Qi Wang¹, Philipp Pirro², Andrii Chumak^1
1 Austria Faculty of Physics, University of Vienna, Vienna, Austria
² Fachbereich Physik and Landesforschungszentrum OPTIMAS, Technische Universität Kaiserslautern, Kaiserslautern, Germany
Spin waves, and their quanta magnons, are of great interest as potential data carriers in future low-energy computing devices [1]. The phase of a spin wave provides an additional degree of freedom, while the scalability of structures and wavelengths down to the nanometer regime [2] are further advantages. Recently, a set of magnonic data processing units was demonstrated. Nowadays, the challenge is the realization of an integrated magnonic circuit.
Here, we present the experimental realization of a nanoscale magnonic directional coupler, which consists of two spin-wave waveguides with 350 nm width, separated by a gap of 320 nm [3]. A U-shaped antenna is used to excite spin waves and space-resolved Brillouin Light Scattering (BLS) spectroscopy is exploited for detection. It is shown that the data is coded into the spin-wave amplitude is guided towards one of its two outputs depending on the signal frequency, magnitude, and on the magnetic field. By controlling these degrees of freedom, the multi-functionality and reconfigurability of the device is achieved.
Furthermore, we introduce a method of the inverse-design magnonics, in which any functionality can be specified first, and a feedback-based computational algorithm is used to obtain the device design. To demonstrate the universality of this approach, linear, nonlinear and non-reciprocal functionalities of magnonic devices are explored using the examples of magnonic (de-)multiplexer, nonlinear switch and circulator.

1. A. V. Chumak et al. Nat. Phys. 11, 453 (2015).
2. Q. Wang et al. Phys. Rev. Lett. 122, 247202 (2019).
3. Q. Wang et al. Nat. Electron. (2020) https://doi.org/10.1038/s41928-020-00485-6.

Nanoscaled magnetic elements patterned from ultralow damping thin films of Yttrium-Iron-Garnet have attracted a lot of interest due to the perspective to use them as building blocks of magnonic circuits for logic and data processing. A key element of those magnonic circuits is the use of nonlinear effects to achieve the desired functionally [1]. From another perspective, also the very fundamental phenomenon of magnonic Bose-Einstein condensation (BEC) relies on the nonlinear magnon-magnon interaction [2]. In nanostructured systems, it can be achieved via a rapid-cooling process [3].
In my presentation, I will discuss recent results from the field on nonlinear magnon interaction in YIG nanostructures obtained by Brillouin light scattering spectroscopy and micromagnetic simulations. I will present a novel nonlinear process which is able to connect spin-wave modes of different quantization of the film thickness in a resonant manner over a comparably broad frequency range. In addition, I will demonstrate that the strong quantization [4] of the magnonic band structure in the nanostructures significantly changes the scattering channels and the observed magnon instability processes. This has important implications, for example on the performance of nonlinear data processing elements as well as on the formation of magnonic BEC’s in nanostructures.

1. Q. Wang et al., “A magnonic directional coupler for integrated magnonic half-adders,” Nature Electronics 43, 264001 (2020).
2. M. Mohseni et al, “Bose–Einstein condensation of nonequilibrium magnons in confined systems,” New J. Phys. 22, 083080, (2020).
3. M. Schneider et al., “Bose-Einstein condensation of quasiparticles by rapid cooling.,” Nature Nanotech 15, 457 (2020).
4. Q. Wang et al., “Spin Pinning and Spin-Wave Dispersion in Nanoscopic Ferromagnetic Waveguides,” Phys. Rev. Lett. 122, 247202 (2019).

Spin waves are proposed as information carriers for next-generation computing devices because of their low power consumption. Moreover, their wave-like nature allows for novel computing paradigms. The conventional method to detect spin waves is based on electrical induction, limiting the down scaling and efficiency complicating eventual implementation. In this talk we demonstrate optical detection of spin waves beyond the diffraction limit using a metallic grating that selectively absorbs laser light. Specifically, we demonstrate the detection of propagating spin waves with wavelengths of 700 nm using a diffraction-limited laser spot with a size of 10 um in 20 nm thick Py strips. Additionally, we show that this grating is selective to the wavelength of the spin wave, providing wavevector-selective spin-wave detection. This should open up new avenues towards the integration of the burgeoning fields of photonics and magnonics, and aid in the optical detection of spin waves in the short-wavelength exchange regime for fundamental research.

P. Gruszecki, M. Zelent, K. Sobucki, M. Krawczyk
Focusing electromagnetic waves has a long history, but recent breakthroughs have been made in the control of waves at sub-wavelengths, thanks to metasurfaces. The realization of the magnonic counterpart of photonic metasurfaces still waiting realization, but there are few promising approaches. Coupling of an uniform ferromagnetic film with small magnetic elements is an one of the possibility [1]. In our recent paper we showed realization of a magnonic Gires-Tournois interferometer based on a two-modes subwavelength resonator [2]. The fast mode of the resonator is strongly coupled with propagating wave, while suitable Fabry-Perot resonance condition is created for the short wavelength slow mode. This weak coupling between modes is sufficient to achieve high sensitivity of the phase of reflected waves to the stripe width, and also to the stripe-film separation, both suitable for construction of the metasurface. In another approach [3], we propose to exploit the strength of the RKKY coupling to tune the phase of the transmitted spin waves. Combining the phase-shift dependency along the interface with the lens equation allow us to demonstrate numerically the metalens for spin waves.
Further development of the ways for steering spin-wave phase are required and their experimental demonstration very welcome, for realization of magnonic lenses, but also development of magnonic devices where spin-wave phase control is of key importance.

1. Au, Y.; Dvornik, M.; Dmytriiev, O.; Kruglyak, V. Nanoscale spin wave valve and phase shifter, Appl. Phys. Lett. 100, 172408 (2012).
2. K. Sobucki, W. Śmigaj, J. Rychły, M. Krawczyk, P. Gruszecki, Resonant subwavelength control of the phase of spin waves reflected from a ferromagnetic film edge, arXiv:2007.15226v5 [cond-mat.mes-hall] (2020).
3. M. Zelent, M. Mailyan, V. Vashistha, P. Gruszecki, O. Y. Gorobets, Y. I. Gorobets, and M. Krawczyk, Spin wave collimation using a flat metasurface, Nanoscale 11, 9743 (2019).

Nick Träger¹, Pawel Gruszecki^2,3, Filip Lisiecki³, Felix Groß¹, Korbinian Baumgaertl⁵, Johannes Förster¹, Markus Weigand^1,4, Hermann Stoll^1,6, Hubert Głowiński³, Piotr Kuświk³, Janusz Dubowik³, Gisela Schütz¹, Dirk Grundler⁵, Maciej Krawczyk², Joachim Gräfe^1
1 Max Planck Institute for Intelligent Systems, Stuttgart, Germany
² Faculty of Physics, Adam Mickiewicz University, Poznan, Poland
³ Institute of Molecular Physics, Poznan, Poland
⁴ Helmholtz-Zentrum Berlin, Germany
⁵ Institute of Materials, EPFL, Lausanne, Switzerland
⁶ Johannes Gutenberg-University Mainz, Germany
Over the last two decades, spin wave generation in various materials and geometries has been intensively studied revealing, for example, spin wave wavelengths in the nanometer regime or spin wave interference effects realizing magnonic logic devices [1, 2]. Experiments and theory have shown that magnonic materials in thin films or nanosized structures exhibit highly anisotropic dispersion relations [1, 3]. Therefore, knowing the exact dispersion is a crucial factor in designing and characterizing nanoscaled magnonic waveguides within devices and their prospective capabilities in magnonic applications.
Initially, we address the challenge of determining the full dispersion relation for a large range of frequencies and wavevectors combined with real space imaging. This is achieved by transitioning to time resolved scanning transmission x-ray microscopy (STXM) with high spatial (< 20 nm) and temporal (< 35 ps) resolution for detection of spin waves. We introduce a modified Sinc function in time by an arbitrary waveform generator as excitation signal, which allows for the generation of adjustable frequency bands.
Moreover, we use the ultimate capabilities of time resolved STXM to observe higher order mode excitation in nanoscaled magnonic waveguides. To this end, a global continuous wave excitation causes spin wave emission from the short edges forming a periodic magnetization pattern, which shows symmetric and antisymmetric higher order spin wave modes. Additionally, we present an evaluation technique, which permits mode selective imaging with both amplitude and phase information at the nanoscale. By combining signal theory, x-ray microscopy and k-selective imaging the investigation of spin wave excitation and propagation properties reveals outstanding possibilities for future magnonic ultra-thin structures.

1. Dieterle, G. et al., Phys. Rev. Lett., 2019. 122(11): p. 117202
2. Kithun, A. et al., J. Phys. D Appl. Phys., 2010. 43(26)
3. Lenk, B. et al., Phys. Rep., 2011. 507(4-5): p. 107-136

In this work we demonstrate the laser induced control over spin-wave (SW) transport in the magnonic crystal (MC) waveguide formed from the semiconductor slab placed on the ferrite film. We considered two MCs based on yttrium iron garnet and n-type gallium arsenide films: with periodical grooves on the yttrium iron garnet film and on the n-type gallium arsenide slab. These fabricated structures were studied by the use of microwave spectroscopy and Brillouin light scattering. We show that in the structure with thickness modulated semiconductor layer the appearance of the magnonic gaps can be induced by the laser radiation. We demonstrate that optical control of the magnonic gaps frequencies width and position is related to variation of charge carriers concentration in GaAs. We attribute the optically induced modification of SW dispersion relation to the screening of E-field proportional to carriers concentration within the GaAs. Our results show possibility of integration of magnonics and semiconductor electronics on the base of YIG/GaAs structures.

Frederic Vanderveken^{1, 2}, Jeroen Mulkers³, Jonathan Leliaert³, Bartel Van Waeyenberge³, Bart Soree^{1, 4, 5}, Odysseas Zografos¹, Florin Ciubotaru¹, and Christoph Adelmann¹

¹Imec, 3001 Leuven, Belgium
²KU Leuven, Departement Materiaalkunde, SIEM, 3001 Leuven, Belgium
³Universiteit Gent, Departement Vastestofwetenschappen, DyNaMat, 9000 Gent, Belgium
⁴KU Leuven, Departement Elektrotechniek, TELEMIC, 3001 Leuven, Belgium
⁵Universiteit Antwerpen, Departement Fysica, 2000 Antwerpen, Belgium
In recent years, the coupling between elastic and magnetic degrees of freedom in magnetic materials has regained much interest due to emerging nanoscale spintronic applications such as logic spin wave majority gates, magnetic field sensors and spin-wave antennas. At microwave frequencies, the magnetoelastic interaction manifests itself in a mutual coupling between spin waves and elastic waves, leading to the formation of magnetoelastic waves. While the behavior of these waves has been studied for plane waves in bulk materials and thin macroscopic films, a detailed study of propagating magnetoelastic waves in nanoscale waveguides is still lacking. At the nanoscale, the behavior of these waves substantially differs in comparison with macroscopic system as the waves in both the elastic and magnetic domain are confined and strongly influenced by the interface. Furthermore, analytical treatment becomes more difficult as the solutions of the system of differential equations also depends on the mode profiles themself, suggesting a numerical approach to study the system.
In this work, the behavior and coupling mechanisms of confined magnetoelastic waves in thin nanoscale waveguides are studied numerically. The numerical calculations employ a newly developed mumax3 extension that simultaneous solves the LLG and elastodynamic equations with free boundary conditions. The calculation results allow the calculation of the dispersion relations as well as the mode profiles of the different magnetoelastic modes. Furthermore, the simulations unveil several interesting phenomena, such as mode specific coupling between the confined elastic and magnetic waves deping on the mode profiles symmetry as well as the influence of normal and shear strain components onto the magnetization. In addition, the magnetoelastic coupling is also shown to be able to effectively tune the spin-wave properties based on the magnetoelastic coupling. The findings of this work pave the wave for a better understanding of the confinement effect onto the magnetoelastic coupling and magnetoelastic waves and opens the horizon for other mechanisms to control and manipulate the magnetization state in microwave nanoscale devices.
This work has been supported by Imec’s industrial affiliate program on beyond-CMOS logic. It has also received funding from the European Union’s Horizon 2020 research and innovation program within the FET-OPEN project CHIRON under grant agreement No. 801055. F.V. acknowledges financial support from the Research Foundation – Flanders (FWO) through grant No. 1S05719N. J.L. is supported by the Research Foundation – Flanders (FWO) through a postdoctoral fellowship.

There is a growing interest in the development of tunable oscillators and detectors operating in terahertz (THz) frequency range. Antiferromagnetic (AFM) materials have natural resonance frequencies of spin excitations (antiferromagnetic resonance or AFMR) lying in this frequency range. Some AFM materials can operate at room temperatures, they do not require any bias magnetic field, and can be tuned by changing their anisotropy. In the talk experimental results of spin-pumping in NiO/Pt structures are present. Different interpretations of spin-pumping in the structure are discussed.