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In recent years, artificial spin ice (ASI) systems have seen burgeoning interest due to their intriguing physics and potential applications in reprogrammable memory, logic and reconfigurable magnonics [1-2]. As the field progresses, direct in-depth comparisons of distinct artificial spin systems are crucial to advancing the field. While studies have investigated the effects of different lattice geometries, little comparison exists between systems comprising continuously connected nanostructures, where spin-waves (SWs) propagate &lt;i&gt;via&lt;/i&gt; dipole-exchange interaction, and systems with nanobars disconnected at vertices where SW propagation occurs &lt;i&gt;via&lt;/i&gt; stray dipolar-field. Here, we study the magnonic response of two kagome spin ices, a continuously connected system (c-ASI) and a disconnected system (d-ASI) with vertex gaps &lt;i&gt;via&lt;/i&gt; Brillouin light scattering (BLS) (see Fig. 1(a) for schematic) and micromagnetic simulation [3]. Array of 25-nm-thick Ni&lt;sub&gt;80&lt;/sub&gt;Fe&lt;sub&gt;20&lt;/sub&gt; (Py) kagome ASI were fabricated using electron-beam lithography and lift-off process. For both samples, the nanobars have length (&lt;i&gt;l&lt;/i&gt;) and width (&lt;i&gt;w&lt;/i&gt;) of 300 nm and 80 nm, respectively and d-ASI has a 50 nm vertex gap (Figs. 1(b,c)). Bias magnetic field (&lt;i&gt;H&lt;/i&gt;) dependent SW spectra (Figs. 1(d,g)) measured by BLS reveal that both c-ASI and d-ASI exhibit frequency minimum, occurring at negative bias magnetic fields, when ramped down from a positive saturation field. A sharp jump in the mode frequencies was observed beyond the minima, linked to bar reversal &lt;i&gt;via&lt;/i&gt; the switching field observed from the magneto-optical Kerr effect loops and the corresponding mixed magnetization state of nanobars aligned parallel and anti-parallel to the magnetic bias field. Furthermore, we elucidate striking differences in spatial mode-localization and mode quantization between dipole-exchange-mediated c-ASI and dipolar-coupled d-ASI (Figs. 1(e,f) and 1(h,i)). These observations are pertinent for the fundamental understanding of artificial spin systems and broader design and engineering of reconfigurable functional magnonic crystals.

![](https://assets.underline.io/uploads/markdown_image/1/image/760d0a123abfca6bca5645ed091efa1f.jpg)

Figure 1(a) Schematic of the BLS measurement geometry. (b,c) SEM images. (d-i) SW mode frequencies as a function of H together with the phase profiles for M1 and M3.



2022 Joint MMM-INTERMAG

Comparison of Spin-Wave Modes in Connected and Disconnected Artificial Spin Ice Nanostructures using Brillouin Light Scattering

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

The 2022 Joint Conference will offer both in-person and prerecorded on-demand content.

For tickets to the event, please register at the following link: https://magnetism.org/register

Please register for the Joint MMM-Intermag conference to join the event.

This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

In recent years, artificial spin ice (ASI) systems have seen burgeoning interest due to their intriguing physics and potential applications in reprogrammable memory, logic and reconfigurable magnonics [1-2]. As the field progresses, direct in-depth comparisons of distinct artificial spin systems are crucial to advancing the field. While studies have investigated the effects of different lattice geometries, little comparison exists between systems comprising continuously connected nanostructures, where spin-waves (SWs) propagate via dipole-exchange interaction, and systems with nanobars disconnected at vertices where SW propagation occurs via stray dipolar-field. Here, we study the magnonic response of two kagome spin ices, a continuously connected system (c-ASI) and a disconnected system (d-ASI) with vertex gaps via Brillouin light scattering (BLS) (see Fig. 1(a) for schematic) and micromagnetic simulation [3]. Array of 25-nm-thick Ni80Fe20 (Py) kagome ASI were fabricated using electron-beam lithography and lift-off process. For both samples, the nanobars have length (l) and width (w) of 300 nm and 80 nm, respectively and d-ASI has a 50 nm vertex gap (Figs. 1(b,c)). Bias magnetic field (H) dependent SW spectra (Figs. 1(d,g)) measured by BLS reveal that both c-ASI and d-ASI exhibit frequency minimum, occurring at negative bias magnetic fields, when ramped down from a positive saturation field. A sharp jump in the mode frequencies was observed beyond the minima, linked to bar reversal via the switching field observed from the magneto-optical Kerr effect loops and the corresponding mixed magnetization state of nanobars aligned parallel and anti-parallel to the magnetic bias field. Furthermore, we elucidate striking differences in spatial mode-localization and mode quantization between dipole-exchange-mediated c-ASI and dipolar-coupled d-ASI (Figs. 1(e,f) and 1(h,i)). These observations are pertinent for the fundamental understanding of artificial spin systems and broader design and engineering of reconfigurable functional magnonic crystals.

![](https://assets.underline.io/uploads/markdown_image/1/image/760d0a123abfca6bca5645ed091efa1f.jpg)

Figure 1(a) Schematic of the BLS measurement geometry. (b,c) SEM images. (d-i) SW mode frequencies as a function of H together with the phase profiles for M1 and M3.

technical paper

The presence of a Dzyaloshinskii-Moriya interaction (DMI) in perpendicularly magnetised thin films stabilises Néel type domain walls of fixed chirality, as opposed to the Bloch walls that are formed in the absence of a DMI [1]. Néel walls of fixed chirality have been shown to be driven efficiently in the same direction by electric currents through spin-orbit torques [2,3]. Later, a large tuning of the DMI with an applied voltage was demonstrated [4], which can be used e.g. for electric field control of magnetic domain wall motion via the modulation of the DMI [5]. Here, we demonstrate an alternative mechanism for the control of domain wall type: using micromagnetic simulations and analytical modelling, we show that the presence of a uniaxial in-plane magnetic anisotropy can also lead to the formation of Néel walls in the absence of a DMI. It is possible to abruptly switch between Bloch and Néel walls via a small modulation of both the in-plane, but also the perpendicular magnetic anisotropy [6]. This opens up a route towards efficient electric field control of the domain wall type with tiny applied voltages, as the magnetic anisotropy can be modulated via the direct voltage controlled magnetic anisotropy (VCMA) mechanism [7], or via magnetoelastic anisotropy induced through coupling to a piezoelectric element [8].

![](https://assets.underline.io/uploads/markdown_image/1/image/7bef5f5df8869fd862ccaab94b5a3626.jpg)

Fig. 1 In a perpendicularly magnetised thin film, domain walls (DWs) are generally of Bloch type (bottom left). The presence of a DMI (top left), stabilises Néel walls. Alternatively, and in-plane magnetic anisotropy can also form Néel walls (bottom right). An increase in the perpendicular magnetic anisotropy switches the DW type back to Bloch (top right).

Switching between Magnetic Bloch and Néel Domain Walls with Anisotropy Modulations

Spin waves are promising to develop post-CMOS and/or green technologies due to their sub-micron wavelengths in the GHz range, their frequency tuning with magnetic or electric fields and their small energy dissipation during propagation. Many spin waves based applications have already been demonstrated for data processing and other microwave devices (filters, resonators, delay lines…). To reach these objectives, magnonic crystals (MC) consisting of a periodic modulation of magnetic properties are among the most promising geometries. One key point for applications with MC is to remove the static bias field as most devices operate at zero field. So technological developments imply to find solutions to obtain well-defined spin waves modes at remanence, with the possibility to modify purposely the spin wave spectrum to achieve different functionalities. While several approaches for reconfigurable states have been proposed [1-4], a “material approach” consisting in exploiting the crystal anisotropy of magnetic materials has been poorly explored up to now. Here we show by micromagnetic simulations that materials such as Co2MnSi Heusler alloy with strong cubic anisotropy can be used to obtain very different spin waves modes at remanence in a simple and classical geometry (Fig. 1 and Fig. 2). In particular, we demonstrate the variation of remanent configuration after saturation with a weak external field H0 (&lt; 50 Oe) in different directions. This allows to decrease the amplitude, extinct or shift the frequencies of the different modes depending if they correspond to edge or volume modes. We will show the advantage of using cubic anisotropy with respect to isotropic material by comparing the amplitude variation of the different modes. Finally, we will also show that proper design of the crystal with respect to the cubic anisotropy directions can achieve a microwave response very robust against magnetic perturbation.

![](https://assets.underline.io/uploads/markdown_image/1/image/53c4c32972d9d1d95c4dd4277757c834.jpg)

Fig. 1: Remanent magnetic states for different H0 directions.The dot edge size is 300 nm and the spacing is 900 nm.


![](https://assets.underline.io/uploads/markdown_image/1/image/5d2d5ca648dfd24fea180bab3925633e.jpg)

Fig. 2: Corresponding FFT spectrum for a RF field along x. The calculated spatial profiles of the spin waves modes at particular frequencies are shown for the configuration A.

Reconfigurable dynamic states in Heusler based magnonic crystals

Optical frequency comb is a light source consisting of a series of discrete and equally spaced frequency lines. Since its inception, optical frequency comb has dramatically improved the accuracy of frequency metrology [1] and thus leads to one half of the Nobel Prize in Physics in 2005. Recently, an analogue of optical frequency comb in an acoustic system has been realized in experiment [2], which provides vast opportunities for both fundamental and applied research in phononics. On the other hand, magnons, as elementary excitations in the ordered magnets, are akin to photons and phonons as bosonic quasiparticles. However, the frequency comb of magnons has not been realized to date. The fundamental difficulty is the smallness of nonlinearity in a normal ferromagnet, which hinders the form of frequency comb structure. In this work [3], we realize a novel magnonic frequency comb induced by the nonlinear coupling between magnetic skyrmion and spin waves excited by a microwave field. Here the nonlinear interaction is significantly enhanced by the topological structure of skyrmions and thus leads to the generation of frequency comb through three-magnon process. The mode spacing of frequency combs is equal to the frequency of the skyrmion breathing mode, which can be efficiently tuned by both magnetic and electric means. The essential physics can be further extended to a wide class of magnetic solitons including antiskyrmions, domain walls, and vortices. Our results provide a novel avenue to study frequency comb physics combining the advantages of magnons and general magnetic textures and may further benefit its applications in the spin-wave calibration and metrology.

![](https://assets.underline.io/uploads/markdown_image/1/image/7f0b08234acae8f54655225ca4fd19be.jpg)

Magnonic Frequency Comb Through Nonlinear Magnon-Skyrmion Scattering

Magnonic Frequency Comb Through Nonlinear Magnon-Skyrmion Scattering

Optical frequency combs are well established and powerful tools for synchronization, stabilization and frequency conversion in both fundamental science and technological applications. In this work, we present experimental observations on the generation of spin-wave frequency combs. Nonlinear four-magnon scattering is known to cause parasitic losses for a driven spin-wave mode with respect to coherence and amplitude. By means of Brillouin light scattering microscopy, we demonstrate that adding a second microwave frequency causes the scattering to populate well-defined states instead of redistributing energy to the entire spin-wave band. Here, the amplitudes and frequencies of both microwave sources can be utilised in order to tune this behaviour. When reducing the sizes of the magnetic conduits, lateral confinement causes a discretisation of the eigenmode spectrum. Therefore, the formation of frequency combs can be observed even when driving the conduits into the nonlinear regime by just a single pumping frequency. The tunability of multimode spin-wave spectra, as demonstrated in our experiments, offers promising technological applications that allow to bridge the frequency gap between CMOS technology and spin waves.

Spin-wave frequency combs

Strong coupling between magnons and different quantum particles was studied extensively for quantum electrodynamics in the past few years. In this system, magnons in magnetic materials with high spin density are utilized where the “coupling strength” is collectively enhanced by square root of number of spins (N) to overcome the weaker coupling strength between individual spins and the microwave field. However, the strongly magnon-magnon [1] couplings in a confined nanomagnet remains to be revealed which is important for on chief intregration of hybride system. Here, we report the interaction between different magnon modes [2] in a magnetic dot array. The intermodal coupling in the strong coupling regime was approached with a maximum coupling strength of 0.745 GHz and cooperativity of 2.5. Furthermore, it is found that the coupling strength is highly dependent on the orientation of bias field. Micromagnetic simulation reveals that the compitetion between external bias field and inter dot dipolar interaction create a movement of domain in between the dot which results the anticrossing. The tunable coupling strength with the strength and orientation of bias field opens the opportunity of extremely controlled hybrid magnonic devices. This findings could greatly enrich the still evolving field of quantum magnonics.

![](https://assets.underline.io/uploads/markdown_image/1/image/1000a626fbb67c2a7ac444f1916e06d6.jpg)

Figure 1: (a) Bias field (Hext) dependent spin wave (SW) absorption spectra of NiFe nanodot lattice are shown at φ = 45°. (b) Distribution of internal at 400 Oe bias field strength along the 45 degree orientation (φ = 45°). The color map for the surface plots and internal field profile with the schematic of the external applied field (Hext) are given at the bottom right corner of the figure.

Strong Magnon-Magnon Coupling in Two-Dimensional Diamond Shaped Ferromagnetic Nanodots Array

Development of structuring technologies for thin-film magnetic micro- and nanostructures make it possible rapid growing number and variety of studies of mechanisms and methods of excitation and control of spin wave (SW) in magnetic materials [1]. Using of the effects of dipole coupling of spin waves in lateral stripes leads to a periodic transfer of power between the stripes, while the spin-wave coupling can be used to implement the modes of spatial-frequency selection SW [2]. In the regime of multimode propagation of a spin-wave signal in a system of laterally coupled magnetic microwave guides, it turns out to be impossible to completely ensure the transfer of the propagating SW energy from one magnetic microstrip to another. In the same time over the last few years, have become a subject of striking interest structures with periodic changes in magnetic or geometric parameters due to the great potential of the application for signal and information processing devices. Such structures are named magnonic crystals (MC). One of the options for using geometric modulation is the formation of a one-dimensional array of antidots in the magnonic structure of laterally connected magnetic stripes [3]. The disadvantage of MCs with geometric modulation in the form of a one-dimensional array of antidots is the fixed parameters after fabrication of the structure, which implies the difficulty in adjusting the SW propagation. Combining the idea of a magnonic crystal and laterally coupled magnetic waveguides avoids the disadvantages of both types of structures. In this work, we investigated the properties of spin-wave transport in a reconfigurable system of magnetic microwave guides connected laterally by a commutation connection in the form of a one-dimensional antidote array. As a material for microwave guides, we used yttrium iron garnet [Y3Fe5O12 (111)] with a saturation magnetization Ms = 1750 G due to the low damping of the spin wave. The modes of simultaneous spatial and frequency selection of spin waves by the Brillouin spectroscopy and by the method of micromagnetic simulation are shown. This work was supported by the Russian Science Foundation (Grant 20-79-10191).

![](https://assets.underline.io/uploads/markdown_image/1/image/1e52f3b7192171a9e9215e6b90add9f3.jpg)

Spin wave dynamics in magnonic coupler with antidote interconnection

When interacting spins in condensed matter order ferromagnetically, their ground-state wave function is topologically trivial. Nonetheless, in two dimensions, ferromagnets can support spin excitations with nontrivial topology, an exotic state known as topological magnon insulator (TMI). In Ref. [1], we theoretically unveil and numerically confirm a ferromagnetic state in three dimensions dubbed second-order TMI, whose hallmarks are excitations at its hinges, where facets intersect. Since ferromagnetism naturally comes with broken time-reversal symmetry, the hinge magnons are chiral, rendering backscattering impossible. Hence, as shown in the figure, they trace out three-dimensional paths about the sample unimpeded by defects and are topologically protected by the spectral gap. They are remarkably robust against disorder and highly tunable by atomic-level engineering of the sample termination. We predict that a van der Waals heterostructure built from chromium trihalide and transition metal dichalcogenide monolayers exhibits second-order magnon topology. Our findings empower magnonics, the harnessing of spin waves as information carriers, with the tools of higher-order topology, a promising route to combine low-energy information transfer free of Joule heating with three-dimensional vertical integration.

![](https://assets.underline.io/uploads/markdown_image/1/image/3539eda5d4812d50ee16d061b7719c73.jpg)

Chiral Hinge Magnons in Second-Order Topological Magnon Insulators

Chiral Hinge Magnons in Second-Order Topological Magnon Insulators

Two-particle interference, i.e., the Hong-Ou-Mandel (HOM) effect [1], is used in many quantum technologies to detect and generate entanglement [2]. HOM has no classical analog and is expressly quantum in nature, making it ideal to study fundamental quantum properties. HOM was first observed with photons in a 50:50 beam splitter (BS) [1] and has been studied in other quantum objects using various interference systems [2], but the effect has not yet been studied in quantum magnonics [3, 4]. Here, we propose a model system for two-magnon interference. The system is an elliptical pillar of 2 antiferromagnetically-coupled ferromagnetic (FM) layers (see Fig. 1 inset) with resonant frequencies ω1 and ω2. With no bias magnetic field (B=0) ω1=ω2 and the FM layers are strongly coupled, but they are virtually independent when B≠0. This means that a time-dependent magnetic field B(t) can dynamically tune the system off-resonance (ω1≠ω2) to excite and detect magnons individually in each FM layer, and on-resonance (ω1=ω2) to allow for interaction of the magnons in two FM layers. The magnonic BS operation is achieved when the system is on-resonance for some time τ such that the probability for single magnon scattering between layers is ½. Our numerical simulations in Fig. 2 show the evolution of the system initially prepared in a two-magnon unentangled state │11〉. After the BS pulse, a maximally entangled N00N state (│20〉+│02〉)/√2 is formed. The relative phase between │20〉 and │20〉 components of the N00N state can be controlled by shaping the profile of the magnetic field B(t). The magnonic BS is a reversible unitary operation so it can also detect different N00N states. In summary, we found that a dynamically tuned magnon resonance can be used to form a magnonic BS that can generate and detect entanglement by two-magnon interference.

![](https://assets.underline.io/uploads/markdown_image/1/image/841d937f4d8f5aa207d43ec90ab1cf52.jpg)

Fig. 1. Dependence of the resonant frequencies on magnetic field B. Solid lines: collective frequencies. Dashed lines and symbols: independent frequencies ω1,2. Gray highlight: strongly coupled region. Inset: elliptical FM/NM/FM pillar geometry.


![](https://assets.underline.io/uploads/markdown_image/1/image/c1fceb94a614c80049f451f3d6e1d1c4.jpg)

Fig. 2. Evolution of unentangled state │11〉 into entangled superposition of states (│20〉+│02〉)/√2 under a magnonic beam splitter operation (gray highlight).

Generation of Entanglement by Two-Magnon Interference

Artificial spin ices are magnetic metamaterials comprising geometrically tiled arrays of interacting nanomagnets. There has been significant recent interest1,2 in these systems as reconfigurable magnonic crystals due to their vast microstate landscape. Here we measure the spin-wave spectra of square artificial spin systems in zero field and isolate spectral effects arising from the microstate and subtle array imperfections. Crucially remanence FMR allows the study of subtle spectral signatures of the local dipolar field without being dwarfed by the applied field3. We examine this in both width-modified, where the microstate may be controlled via global field4, and normal square ASI comprising identical bars. Width modification allows well-defined transitions between vertex states by switching only selected bars. These controlled states allow the identification of precise frequency shifts and peak amplitude changes associated with specific microstates, verified via a combination of MOKE magnetometry and micromagnetic simulations. Having established the fitting procedure in the ordered states of the width-modified arrays, we apply it to the more challenging problem of disordered microstates in square ASI comprising identical bars. Here reversal is defect-mediated with avalanche-type 1D chain reversal, where a combination of all vertex types appear in intermediate states. We measure remanence FMR and show excellent agreement with micromagnetic simulation and local dipolar field from the pure microstates. We illustrate the great potential of spectral-fingerprinting remanence spin-wave spectroscopy via distinct spectral readout of three microstates with identical (zero) magnetisation. These states are indistinguishable from any magnetometry based technique, but easily discernible via remanence FMR.

![](https://assets.underline.io/uploads/markdown_image/1/image/3f06240cdf5040700626995955083270.jpg)

(a) SEM of width-modified square, (b) MOKE magnetometry hysteresis loop showing remanence measurement procedure, (c) Remanence FMR spectral heatmap of width-modified square in monopole orientation showing frequency shift due to local dipolar landscape, (d) Remanence FMR spectral heatmap of normal square sample through reversal, (e) Remanence FMR spectra of as-grown state with MFM of microstate.

Spectral-fingerprinting: Microstate readout via remanence ferromagnetic resonance in artificial spin systems

Strongly-interacting artificial spin systems are moving beyond mimicking naturally-occurring materials to find roles as versatile functional platforms, from reconfigurable magnonics to designer magnetic metamaterials. Typically artificial spin systems comprise nanomagnets with a single magnetisation texture: collinear macrospins or chiral vortices. By tuning nanoarray dimensions we achieve macrospin/vortex bistability and demonstrate a four-state metamaterial spin-system ‘Artificial Spin-Vortex Ice’ (ASVI). ASVI is capable of adopting Ising-like macrospins with strong ice-like vertex interactions, in addition to weakly-coupled vortices with low stray dipolar-field. The enhanced bi-texture microstate space gives rise to emergent physical memory phenomena, with ratchet-like vortex training and history-dependent nonlinear training dynamics. We observe vortex-domain formation alongside MFM tip vortex-writing. Tip-written vortices dramatically alter local reversal and memory dynamics. Vortices and macrospins exhibit starkly-differing spin-wave spectra with analogue-style mode-amplitude control via vortex training and mode-frequency shifts of df = 3.8 GHz. We leverage spin-wave ‘spectral fingerprinting’ for rapid, scaleable readout of vortex and macrospin populations over complex training-protocols with applicability for functional magnonics and physical memory.

![](https://assets.underline.io/uploads/markdown_image/1/image/6fe74ca92f4f20fc19dc1d93439c81eb.jpg)

a-d) FMR heatmaps of ASVI taken during 0-30 training loop field-sequence. x-shaped vortex modes emerge with increadsing intensity as training progresses.
e) Corresponding MFM images showing vortex nucleation and vortex domain growth.

![](https://assets.underline.io/uploads/markdown_image/1/image/8fb28a1a5f1c817f24cd18ee2b3e7681.jpg)

FMR spectra taken at constant -1.2 mT bias field after successive training loops. Macrospin magnon mode-amplitude decreases through training as vortex magnon mode-amplitude increases.

Comparison of Spin-Wave Modes in Connected and Disconnected Artificial Spin Ice Nanostructures using Brillouin Light Scattering

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