Papers - Nii, Yoichi

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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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<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

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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

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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

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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