Magnon-induced switching of nanomagnets is one of the key milestones to realize magnon-based in-memory computing. Spin waves (magnons) have been reported to move domain walls [1] and induce the reversal of Ni81Fe19
(Py) nanostripes (NSs) fabricated on YIG [2]. We report the observation of magnon-induced reversal of bistable nanomagnets
by time-resolved Brillouin light scattering microscopy. We used Py nanostripes separated from YIG via an intermediate insulating layer (Fig. 1). Thereby, we suppressed the interlayer
exchange interaction between Py and YIG. A coplanar waveguide (CPW) was fabricated on top of an array of Py NSs. The device performed as a grating coupler [3]. Short-wave magnons
were emitted into YIG which then propagated underneath remotely positioned nanostripes. We saturated the device at +80 mT and then decreased the field to -24 mT. We measured the
thermal magnon spectrum of the unswitched Py NSs before magnon emission. After magnon excitation we observed characteristic modifications in the magnon spectrum (Fig. 2). Beyond a
critical power level, peak A and peak B vanished and a new peak B’ appeared. These specific modifications indicated the reversal of the nanomagnets and occurred for both the pulsed and
continuous wave magnon excitation. Our experimental data suggest that dipolar coupling is sufficient to achieve magnon-induced switching. We report experiments performed in the linear
and non-linear regime of magnon excitation. The latter experiments are important for the realization of recurrent neural networks based on ferromagnet/ferrimagnet hybrid structures as
proposed in Ref. [4]. Our results suggest that the storage of magnon signals in reversed magnetic bits become possible thus paving the way towards magnon-based in-memory computing
platforms. We acknowledge the financial support from SNSF via Grant No. 197360.
**References** [1] J. Han, P. Zhang, J.T. Hou, Science, Vol. 366, 1121-1125 (2019)
[2] K. Baumgaertl, D. Grundler, submitted
[3] H. Yu, G. Duerr, R. Huber, Nature Communications, Vol. 4, 2702 (2013)
[4] Á. Papp, W. Porod, G. Csaba, Nature Communications, Vol. 12, 6422 (2021) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/1653d0542e758234cbf15706681182ba.png) 

Sketch of the device. An oscillating current is injected in the CPW to excite magnons which propagate in the bare YIG until reaching separately positioned Py nanostripes. Each material
thickness is indicated between parenthesis. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/7dbebbbdfc15c50c042108e45acd650e.png) 

BLS thermal magnon spectra on Py stripes before (magenta) and after (black) magnon excitation at -24 mT.

Reversal of nanostructured ferromagnets by magnon pulses in underlying yttrium iron garnet

Kagome artificial spin ice (KASI) is a network of Ising type nanobars on a kagome lattice [1,2]. Great progress has been made in understanding defects including disordered states in KASI via quasistatic imaging techniques [1,3]. However, the magnetodynamic study of disordered states has been confined to global magnetodynamic (e.g. broadband spin wave spectroscopy) and micromagnetic simulations [4]. Furthermore, the simulation study on KASI has shown the presence of novel magnetodynamic microstates that offer excellent insights into the KASI’s disordered regime [4]. From the fundamental physics side, dynamically controlled microstates may offer a way to create Dirac strings via microwave-assisted switching interior to the KASI lattice in a controlled manner and study the disordered regime systematically. From the magnonic application perspective, experimental studies of microstates in KASIs are key towards their usage as a new type of microwave filter [5] and reprogrammable magnonic crystal [6]. We investigate spin dynamics of a KASI consisting of Ni81Fe19 nanomagnets arranged on an interconnected kagome lattice using broadband spin wave spectroscopy (Fig. a,b), magnetic force microscopy (MFM) (Fig. c), and micro-focus Brillouin light scattering (BLS) microscopy (Fig. e-f). Micro-focus BLS performed on magnetically disordered states exhibit a series of magnon resonances that depend on topological defect configurations that we image by magnetic force microscopy. Nanomagnets on a Dirac string and between a monopole-antimonopole pair show pronounced modifications in magnon frequencies. Our work is key for the creation and annihilation of Dirac strings via microwave-assisted switching and reprogrammable magnonics based on ASIs.

**References:**

1) S. Ladak, D. Read, G. Perkins, L. Cohen, and W. Branford, Nature Physics 6, 359 (2010). 2) R. Wang, C. Nisoli, R. Freitas, J. Li, W. McConville, B. Cooley, M. Lund, N. Samarth, C. Leighton, V. Crespi, et al., Nature 439, 303 (2006). 3) E. Mengotti, L. J. Heyderman, A. F. Rodriguez, F. Nolting, R. V. Hugli, and H.-B. Braun, Nat. Phys. 7, 68 (2011). 4) V. Bhat, F. Heimbach, I. Stasinopoulos, and D. Grundler, Physical Review B 93, 140401 (2016). 5) X. Zhou, G.-L. Chua, N. Singh, and A. O. Adeyeye, Advanced Functional Materials 26, 1437 (2016). 6) M. Krawczyk and D. Grundler, J. Phys.: Condens. Matter 26, 123202 (2014).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/a84d0d93b7a280959772b4805fbd9680.jpg)
Grayscale AESWS spectra map of KASI for a field protocol of (a) -90 mT → +90 mT and (b) -90 mT → 45 mT → 0 mT. (c) Magnetic force microscopy images of KASI after a field protocol of −100 mT → +44 mT → 0 mT. Stray fields detected by MFM are colored in blue and red consistent with charges Q = +q and Q = −q, respectively. (d) Sketches of charge configurations (blue and red spheres) and magnetization vectors M (arrows) for configurations C1 to C5. (e) BLS intensities measured at the central position on horizontal nanobars belonging to the configurations displayed above the spectra at µ0H = 25 mT. (f) Spatially resolved BLS intensity maps at 25 mT for fixed frequencies in configurations C1 to C5 displayed above.

 Direct Observation of Magnon Modes In Kagome Artificial Spin Ice with Topological Defects

Spin waves (SWs) are promising quanta for transmission and processing of information for future communication devices. Excitation and detection of beyond 20 GHz magnons with sub-100 nm wavelength are of key importance [1]. We report on the magnonic grating coupler (MGC) effect [2] for which prepared specific nanopillars. Depending on the type of MGC lattice and nanopillar diameters, magnons are emitted resonantly on the chip in a multitude of directions with different efficiency. We demonstrate the interference effect of short-wave magnons independent of propagation directions. We engineered grating couplers for broadband magnon emission by preparing 100-nm-thick permalloy nanopillars in a periodic square lattice and an aperiodic (Penrose P3) quasicrystalline arrangement [3] on insulating ferrimagnetic yttrium iron garnet (YIG) films. Characteristic lengths amounted 480 nm. Several sub-micron-wide coplanar waveguides (CPWs) with separations of 12 micrometers were integrated on top of the MGC samples to excite/detect SWs by broadband spectroscopy. The MGC sample based on the quasicrystalline arrangement allowed for SW emission omnidirectionally with a broad range of wave vectors. SWs of up to 25.3 GHz were detected in the square lattice-based MGC sample. These parameters were achieved in very small magnetic fields of only a few mT in contrast to previous studies on grating couplers [2]. SWs simultaneously excited by two emitters interfered constructively/destructively at a separate detector antenna depending on the phase difference of the two phase-coherent SWs. Thereby a binary 1/0 output operation could be realized with 70 nm-wavelength magnons over distances of more than 10 micrometers. The applied field direction and specific microwave frequency defined the SW emission direction for the interference experiment. Our study and findings on sub-100 nm magnons are important for future magnonic devices targeting at information processing via wave-logic on the nanoscale. The work was supported by SNSF grant 163016.

**References:**

[1] A.V. Chumak, V. I. Vasyuchka, A. A. Serga et al., Nature Physics, Vol.11, p. 453 (2015) [2] H. Yu, G. Duerr, R. Huber et al., Nat. Commun., Vol. 4, p. 2702 (2013) [3] S. Watanabe, V.S. Bhat, K. Baumgaertl et al., Adv. Funct. Mater., Vol. 30, p. 2001388 (2020)

**Images:**

 Towards Wave-based Logic Operation using Multi-directional sub-100 nm Magnons

Chiral magnets hosting non-collinear spin textures such as skyrmion lattices (SkL) [1] provide novel functionality in magnonics because of the formation of bandgaps in the magnon band structure and asymmetric magnon dispersion relations [2]. Multiple SkL phases were observed in the bulk chiral magnet Cu2OSeO3, e.g. high temperature SkL near TC, low temperature SkLs when the magnetic field was applied along an easy axis and meta-stable SkL when Cu2OSeO3 was quenched by fast cooling down [3-5]. Their dynamic modes are hard to be discriminated in large bulk samples because they coexist with conical and helical phases. Conventionally detected spin excitations (magnons) show complex spectra with a multitude of resonances due to multiple domains of different phases and boundary conditions. It is challenging to understand the one-to-one correspondence between an individual SkL phase and its magnon spectra. Here we report the local spectroscopy of magnons by scanning Brillouin light scattering (BLS) microscopy on Cu2OSeO3 in SkL phases. Thanks to the high sensitivity of BLS, we resolved the dynamic behavior of all reported SkL phases. Clockwise, counterclockwise and breathing modes of hexagonal SkL were detected by BLS proving simultaneous thermal excitation of all three modes which exhibit different polarization characteristics. Using the magnon modes, we were able to locate SkL phases and investigate the stabilization mechanism of non-collinear multi-domains. Our observations of locally excited thermal magnons in Cu2OSeO3 deepen the understanding of the SkL dynamics in chiral magnet. They pave the way for further design of magnonic devices based on chiral magnets. We acknowledge the finical support from Swiss National Science Foundation (SNSF) via Sinergia Network NanoSkyrmionics CRSII5 171003.

**References:**

[1] S. Mühlbauer, B. Binz and F. Jonietz, etc. Science, 323, 915 (2009). [2] M. Garst, J. Waizner and D. Grundler, J. Phys. D. Appl. Phys., 50, 293002 (2017). [3] A. Chacon, L. Heinen, M. Halder, etc. Nat. Phys. 14, 936-941 (2018). [4] F. Qian, L.J. Bannenberg, H. Wilhelm, etc. Sci. Adv. 4, eaat7323 (2018). [5] R. Takagi, Y. Yamasaki, T. Yokouchi, etc. Nat. Commun., 11, 5685 (2020).

**Images:**

 Spin Dynamics of Skyrmion Lattices in a Chiral Magnet Resolved by Micro-Focus Brillouin Light Scattering

Ferromagnetic nanotubes generate continuous interest due to non-collinear spin structures which are tailored by the curvature of the ferromagnetic film [1]. If arranged vertically on a chip [2], they allow for high-density arrays and 3D device architectures in spintronics and magnonics. For their realization, coating of filamentary nanotemplates would be advantageous. However, conformal deposition of ferromagnetic metals is still a challenge. We report the fabrication of Ni nanotubes by atomic layer deposition (ALD) applied to single-crystalline semiconductor nanowires. We used nickelocene as a precursor, water as the oxidant agent and an in-cycle plasma enhanced reduction step with hydrogen. The optimized ALD pulse sequence, combined with a post-processing annealing treatment, allowed us to prepare 30 nm thick metallic Ni layers with a low resistivity of 8 μΩcm at room temperature and good conformality both on the planar substrates and nanotemplates [3]. We prepared several micrometer-long Ni nanotubes with diameters ranging from 120 to 330 nm. We report on the correlation between ALD growth and functional properties of individual Ni nanotubes characterized in terms of magneto-transport at room temperature and the confinement of spin wave modes explored by micro-focus Brillouin light scattering. For the relative variation of resistance R of a nanotube in small magnetic fields we find a value of up to 1.3 % which we attribute to the anisotropic magnetoresistance (AMR) effect. This value is larger by about 30 % compared to the best low-field AMR effect of Ni nanotubes studied previously [4,5]. In the ALD process applied to semiconductor nanowires in Refs. [4,5] the plasma enhancement of the hydrogen reduction step was not available. Together with the spin-wave resonances observed in the Ni nanotubes our findings promise Ni-based 3D device architectures for spintronic and magnonic devices prepared by ALD and operated in the GHz frequency regime. Funding by the German Science Foundation DFG via GR1640/5-2 in SPP1538 Spin caloric transport and SNF via grants 163016, BSCGI0157705, and NCCR QSIT is gratefully acknowledged.References: [1] R. Streubel, P. Fischer, F. Kronast et al., J. Phys. D: Appl. Phys., Vol. 49, p. 363001 (2016). [2] Y.-P. Wang, Z.-J. Ding, Q.-X. Liu et al, J. Mater. Chem. C, Vol. 4, p. 11059 (2016). [3] M.C. Giordano, K. Baumgaertl, S. Escobar Steinvall et al., ACS Appl. Mater. Interfaces (2020), https://doi.org/10.1021/acsami.0c06879. [4] D. Rüffer, R. Huber, P. Berberich et al., Nanoscale, Vol. 4, p. 4989 (2012). [5] D. Rüffer, M. Slot, R. Huber et al., APL Mater., Vol. 2, p. 076112 (2014). 

Nickel nanotubes with low resistivity and coherent magnetization dynamics for 3D spintronics and magnonics prepared by atomic layer deposition

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Presentations

Reversal of nanostructured ferromagnets by magnon pulses in underlying yttrium iron garnet

Direct Observation of Magnon Modes In Kagome Artificial Spin Ice with Topological Defects

Towards Wave-based Logic Operation using Multi-directional sub-100 nm Magnons

Spin Dynamics of Skyrmion Lattices in a Chiral Magnet Resolved by Micro-Focus Brillouin Light Scattering

Nickel nanotubes with low resistivity and coherent magnetization dynamics for 3D spintronics and magnonics prepared by atomic layer deposition

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