER SCIENCE BV) 536 372 - 376 2018.05

Accepted, ISSN 0921-4526

We have investigated the temperature dependence of magnetic dynamics in a spinel-type vanadium oxide MnV2O4 by inelastic neutron scattering. The scattering intensity of excitation around 20 meV disappears in the collinear intermediate-temperature cubic-ferrimagnetic phase, which reveals that this excitation should be peculiar to the orbital ordered phase. We have found a weakly dispersive mode emergent from a non-integer wavevector (1.4,1.4,0) at 56 K, which lies in the cubic-ferrimagnetic phase between non-coplanar ferrimagnetic and paramagnetic phases. This indicates that the probable presence of an incommensurate instability in the simple collinear structure.

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Magnetically Controlled Surface Acoustic Waves on Multiferroic BiFeO3

Y. Ishii, R. Sasaki, Y. Nii, T. Ito, Y. Onose

Physical Review Applied (American Physical Society) 9 ( 3 ) 2018.03

Accepted, ISSN 2331-7019

We fabricate a surface acoustic wave (SAW) device on a multiferroic BiFeO3 crystal while SAW devices are usually fabricated on nonmagnetic piezoelectrics and commercially available as bandpass filters. By using the time-domain technique, we demonstrate the SAW excitation on BiFeO3. The amplitude and phase of the SAW signal are modulated by the external magnetic field reflecting the multiferroicity of BiFeO3. The magnetic controllability of the multiferroic SAW device seems useful for the further functionalization of the SAW device.

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Surface acoustic wave on a multiferroic material CuB2O4

Sasaki Ryo, Nii Yoichi, Onose Yoshinori

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan) 73.1 2312 - 2312 2018

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Magnetotransport properties in the noncentrosymmetric itinerant ferromagnet Cr11Ge19

N. Jiang, Y. Nii, R. Ishii, Z. Hiroi, Y. Onose

PHYSICAL REVIEW B (AMER PHYSICAL SOC) 96 ( 14 ) 2017.10

Accepted, ISSN 2469-9950

We have investigated the anomalous Hall effect and magnetoresistance of the noncentrosymmetric itinerant ferromagnet Cr11Ge19. We have found a characteristic temperature (similar to 30 K) below the ferromagnetic transition temperature T-C = 85 K. While a conventional anomalous Hall conductivity that is proportional to the magnetization smoothly grows below T-C, the anomalous Hall conductivity shows an additional enhancement below the characteristic temperature. Concomitantly, the AMR emerges and the magnitude of negative magnetoresistance begins to increase toward low temperature. Because there is no anomaly in the temperature dependence of magnetization around the characteristic temperature, the origin of these observations in transport properties is ascribed to some electronic structure with the energy scale of 30 K. We speculate this is caused by the spin splitting due to breaking of spatial inversion symmetry.

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Spin-Orbital Correlated Dynamics in the Spinel-Type Vanadium Oxide MnV2O4

Keisuke Matsuura, Hajime Sagayama, Amane Uehara, Yoichi Nii, Ryoichi Kajimoto, Kazuya Kamazawa, Kazuhiko Ikeuchi, Sungdae Ji, Nobuyuki Abe, Taka-hisa Arima

PHYSICAL REVIEW LETTERS (AMER PHYSICAL SOC) 119 ( 1 ) 2017.07

Accepted, ISSN 0031-9007

We investigate the magnetic dynamics in the spinel-type vanadium oxide MnV2O4. Inelastic neutron scattering around 10 meV and a Heisenberg model analysis have revealed that V3+ spin-wave modes exist at a lower-energy region than previously reported. The scattering around 20 meV cannot be reproduced with the spin-wave analysis. We propose that this scattering could originate from the spin-orbital coupled excitation. This scattering is most likely attributable to V3+ spin-wave modes, entangled with the orbital hybridization between t(2g) orbitals.

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Magnetoelectrical control of nonreciprocal microwave response in a multiferroic helimagnet

Y. Iguchi, Y. Nii, Y. Onose

NATURE COMMUNICATIONS (NATURE PUBLISHING GROUP) 8 2017.05

Accepted, ISSN 2041-1723

The control of physical properties by external fields is essential in many contemporary technologies. For example, conductance can be controlled by a gate electric field in a field effect transistor, which is a main component of integrated circuits. Optical phenomena induced by an electric field such as electroluminescence and electrochromism are useful for display and other technologies. Control of microwave propagation is also important for future wireless communication technology. Microwave properties in solids are dominated mostly by magnetic excitations, which cannot be easily controlled by an electric field. One solution to this problem is to use magnetically induced ferroelectrics (multiferroics). Here we show that microwave nonreciprocity, that is, different refractive indices for microwaves propagating in opposite directions, could be reversed by an external electric field in a multiferroic helimagnet Ba2Mg2Fe12O22. This approach offers an avenue for the electrical control of microwave properties.

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Microwave Magnetochiral Effect in the Non-centrosymmetric Magnet CuB2O4

Yoichi Nii, Ryo Sasaki, Yusuke Iguchi, Yoshinori Onose

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN (PHYSICAL SOC JAPAN) 86 ( 2 ) 2017.02

Lead author, Corresponding author, Accepted, ISSN 0031-9015

We have investigated microwave nonreciprocity in the non-centrosymmetric magnet CuB2O4. We simultaneously observed nonreciprocities with different origins: the classical magnetic dipolar nonreciprocity and the magnetochiral (MCh) nonreciprocity. By rotating a magnetic field in a tetragonal plane, we clearly unveiled a qualitative difference between them. The MCh signal reveals chiral transitions from one enantiomer to the other via an intermediate achiral state. We showed that the magnetoelectric effect plays an essential role in the emergence of the microwave MCh effect. Our demonstration may give a new insight into the classical dipolar-type and emergent spin-orbit-type microwave nonreciprocities in media with broken time-reversal symmetry and spatial inversion symmetry.

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Nonreciprocal propagation of surface acoustic wave in Ni/LiNbO3

R. Sasaki, Y. Nii, Y. Iguchi, Y. Onose

PHYSICAL REVIEW B (AMER PHYSICAL SOC) 95 ( 2 ) 2017.01

Accepted, ISSN 2469-9950

We investigated surface acoustic wave propagation in a Ni/LiNbO3 hybrid device. We found that the absorption and phase velocity are dependent on the sign of the wave vector, which indicates that the surface acoustic wave propagation has nonreciprocal characteristics induced by simultaneous breaking of time-reversal and spatial inversion symmetries. The nonreciprocity was reversed by 180 degrees rotation of the magnetic field. The origin of the nonreciprocity is ascribed to interference of shear-type and longitudinal-type magnetoelastic couplings.

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Surface acoustic wave propagation in a multiferroic material BiFeO_3

Ishii Yuta, Sasaki Ryo, Nii Yoichi, Ito Toshimitsu, Onose Yoshinori

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan) 72.1 2348 - 2348 2017

マルチフェロイック物質では複数の自由度が同時に強的な秩序を示す。またこの複数の秩序を外場などで制御することによって、新たな物質機能やデバイスへの応用が期待される。私たちはマルチフェロイック物質における圧電性とその応用である表面弾性波デバイスに注目した。私たちはBiFeO_3の単結晶を使った表面弾性波デバイスを作成し、マルチフェロイック物質において初めて、表面弾性波の伝搬を観測した。また磁気弾性結合を介したことによる、表面弾性波の磁場依存性も確認することができた

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Application of strain to orbital-spin-coupled system MnV2O4 at cryogenic temperatures within a transmission electron microscope

Y. Murakami, T. Suzuki, Y. Nii, S. Murai, T. Arima, R. Kainuma, D. Shindo

MICROSCOPY (OXFORD UNIV PRESS) 65 ( 3 ) 223 - 232 2016.06

Accepted, ISSN 2050-5698

The impact of mechanical stress on the morphology of crystallographic and magnetic domains in shape-controlled specimens of an orbital-spin-coupled system, MnV2O4, was examined by cryogenic Lorentz microscopy. Because of the difference in thermal expansion coefficients of MnV2O4 and the supporting Mo mesh, compression on the order of 0.01% was applied to the thin-foil specimens near the structural/magnetic phase transformation temperatures. The extent of compression was comparable to the lattice striction associated with the cubic-to-tetragonal phase transformation in MnV2O4. The applied strain thus clearly influenced the morphology of crystallographic domains (i.e. twinning configuration in the tetragonal phase) produced during cooling. The magnetic domain structure was entirely dependent on the configuration of twinning in the tetragonal phase. The observations in this study provided useful information for understanding the relationship between the crystallographic domains and the magnetic domains in MnV2O4.

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Critical phenomena of emergent magnetic monopoles in a chiral magnet

N. Kanazawa, Y. Nii, X. -X. Zhang, A. S. Mishchenko, G. De Filippis, F. Kagawa, Y. Iwasa, N. Nagaosa, Y. Tokura

NATURE COMMUNICATIONS (NATURE PUBLISHING GROUP) 7 11622 2016.05

Accepted, ISSN 2041-1723

Second-order continuous phase transitions are characterized by symmetry breaking with order parameters. Topological orders of electrons, characterized by the topological index defined in momentum space, provide a distinct perspective for phase transitions, which are categorized as quantum phase transitions not being accompanied by symmetry breaking. However, there are still limited observations of counterparts in real space. Here we show a real-space topological phase transition in a chiral magnet MnGe, hosting a periodic array of hedgehog and antihedgehog topological spin singularities. This transition is driven by the pair annihilation of the hedgehogs and antihedgehogs acting as monopoles and antimonopoles of the emergent electromagnetic field. Observed anomalies in the magnetoresistivity and phonon softening are consistent with the theoretical prediction of critical phenomena associated with enhanced fluctuations of emergent field near the transition. This finding reveals a vital role of topology of the spins in strongly correlated systems.

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Electric control of nonreciprocal microwave response in ferroelectric spiral magnet

Iguchi Yusuke, Nii Yoichi, Onose Yoshinori

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan) 71 721 - 721 2016

多くの磁性体における磁化ダイナミクスはマイクロ波領域の交流磁場によって励起できる。そのため、マイクロ波導波路上に磁性体をおくとマイクロ波の吸収が観測される。この吸収強度はマイクロ波透過方向に対して相反的であるが、自発電気分極を持った磁性体を使うことで、非相反性が現れる。今回我々は、電場で電気分極方向を反転できるヘキサフェライトを使い、電場によるマイクロ波非相反伝搬の制御に初めて成功した。

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22aAU-2 Microwave response of a noncentrosymmetric magnet CuB_2O_4

Nii Y., Sasaki R., Iguchi Y., Onose Y.

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan (JPS)) 71 1279 - 1279 2016

ISSN 2189-079X

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19aAU-11 Magnetoelastic coupling and nonreciprocity of surface acoustic wave on Ni/LiNbO_3

Sasaki R., Nii Y., Iguchi Y., Onose Y.

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan (JPS)) 71 1006 - 1006 2016

ISSN 2189-079X

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Uniaxial stress control of skyrmion phase

Y. Nii, T. Nakajima, A. Kikkawa, Y. Yamasaki, K. Ohishi, J. Suzuki, Y. Taguchi, T. Arima, Y. Tokura, Y. Iwasa

NATURE COMMUNICATIONS (NATURE PUBLISHING GROUP) 6 8539 2015.10

Lead author, Corresponding author, Accepted, ISSN 2041-1723

Magnetic skyrmions, swirling nanometric spin textures, have been attracting increasing attention by virtue of their potential applications for future memory technology and their emergent electromagnetism. Despite a variety of theoretical proposals oriented towards skyrmion-based electronics (that is, skyrmionics), few experiments have succeeded in creating, deleting and transferring skyrmions, and the manipulation methodologies have thus far remained limited to electric, magnetic and thermal stimuli. Here, we demonstrate a new approach for skyrmion phase control based on a mechanical stress. By continuously scanning uniaxial stress at low temperatures, we can create and annihilate a skyrmion crystal in a prototypical chiral magnet MnSi. The critical stress is merely several tens of MPa, which is easily accessible using the tip of a conventional cantilever. The present results offer a new guideline even for single skyrmion control that requires neither electric nor magnetic biases and consumes extremely little energy.

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X-ray magnetic circular dichroism study of an orbital ordered state in the spinel-type vanadium oxide A V2 O4 (A= Mn, Fe)

K. Matsuura, H. Sagayama, Y. Nii, N. D. Khanh, N. Abe, T. Arima

Physical Review B - Condensed Matter and Materials Physics (American Physical Society) 92 ( 3 ) 2015.07

Accepted, ISSN 1550-235X

Spinel-type vanadium oxides FeV2O4 and MnV2O4 are considered to exhibit different types of V3+ orbital order from each other. In order to tackle this issue, the temperature dependence of x-ray absorption spectrum (XAS) and x-ray magnetic circular dichroism (XMCD) spectrum are investigated in both spinel compounds. The application of the sum rule to the obtained V edge XMCD spectra and the comparison with crystal-field multiplet calculations suggest a real orbital ordered state with quenched orbital angular momenta in both FeV2O4 and MnV2O4. A dissimilarity in XAS spectra between these compounds would originate from a quantitative difference of trigonal distortion.

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X-ray magnetic circular dichroism study of an orbital ordered state in the spinel-type vanadium oxide AV(2)O(4) (A = Mn, Fe)

K. Matsuura, H. Sagayama, Y. Nii, N. D. Khanh, N. Abe, T. Arima

PHYSICAL REVIEW B (AMER PHYSICAL SOC) 92 ( 3 ) 35133 2015.07

Accepted, ISSN 2469-9950

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16aCD-8 Large elastic response in a chiral magnet MnGe

Nii Y., Kanazawa N., Zhang X. -X., Nagaosa N., Tokura Y., Iwasa Y.

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan (JPS)) 70 1661 - 1661 2015

ISSN 2189-079X

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19pCD-2 Temperature dependence of magnetic excitation in spinel oxide MnV_2O_4

Matsuura K., Sagayama H., Nii Y., Arima T., Kamazawa K., Kajimoto R.

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan (JPS)) 70 2198 - 2198 2015

ISSN 2189-079X

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Elastic Stiffness of a Skyrmion Crystal

Y. Nii, A. Kikkawa, Y. Taguchi, Y. Tokura, Y. Iwasa

PHYSICAL REVIEW LETTERS (AMER PHYSICAL SOC) 113 ( 26 ) 267203 2014.12

Lead author, Corresponding author, Accepted, ISSN 0031-9007

We observe the elastic stiffness and ultrasonic absorption of a Skyrmion crystal in the chiral-lattice magnet MnSi. The Skyrmion crystal lattice exhibits a stiffness 3 orders of magnitude smaller than that of the atomic lattice of MnSi, being as soft as the flux line lattice in type-II superconductors. The observed anisotropic elastic responses are consistent with the cylindrical shape of the Skyrmion spin texture. Phenomenological analysis reveals that the spin-orbit coupling is responsible for the emergence of anisotropic elasticity in the Skyrmion lattice.

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TEM studies of domain formation mechanisms in MnV2O4

Y. Murakami, Y. Nii, T. Arima, D. Shindo, K. Yanagisawa, A. Tonomura

JOURNAL OF ALLOYS AND COMPOUNDS (ELSEVIER SCIENCE SA) 577 S731 - S735 2013.11

Accepted, ISSN 0925-8388

Crystallographic and magnetic domains produced in a spinel-type compound MnV2O4, which exhibits a type of giant magnetostriction attributed to twin boundary motion, have been studied using transmission electron microscopy techniques. Although MnV2O4 undergoes a displacive cubic-to-tetragonal transformation upon cooling, it does not show a well-defined habit plane (i.e. the plane with a specific index that is favored for minimizing the transformation) due to the small elongation/contraction in the lattice. Electron holography demonstrates a considerable reduction in the magnetic signal by cooling the tetragonal phase to 40 K. Despite the elimination of micrometer-scale ferrimagnetic domains, weak magnetic contrast still remained, indicating small residual magnetic domains in particular portions, such as in the crosshatch of twinning pairs. (C) 2012 Elsevier B.V. All rights reserved.

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Interplay among spin, orbital, and lattice degrees of freedom in a frustrated spinel Mn3O4

Y. Nii, H. Sagayama, H. Umetsu, N. Abe, K. Taniguchi, T. Arima

PHYSICAL REVIEW B (AMER PHYSICAL SOC) 87 ( 19 ) 2013.05

Accepted, ISSN 1098-0121

We have investigated magnetic-field (H parallel to[100]) effect on a spin-frustrated tetragonally distorted spinelMn3O4 by measurements of magnetization, dielectric constant, and strain as well as synchrotron x-ray and neutron diffraction. In two low-temperature phases below 40 K, long-wavelength lattice modulations coupled with magnetic order have been confirmed, revealing spin-lattice coupling caused by the exchange striction in the geometrically frustrated Mn3+ spin moments on the distorted pyrochlore lattice. A magnetic-field-induced phase transition has been found at low temperatures, which involves an abrupt change in both structural and magnetic configurations. The magnetostructural phase transition by the application of a magnetic field H parallel to[100] is discussed in terms of interplay among spin, orbital, and lattice degrees of freedom.

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Orbital-spin-coupled fluctuations in spinel vanadate MnV2O 4

Yoichi Nii, Nobuyuki Abe, Taka-Hisa Arima

Physical Review B - Condensed Matter and Materials Physics 87 ( 8 ) 2013.02

Accepted, ISSN 1098-0121

The elastic properties of a spinel vanadate MnV2O4 that has an orbital degree of freedom in the triply degenerate t2g orbital at V3+(d2) sites are investigated by ultrasonic pulse-echo measurement. Considerable softening of the shear elastic constant (C 11-C12)/2 is observed as the temperature approaches the first-order transition from the high-temperature orbital-disordered cubic phase to the low-temperature orbital-ordered tetragonal phase. The softening is attributed to fluctuations between dyz and dzx. Moreover, the elastic anomaly is found to be sensitive to the external magnetic field, revealing a coupling between spin and orbital fluctuations. The elastic anomaly is well reproduced theoretically based on Landau theory and elucidates a characteristic precursor phenomenon in MnV2O4. © 2013 American Physical Society.

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Orbital-spin-coupled fluctuations in spinel vanadate MnV2O4

Yoichi Nii, Nobuyuki Abe, Taka-hisa Arima

PHYSICAL REVIEW B (AMER PHYSICAL SOC) 87 ( 8 ) 2013.02

Accepted, ISSN 2469-9950

The elastic properties of a spinel vanadate MnV2O4 that has an orbital degree of freedom in the triply degenerate t(2g) orbital at V3+(d(2)) sites are investigated by ultrasonic pulse-echo measurement. Considerable softening of the shear elastic constant (C-11 - C-12)/2 is observed as the temperature approaches the first-order transition from the high-temperature orbital-disordered cubic phase to the low-temperature orbital-ordered tetragonal phase. The softening is attributed to fluctuations between d(yz) and d(zx). Moreover, the elastic anomaly is found to be sensitive to the external magnetic field, revealing a coupling between spin and orbital fluctuations. The elastic anomaly is well reproduced theoretically based on Landau theory and elucidates a characteristic precursor phenomenon in MnV2O4. DOI: 10.1103/PhysRevB.87.085111

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Orbital structures in spinel vanadates AV(2)O(4) (A = Fe, Mn)

Y. Nii, H. Sagayama, T. Arima, S. Aoyagi, R. Sakai, S. Maki, E. Nishibori, H. Sawa, K. Sugimoto, H. Ohsumi, M. Takata

PHYSICAL REVIEW B (AMER PHYSICAL SOC) 86 ( 12 ) 2012.09

Accepted, ISSN 1098-0121

Spinel FeV2O4 exhibits successive structural phase transitions, reflecting the interplay between the Fe2+ (3d(6)) and V3+ (3d(2)) ions, both of which have orbital and spin degrees of freedom. The temperature-dependent orbital shapes of Fe2+ and V3+ were investigated by means of single-crystal structure analysis, and were compared with those in MnV2O4, where only the V3+ ions are Jahn-Teller active. The highest-temperature transition from the cubic to the high-temperature tetragonal phase was driven by a ferroic Fe2+ 3z(2)-r(2) orbital order (OO). At 110 K, where the ferrimagnetic transition takes place, the magnetic order modified the orbital shape through intratomic spin-orbit coupling, causing an orthorhombic distortion. The V3+ orbital order (V-OO) contributed to the lowest temperature transition from the orthorhombic to the low-temperature tetragonal phase. The V-OO in FeV2O4 was qualitatively different from that in MnV2O4. We propose that ferro-OO contains a complex orbital in FeV2O4 in contrast to the V-OO of real orbitals observed in MnV2O4.

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Magnetic shape memory effect in orbital-spin-coupled system MnV2O4

Y. Nii, N. Abe, K. Taniguchi, T. Arima

APPLIED PHYSICS LETTERS (AMER INST PHYSICS) 100 ( 5 ) 2012.01

Accepted, ISSN 0003-6951

Magnetic-field(H)-induced shape memory effect without temperature variation is demonstrated by a combination of H-sweep and H-rotation in an orbital-spin-coupled spinel-type insulating system MnV2O4. A rotation of the direction of a magnetic field of 5 T in the low-temperature tetragonal phase gives rise to a large macroscopic strain up to 1%, which stems from a 90 degrees rotation of tetragonal domains. The results show several possible ways of macroscopic shape control in this class of matter, and may open a possibility of high-speed actuators free from the eddy current. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3681581]

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Magnetic domain structure in the orbital-spin-coupled system MnV2O4

Y. Murakami, Y. Nii, T. Arima, D. Shindo, K. Yanagisawa, A. Tonomura

PHYSICAL REVIEW B (AMER PHYSICAL SOC) 84 ( 5 ) 2011.08

Accepted, ISSN 1098-0121

TEM studies have revealed the temperature dependence of the magnetic domain structure in the crystal MnV2O4, which has attracted considerable attention because of the intriguing coupling of structural orders (magnetism, orbital, and lattice) and potential applications of these phenomena. Although this crystal forms well-defined magnetic domains because of long-range ferrimagnetic order, in both the cubic and the tetragonal phases, the magnetic information is dramatically reduced at temperatures below similar to 42 K. The results provide useful information for understanding magnetic anomalies, such as glassy behavior in the thermomagnetization curves, which have been reported in this temperature range.

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Effect of randomness on ferroelastic transitions: Disorder-induced hysteresis loop rounding in Ti-Nb-O martensitic alloy (vol 82, 214104, 2010)

Yoichi Nii, Taka-hisa Arima, Hee Young Kim, Shuichi Miyazaki

PHYSICAL REVIEW B (AMER PHYSICAL SOC) 82 ( 21 ) 2010.12

Accepted, ISSN 1098-0121

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