New fascinating phenomena involving coupled spin waves and phonons are observed at the magnetoelastic resonance as acoustically-assisted spin pumping [1, 2] and magnetization swiching [3, 4]. In this context, Surface Acoustic waves (SAW), a mature technology widely used in today's sensors, filters and microwave circuitry trigger efficiently magnetization dynamics: resonant effects are obtained with appropriate intensities and directions of the external magnetic field (Bext). This tunability is an asset for magnonic applications of SAW-FMR (ferromagnetic resonance). Recently, SAW-FMR has been obtained in Fe epitaxially grown on GaAs(001), a spintronic and magnonic compatible magnetoelastic and piezoelectric heterostucture.[5] Here, we show that SAW propagation in Fe thin films is strongly affected by the SWs modes dispersion in the reciprocal space, even at very low K-values. It turns out that the Bext-dependence of the sound velocity depends a lot of the angle between Bext and the SAW vector directions. In particular, the resonant SAW-FMR is possible only in the backward configuration. Furthermore, we give a theoretical description of the observed phenomena by introducing calculated dispersion curves into a phenomenological and approximated approach derived by LLG equations. [6,7] This approach catches the physics of SAW propagation in a Fe thin films, as attested by the comparison with quasi-exact elastic calculations. We give a very good description of the SAW-SW interaction that facilitates the design of magnonic devices where SWs are generated by SAW Interdigital transducers (IDT) and not by much more energy-dissipative inductive antenna.

Effects of Spin Wave Dispersion on Surface Acoustic Wave Velocity

Spintronics research concentrated its efforts on handling spin polarization current leading to the success-story of giant magnetic resistance spin valves [1-2]. More recently, pure spin currents, i.e. a spin flux without charge flux, are at the centre of interest since they do not produce Joule heating, reducing power dissipation in spintronics devices, as in STT-MRAM [3]. These applications require an increase in the emission and detection efficiencies via a full understanding of the complex physics behind pure spin currents. Spin current can be detected by inverse spin Hall effect (ISHE) that corresponds to the conversion of a spin current into a detectable charge current by measuring the voltage on a normal metal presenting a strong spin-orbit interaction. Pure spin current emission can be obtained by temperature gradients, by charge currents and by magnetization dynamics. This last mechanism permits to generate pure spin current via ferromagnetic resonance in a ferromagnetic material [4]. We will present acoustic spin pumping experiments in Co/Pt bilayers, using surface acoustic waves (SAW) to induce the magnetization precession in the Co layer taking advantage of resonant magnetoelastic coupling. We will report a peculiar behaviour of SAW-FMR assisted spin pumping in a multiferroic system where Co/Pt bilayer is in contact with a ferroelectric substrate LiNbO3 [5]. We will show that the substrate on which the bilayer is deposited plays a role in the spin current excitation processes. We will show that the electrical polarization vector affects the ISHE voltage: the inversion of the magnetization and of the SAW propagation vector modifies the spin pumping efficiency. We suggest that this polarization is an important factor in the spin pumping excitation processes. The nature of the coupling with the magnetic polarization may be due to a magnetoelectric coupling at the interface between LiNbO3 and Co, inducing non equilibrium spin density across the whole thin film [6] and affecting spin pumping efficiency.

**References:**

[1] M. N. Baibich, J. M. Broto, A. Fert, F. N. Van Dau, F. Petroff, P. Eitenne, G. Creuzet, A. Friederich, and J. Chazelas, Phys. Rev. Lett. 61, 2472 (1988). [2] J. M. George, L. G. Pereira, A. Barthelemy, F. Petroff, L. Steren, J. L. Duvail, A. Fert, R. Loloee, P. Holody, and P. A. Schroeder, Phys. Rev. Lett. 72, 408 (1994). [3] S. Maekawa, S. O. Valenzuela, E. Saitoh, and T. Kimura, Spin Current (Oxford University Press, 2017) pp. 1-520. [4] M. Weiler, H. Huebl, F. S. Goerg, F. D. Czeschka, R. Gross, and S. T. B. Goennenwein, Phys. Rev. Lett. 108, 176601 (2012). [5] P. Rovillain, R. Cardoso de Oliveira, M. Marangolo, and J.-Y. Duquesne Phys. Rev. B 102, 184409 (2020). [6] C.-L. Jia, T.-L. Wei, C.-J. Jiang, D.-S. Xue, A. Sukhov,and J. Berakdar, Phys. Rev. B 90, 054423 (2014).

**Images:**

 Nonsymmetric spin pumping in a multiferroic heterostructure using surface acoustic wave

New fascinating phenomena involving coupled spin waves and phonons are observed at the magneto-elastic resonance: magnetic field dependent spin Seebeck effect [1], acoustically-assisted spin pumping [2, 3] and magnetization swiching [4, 5]. In this context, it has been shown that an efficient mean to excite phonons is by surface acoustic waves (SAW). The integration of SAW-FMR (ferromagnetic resonance) mechanism in spintronic and magnonic devices are claimed to operate with very low power. However, most ferromagnetic materials have precession frequencies well above 5 GHz, i.e. above the highest frequency reached by today's SAW technologies. This restricts the research on SAW-FMR to low magnetization and/or low magnetic anisotropy materials, such as (Ga,Mn)As or (GaMn)(As,P), Ni, and YIG. Here, we show that thin films of Fe epitaxially grown on GaAs(001), a spintronic and magnonic compatible magnetoelastic and piezoelectric heterostucture, give the opportunity to obtain SAW-FMR, at room temperature, below 1 GHz despite the high Fe magnetization. This is obtained by exploiting the softening of the magnetic stiffness when the field is applied along a hard axis, at moderate intensity (60 mT). Slight variations of the magnetic field direction, less than 0.1 deg off, increase dramatically magnetization precession frequency leading to off-resonance interaction and suppressing SAW attenuation. The observed phenomena are described by adopting a very simple magnetization dynamics model that catches the physics of the interaction. The observation of resonant, low frequency and highly directional magnetoelastic coupling in Fe suggests that SAW mature technology offers the opportunity to magnonics and spintronics to handle magnetization precession in a remote way, without using any dissipative electron current. [6]References: [1] Takashi Kikkawa, et al., Phys. Rev. Lett. 117, 207203, (2016) [2] M. Weiler, et al. Phys. Rev. Lett. 108, 176601, (2012) [3] Ken ichi Uchida, et al., Solid State Communications 198, 26, (2014) [4] L. Thevenard,et al. Phys. Rev. B 93, 134430, (2016) [5] P. Kuszewski, et al. Journal of Physics: Condensed Matter 30, 244003 (2018) [6] J.-Y. Duquesne, P. Rovillain, et al. Phys. Rev. Appl. 12, 024042 (2019)

Sub-GHz resonant magnetoelastic coupling in epitaxial Fe thin films

**Session chair:** Jaroslaw Klos, Uniwersytet im Adama Mickiewicza w Poznaniu


HOK - Spin Waves and Magnetization Dynamics I

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SHORT BIO

Presentations

Effects of Spin Wave Dispersion on Surface Acoustic Wave Velocity

Nonsymmetric spin pumping in a multiferroic heterostructure using surface acoustic wave

Sub-GHz resonant magnetoelastic coupling in epitaxial Fe thin films

Sub-GHz resonant magnetoelastic coupling in epitaxial Fe thin films

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