Magnon-phonon couplingAug 19, 2020 (Wed)Interaction between phonons and magnons has recently attracted a lot of interest in the spintronics
field. The interaction can give rise to novel spintronics functionalities such as non-reciprocaltransport, enhancement of spin Seebeck effect and spin current generation due to lattice vibration.
This topical meeting provides an opportunity to discuss on the such latest topics. We seek a wide
range of opinions on the direction and future prospects of novel spintronics based on magnonphonon interaction.

13:00-13:10 Opening

15:10-15:30 Break

13:10-13:50

15:30-16:10

Jorge Puebla(RIKEN CEMS)

Yukio Nozaki(Keio-U)“Spin current generation andnon-reciprocity via magnonphonon”“Quantitative study on spinvorticity coupling”

13:50-14:30

16:10-16:50

Kei Yamamoto(JAEA)

Mamoru Matsuo(KITS)“Non-reciprocal pumping of
surface acoustic waves by spinwave”“Spin transport driven byfluid and elastic motions”

14:30-15:10

16:50-17:30

Yoichi Nii(Tohoku-U)

Takashi Kikkawa(U-Tokyo)“Nonreciprocal phenomena ofphonon in noncentrosymmetric magnets”“Magnon-phonon
hybridization in thermal spintransport”

17:30-17:40 Closing

https://cems.riken.jp/topicalmeeting/002_magnon-phonon/

"Spin current generation and non-reciprocity via magnonphonon coupling"
Jorge Puebla1
1
RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japanjorgeluis.pueblanunez@riken.jp
In this talk we present our recent work regarding magnon-phonon coupling. We briefly
introduce the mechanism of acoustic spin pumping [1] and demonstrate a spin to charge current
conversion via magnon-phonon coupling and inverse Edelstein effect [2]. We further show thatin the presence of acoustic reflectors forming an acoustic cavity, we observe an enhancement inacoustic spin pumping of about 3 times in the Ni/Cu/Bi2O3 heterostructure [3]. We also
demonstrate the non-reciprocal absorption of surface acoustic waves when interacting withanisotropic magnets. Under optimized conditions based on theoretical predictions we achieve up
to 100% non-reciprocity in a Ta/CoFeB(1.6 nm)/MgO heterostructure [4].

Figure 1: (a) Acoustic spin pumping voltage of a Ni/Cu/Bi2O3 heterostructure in a surface acoustic
wave (SAW) device with (blue open circles) and without (red open squares) acoustic cavity. External
in-plane magnetic field is applied at= 45 , −135 . (b) In-plane magnetic field angle dependenceof non-reciprocity ration of SAW absorption in a Ta/CoFeB(1.6 nm)/MgO heterostructure.

[1] J. Puebla, M. Xu, B. Rana, K. Yamamoto, S. Maekawa, Y. Otani, Journal of Physics D:
Applied Physics 53 (26), 264002 (2020)[2] M. Xu, J. Puebla, F. Auvray, B. Rana, K. Kondou, Y. Otani, Physical Review B 97 (18),180301® (2018)[3] Y. Hwang, J. Puebla, M. Xu, A. Lagarrigue, K. Kondou, Y. Otani, Appl. Phys. Lett. 116,252404, Editor’s choice (2020)[4] M. Xu, K. Yamamoto, J. Puebla, K. Baumgaertl, B. Rana, K. Miura, H. Takahashi, D.
Grundler, S. Maekawa, Y. Otani, Science Advances, in press (2020)

Non-reciprocal pumping of surface acoustic waves by spinwave resonance
K. Yamamoto1,2, W. Yu3, J. Puebla2, M. Xu2,4, S. Maekawa2,5,1 and G. Bauer3,61Advance Science Resaerch Center, Japan Atomic Energy Agency, Tokai 319-1195,
Japan
2
RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan
3
Institute for Materials Research, Tohoku University, Sendai 980-8755, Japan
4
Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa,Chiba 277-8581, Japan
5Kavli Institute for Theoretical Sciences, University of Chinese Academy of Sciences,
Beijing 100049, People’s Republic of China
6Advanced Institute for Materials Research, Tohoku University, Sendai 908-8577,
Japan
Surface acoustic waves (SAWs) are accompanied by rotations of individual atoms near thesurface. There is a locking between the axis of rotation and the direction of propagation, which
implies that left- and right-moving SAWs have opposite senses of the surface atom rotation. The
coupling between the rotation and magnetisation dynamics is expected to depend on whether theaxis of rotation is parallel or anti-parallel to the equilibrium magnetisaion. Recent experiments on
SAW transmission across magnetic scatterers [1,2] demonstrated that this coupling asymmetry
manifests itself as a strong non-reciprocal transmission of SAWs.
In this talk, we theoretically study the SAW nonreciprocity induced by its coupling to magnetisation
dynamics. Starting from the coupled equations of motion for
acoustic and spin waves, we show that the interaction ischaracterised by two effective coupling constants, which
depend on the angle between the equilibrium magnetisationand the SAW wave vector. We analytically show that the twocancel each other at certain angles, leading to vanishing of
the net interaction between magnons and phonons. The
formalism is first applied to the aforementioned transmission
experiments, then used to predict the inverse effect in which
non-reciprocal generation of SAWs is achieved by externally Figure 1: Non-reciprocal generation
driving spin waves.
of SAWs by spin wave resonance[1] R. Sasaki, Y. Nii, Y. Iguchi and Y. Onose, Phys. Rev. B 95, 020407 (2017).
[2] M. Xu, K. Yamamoto, J. Puebla, K. Baumgaertl, B. Rana, K. Miura, H. Takahashi, D.
Grundler, S. Maekawa and Y. Otani, Science Advances, in press (2020).

Nonreciprocal phenomena of phonon in
noncentrosymmetric magnets
Y. Nii1,2, R. Sasaki3, Y. Hirokane4, H. Masuda1, T. Koretsune5, D. Ishikawa6,A. Baron6, and Y. Onose1
1
Institute of Materials Research, Tohoku University, Sendai 980-8577, Japan
2
PRESTO, Japan Science and Technology Agency (JST), Kawaguchi 332-0012, Japan
3
RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan
4
Department of Basic Science, The University of Tokyo, Tokyo 153-8902, Japan
5
Department of Physics, Tohoku University, Sendai 980-8577, Japan
6
Materials Dynamics Laboratory, RIKEN SPring-8 Center, Hyogo 679-5148, Japan
Relativistic spin-orbit coupling (SOC) plays essential roles in condensed matter physics for
emergence of exotic quantum states. Entangled spin-orbital state often induces topological spin
textures in either real or momentum spaces, and results in emergent electromagnetic responses.
Here, we consider its phononic analogue in noncentrosymmetric magnets. Regarding “phononangular momentum” [1] as the pseudo-spin degree of freedom of phonon, we unveil novel
phenomena emerging from phononic-SOC.
First, we show phononic version of Rashba-like splitting in a chiral medium MnSi. By
combining inelastic X-ray scattering and first-principle calculations, we unveiled hedgehog
type acoustic phonon band splitting originating from the crystal chirality. Secondly, we show
nonreciprocal phonon transport in multiferroics [2,3]. By investigating surface acoustic wave
(SAW) propagation in a Ni/LiNbO3 hybrid multiferroic device, we revealed velocity and decayrate becomes inequivalent for counter-propagating SAWs. In a prototypical multiferroic
TbMnO3, on the other hand, nonreciprocal phonon transport has been observed as thermal
rectification effect [3]. To the best of our knowledge, this is the first observation of thermal
diode effect using a single bulk material. Finally, we demonstrate phonon induced
magnetization control in Ni/LiNbO3 device [4]. We show that phonon angular momentum
inherent in SAW can selectively switch magnetization direction. All these results may offer
potential applications in phononics and spintronics field based on the concept ofphononic-SOC.
[1] L. Zhang and Q. Niu, Phys. Rev. Lett., 112, 085503 (2014)[2] R. Sasaki, Y. Nii, Y. Iguchi, and Y. Onose, Phys. Rev. B 95, 020407(R) (2017)[3] Y. Hirokane, Y. Nii, H. Masuda, and Y. Onose, arXiv:2004.03801[4] R. Sasaki, Y. Nii, and Y. Onose, arXiv:2007.03192

Quantitative study on spin-vorticity coupling
Y. Nozaki1,2, S. Tateno1, Y. Kurimune1, M. Matsuo3,4,5 and S. Maekawa4,3
1
Department of Physics, Keio University, Japan
2
Center for Spintronics Research Network, Keio University, Japan
3
Kavli Institute for Theoretical Sciences, University of Chinese Academy of Sciences,China
4RIKEN Center for Emergent Matter Science (CEMS), Japan
5Advanced Science Research Center, Japan Atomic Energy Agency, Japan
Since a novel type of spin current (SC) generation via spin-vorticity coupling (SVC) in asurface acoustic wave (SAW) was theoretically predicted by Matsuo et al. [1], experimental
studies on the SVC had been reported by several groups. In our research group, we succeeded todemonstrate a spinwave resonance (SWR) excitation via alternating SC generated in a NiFe/Cubilayer when the Rayleigh-type SAW was applied [2]. To improve the understanding in the
microscopic mechanism of the angular momentum conversion between microscopic electron spin
and macroscopic angular momentum in the SAW, it is significant to obtain the magnitude andspatial distribution of the SAW in the bilayer system.
In this presentation, we will show you our recent experimental studies on (i) highly
nonreciprocal SWR excited using magnetoelastic coupling in Ni/Si bilayer [3], (ii) reciprocal
SWR excited using gyromagnetic coupling in NiFe single layer [4], and (iii) electricalmeasurement of alternating SC in NiFe/Cu bilayer [5]. We found that the nonreciprocity of theSWR owing to a shear strain component was strongly enhanced by embedding the Ni far fromthe surface. From the variation of the nonreciprocity on the thickness of Si covered on the Ni, we
can estimate the depth profile of the relative amplitude of the shear strain component with respect
to the longitudinal strain component that gives the spatial distribution of the SAW. Moreover, apicometer order SAW amplitude at the surface of the NiFe film was experimentally evaluatedfrom the amplitude of SWR excited via gyromagnetic effect whose amplitude was simply givenby the vorticity of SAW. Finally, from the comparison between the amplitude of the alternating
SC in NiFe/Cu bilayer and the SAW amplitude evaluated, we found that the conversion efficiencyof the angular momentum from the SAW to the electron spin was much larger than in the spincurrent generation using a vorticity of liquid metal [6].
[1] M. Matsuo, J. Ieda, K. Harii, E. Saitoh, and S. Maekawa, Phys. Rev. B 87, 180402(R) (2013).
[2] D. Kobayashi, T. Yoshikawa, M. Matsuo, R. Iguchi, S. Maekawa, E. Saitoh, and Y. Nozaki
Phy. Rev. Lett. 119, 077202 (2017).
[3] S. Tateno and Y. Nozaki, Phys. Rev. Applied 13, 034074 (2020).
[4] Y. Kurimune, M. Matsuo and Y. Nozaki, Phys. Rev. Lett. 124, 217205 (2020).
[5] S. Tateno, M. Matsuo and Y. Nozaki, submitted to Phys. Rev. B.
[6] M. Matsuo, Y. Ohnuma, and S. Maekawa, Phys. Rev. B 96, 020401(R) (2017).

Spin transport driven by fluid and elastic motions
M. Matsuo
Kavli Institute for Theoretical Sciences (KITS), University of Academy of Sciences(UCAS), Beijin Beijing 100190, China
CAS Center for Excellence in Topological Quantum Computation, UCAS, Beijin Beijing
100190, China
RIKEN Center for Emergent Matter Sicence (CEMS), Wako, 351-0198, Japan
Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, 319-1195,
Japan
Spin current is a key concept in spintronics. Spin-current generation has been achieved byusing angular momentum conversion among magnetization, photons, the orbital motion ofelectrons, and spin angular momentum. Recently, the interconversion between the mechanicalangular momentum of vortical objects and spin has attracted much attention [1,2,3,4]. In this talk,I will present our recent results on spin transport driven by fluid and elastic motions as follows.
In a pipe flow of liquid metal, fluid vorticity couples to electron spins, resulting in a spincurrent generation [3]. This effect was demonstrated in a turbulent flow regime [2]. Very recently,
the fluid-driven spin current is observed in pipe flows at a wide range of Reynolds number [5,6],
as the theoretical predictions [3].
Similar spin-vorticity effects are also predicted using the surface acoustic wave injected into
small spin-orbit materials such as Cu [1]. Conventionally, a small spin-orbit material is notconsidered as suitable for a spin-current source because the spin Hall effect relies on the large
spin-orbit coupling. We demonstrated the spin-vorticity coupling allows us to utilize the small
spin-orbit materials as a good spin current source [4]. Besides, the spin-vorticity coupling thatappeared in elastic motion is found to be used for the spinwave excitation in a NiFe thin film [7].
Finally, I also present our recent theoretical work on a magnon current generation [8]. We
theoretically show that a torsional oscillation of a ferromagnetic insulator can yield theDzyaloshinskii-Moriya interaction, and lead to the detectably-large magnon current.
[1] M. Matsuo et al., Phys. Rev. B 87, 180402 (R) (2013).
[2] R. Takahashi et al., Nat. Phys. 12, 52 (2016).
[3] M. Matsuo et al., Phys. Rev. B 96, 020401 (R) (2017).
[4] D. Kobayashi et al., Phys. Rev. Lett. 119, 077202 (2017).
[5] R. Takahashi et al., Nat. Comm. 11, 3009, (2020).
[6] H.T. Kazerooni et al., Phys. Rev. Applied 14, 014002 (2020).
[7] Y. Kurimune et al., Phys. Rev. Lett. 124, 217205 (2020).
[8] J. Fujimoto and M. Matsuo, Phys. Rev. B (R) in press.

Magnon-phonon hybridization in thermal spin transport
Takashi Kikkawa1
1
Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan

Magnetoelastic coupling, the interaction between spin waves (magnons) and lattice waves(phonons), was first investigated more than half a century ago [1] and has renewed attention in
spintronics. By this coupling, magnons and phonons, in the vicinity of the crossings of theirdispersion relations, are hybridized into quasiparticles called magnon polarons that share mixed
magnonic and phononic characters [2]. Magnon polarons can convey spin information withvelocities close to those of phonons, much faster than the magnon velocities in the dipolar magnon
regime [3]. Besides, thanks to the long-lived phononic constituent, magnon polarons may havelonger lifetimes than pure magnons and may enhance spin-current related phenomena. Here wereport the magnon-polaron-induced enhancement of the spin Seebeck effect (SSE), i.e., thermalspin-current generation, and the spin Peltier effect (SPE: the reciprocal of SSE) in magneticgarnets (Y3Fe5O12 and Lu2BiFe4GaO12) [4-7]. The anomalies appear when the magnon andphonon dispersion curves touch, which maximizes the phase space of magnon-polaron formation.
The experimental results are well reproduced by a Boltzmann equation, in which the magnonphonon hybridization is taken into account. The sharp structures can thereby be attributed to thespin current carried by magnon polarons exhibiting longer lifetimes than pure magnons, whichleads to the increased spin-current (heat-current) generation in the SSE (SPE).
The work was done in collaboration with R. Yahiro, R. Ramos, K. Oyanagi, T. Hioki, Y.
Hashimoto, Z. Qiu, K. Uchida, E. Saitoh, G. E. W. Bauer, K. Shen, B. Flebus, and R. A. Duine.
[1] C. Kittel, Phys. Rev. 110, 157 (1958).
[2] A. Kamra, H. Keshtgar, P. Yan, and G. E. W. Bauer, Phys. Rev. B 91, 104409 (2015).
[3] N. Ogawa, W. Koshibae, A. J. Beekman, N. Nagaosa, M. Kubota, M. Kawasaki, and Y.
Tokura, Proc. Natl. Acad. Sci. USA 112, 8977 (2015).
[4] T. Kikkawa, K. Shen, B. Flebus, R. A. Duine, K. Uchida, Z. Qiu, G. E. W. Bauer, and E.
Saitoh, Phys. Rev. Lett. 117, 207203 (2016).
[5] B. Flebus, K. Shen, T. Kikkawa, K. Uchida, Z. Qiu, E. Saitoh, R. A. Duine, and G. E. W.
Bauer, Phys. Rev. B 95, 144420 (2017).
[6] R. Ramos, T. Hioki, Y. Hashimoto, T. Kikkawa, P. Frey, A. J. E. Kreil, V. I. Vasyuchka, A.
A. Serga, B. Hillebrands, and E. Saitoh, Nat. Commun. 10, 5162 (2019).
[7] R. Yahiro, T. Kikkawa, R. Ramos, K. Oyanagi, T. Hioki, S. Daimon, and E. Saitoh, Phys. Rev.
B 101, 024407 (2020).