Papers - Nii, Yoichi

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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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<p>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。</p>

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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

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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

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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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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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