YoshiChika OTANI, Magnon-Phonon Coupling in Spintronics

ISSP, the University of Tokyo, Kashiwa 277-858, Japan
CEMS RIKEN Wako 351-0198, Japan

Attention, le séminaire aura lieu à 10h00 (amphi Blandin du LPS au lieu de 14h00 comme d’habitude et sera diffusé
sur zoom: https://zoom.us/j/97017697946?pwd=QXlBOUhaYmFEQk5FeTBaYWs2aFhPZz09.

Methods for generating spin current and its conversion to electrical charge have been vigorously
studied in recent years, mainly based on physical phenomena at the nanoscale [1]. In the presence of
magnetic materials, spin currents can be generated by magnon-photon or magnon-phonon coupling.
Magnon-phonon coupling can be achieved by passing surface acoustic waves (SAW) across
ferromagnetic layers due to the magnetoelastic effect [2], driving magnetization dynamics such as
ferromagnetic resonance (FMR). This process is known as acoustic FMR(A-FMR), and is
analogous to the most common FMR driven by electromagnetic wave photons. Both FMR and A-
FMR are used as a mechanism for the spin current generation, which can be injected into adjacent
nonmagnetic layers, a process better known as spin pumping [3]. We have recently demonstrated
the spin to charge current conversion via magnon-phonon coupling and inverse Edelstein effect at
the hybrid device Ni/Cu(Ag)/Bi2O3. The results show four-fold butterfly shape signals, the acoustic
spin pumping fingerprint (A-SP) [4]. We have also observed the magneto-rotation coupling in a
similar system, ultra-thin film Ta/CoFeB(1.6 nm)/MgO, which induces nonreciprocal acoustic wave
attenuation with an unprecedented ratio up to 77% and theoretically predicted 100% rectification at
the optimized condition [5]. We demonstrated strongly nonreciprocal acoustic attenuation in the
power and resonance field separately. These intriguing nonreciprocal features of the presented
acoustic devices suggest an extraordinary versatility of acoustomagnetic applications. The marked
angular dependence of the nonreciprocal ratio indicates an efficient and adjustable acoustic rectifier.
The systematic change of non-reciprocity in the resonance field presents its capability as a new
route for the characterization of DMI. Our results indicate that surface acoustic wave provides an
efficient tool to generate spin currents and shows its potential to develop spintronic devices.

[1] Y. Otani, M. Shiraishi, A. Oiwa, E. Saitoh, and S. Murakami, Nature Physics 13, 829 (2017).
[2] M. Weiler, L. Dreher, C. Heeg, H. Huebl, R. Gross, M. S. Brandt, and S. T. B. Goennenwein,
Physical Review Letters 106, 117601 (2011).
[3] Y. Tserkovnyak, A. Brataas, and G. E. W. Bauer, Physical Review B 66, 224403 (2002).
[4] M. Xu, J. Puebla, F. Auvray, B. Rana, K. Kondou, Y. Otani, Phys. Rev. B. 97, 180301 (2018).
[5] M. Xu, K. Yamamoto, J. Puebla, K. Baumgaertl, B. Rana, K. Miura, H. Takahashi, D. Grundler, S.
Maekawa, and Y. Otani, Sci. Adv. 6, 1724-1~4 (2020).