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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.&lt;br/&gt;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.&lt;br/&gt;Here, the amplitudes and frequencies of both microwave sources can be utilised in order to tune this behaviour.&lt;br/&gt;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.&lt;br/&gt;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.

2022 Joint MMM-INTERMAG

Spin-wave frequency combs

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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MMM 2020

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.

technical paper

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

Classifying or predicting complex time-dependent signals is a challenging computational task. Reservoir computing (RC) is an efficient neuromorphic computing approach that is ideally suited to such tasks, and is typically implemented in software using a recurrent neural network (RNN) with fixed synaptic weights (the reservoir) connected to a single, trainable readout layer. However, more efficient implementations of RC are possible if the software RNN reservoir is substituted with a physical system with the correct properties, such as non-linear response to input signals and inherent memory [1]. In this work, we exploit the chaotic and non-linear dynamics of a single magnetic domain wall (DW) trapped between two anti-notches in a Ni nanostrip as a hardware-based reservoir. We have modelled the structure (inset of Fig. 1) using both a simple 1D model and micromagnetic simulations with an oscillating field. As shown in Fig. 1, complex oscillatory dynamics over a range of field amplitudes are observed. A chaotic and multi-period regime occurs between 0.4 kA/m and 1.2 kA/m and a non-linear regime at higher fields. Thus, the DW oscillator can be used to transform an input magnetic field to the DW position output suitable for RC. A time-multiplexed input signal is injected into the DW oscillator by modulating the applied field magnitude, and we show how this approach allows the device to perform classification tasks as a reservoir.First, we demonstrate how the scaling to convert input data into applied fields affects the classification accuracy of the sine or square wave task, showing that the best recognition rate is obtained at the edge of a chaotic regime of oscillation (Fig.2). We also demonstrate that the DW oscillator can perform complex tasks, such as spoken digit recognition, and that coupling multiple DWs together enhances the computational capabilities of the reservoir. Our work opens new avenues for research into neuromorphic computing using nanomagnetic hardware.

![](https://assets.underline.io/uploads/markdown_image/1/image/b1c2f88ba8e32e54699a48e2fafd9388.jpg)

Bifurcation plot showing the DW position once per cycle for a given applied field using the 1D model. Inset: Schematic of the system.

![](https://assets.underline.io/uploads/markdown_image/1/image/7c99dd1e515fbc149d6687d21ac4a5b5.jpg)

Classification accuracy of the Sine-Square task over a range of field centre (H0) and field width (ΔH) values.

Neuromorphic computation with a single magnetic domain wall

Magnetic skyrmions can be transferred from it’s position to a desired location using spin torques [1] and there were proposals of skyrmion reacetrack memory and logic gate applications [2]. But, the instabilities and stochastic behavior of Skyrmions due to pinning [3], temperature and applied current, and skyrmion Hall angle (SkHA) [4] makes the realization of these applications challenging. Instead, stochastic skyrmion behaviour can be used to create new computing architectures, e.g., probabilistic and reservoir computing [5,6]. The major aspect of this computation is having a constant uncorrelated signal [5] that results from a reshuffle of the skyrmions. In this study, the stochastic property in a shuffling chamber will be investigated to create uncorrelated signals. The study will guide for the creation of a better probabilistic computing devices. We use the shuffling chamber (Fig. 1) and the current density for the geometry was generated in Mathematica and masking was used to implement it in micromagnetic simulator Mumax3 [7]. At first, the height of the paths were changed to 320, 360, 400 and 520 nm, this resulted in increase on the longevity of the final skyrmion path, and the skyrmion’s projectile motion decreased, accordingly. As the height of the rectangle increases, distance between the Skyrmion and the output borderline increases too. At first, the Skyrmion motion was observed under 0 K temperature, and without a grain. The skyrmions had a projectile and predictive motion. Then, the system was run using different temperatures, grain, and different currents to get a stochastic Skyrmion motion. In our simulations, the skyrmion under higher temperature was unstable at the output of the chamber, but by using higher anisotropy constant (Ku) at the output edge of the chamber, the instability was eliminated. The correlation between input and output paths were analyzed using Pearson correlation coefficient (PCC). We vary the temperature from 0 K to 100 K temperatures and find that the PCC changes from -0.055 to -0.04.

![](https://assets.underline.io/uploads/markdown_image/1/image/09252b3e4c415b8637e54d793b896946.jpg)

Fig. 1

![](https://assets.underline.io/uploads/markdown_image/1/image/186c33c8eb11dd907aa054cce316e78c.jpg)

Fig .2

Stochastic Property of Skyrmionic Shuffling Device

We introduce a new class of primitive building blocks for realizing quantum logic elements based on nanoscale magnetization textures called skyrmions. In a skyrmion qubit, information is stored in the quantum degree of helicity, and the logical states can be adjusted by electric and magnetic fields, offering a rich operation regime with high anharmonicity. By exploring a large parameter space, we propose two skyrmion qubit variants depending on their quantized state. We discuss appropriate microwave pulses required to generate single-qubit gates for quantum computing, and skyrmion multiqubit schemes for a scalable architecture with tailored couplings. Scalability, controllability by microwave fields, operation time scales, and readout by nonvolatile techniques converge to make the skyrmion qubit highly attractive as a logical element of a quantum processor.

![](https://assets.underline.io/uploads/markdown_image/1/image/9cf0d32a1ba0d161d137ecbbaa42774d.jpg)

Skyrmion Qubits: A New Class of Quantum Logic Elements Based on Nanoscale Magnetization

Vortices are familiar phenomena in fluids, distinguishable for example in tornadoes, hurricanes, whirlpools and rings. In ferromagnets, vortices are naturally forming flux-closure states characterized by a curling of the magnetic moments around a very stable and narrow core [1]. The study of vortices is motivated by both fundamental and technological interest. Indeed, their topological properties endow vortices with rich and surprising dynamics, such as the ultrafast reversal of the vortex core [2], making them fascinating objects of study. These properties also establish them as promising candidates for applications in data storage and signal processing. Over the past decades, vortices have been extensively studied in laterally confined thin-film elements, where the magnetization is accessible with two-dimensional imaging methods. Recent developments have enabled non-destructive imaging of three-dimensional magnetic structures at the nanoscale. In particular, using hard X-ray tomography [3], it is now possible to directly image the internal structure within bulk micromagnets. For example, in GdCo2 pillars, we could image the structure surrounding micromagnetic singularities, known as Bloch points, which were predicted over 50 years ago. Recently, we have uncovered three-dimensional structures forming closed vortex loops. The cross-section of these loops consists of a vortex–antivortex pair and, based on magnetic vorticity (a quantity analogous to hydrodynamic vorticity), we identified these configurations as magnetic vortex rings [4]. Such structures have been predicted to exist as transient states in exchange ferromagnets [5]; in contrast, the observed vortex rings exist as static configurations, and we attribute their stability to the dipolar interaction. In addition, we observe stable vortex loops intersected by point singularities at which the magnetization within the vortex and antivortex cores reverses. Such structures can exist because a non-zero divergence of the magnetization is allowed in ferromagnets; they have no analogue in incompressible fluids. Incidentally, it is known since the 1970s that ‘Bloch lines’ can assume complex structures, but these could only be inferred from surface observations combined with theory. Our results shed new light onto the rich physics of bulk magnets and open possibilities for further studies of complex three-dimensional solitons, enabling the development of applications based on three-dimensional magnetic structures. Thus, the title of this talk could as well be, ‘Magnetic Vortices: Back to the Third Dimension’.

Magnetic Vortices: into the Third Dimension

Much effort has focused on achieving micron-scale spin propagation using diffusive spin waves, but their exponential decay with distance makes efficient spin transport challenging. An alternative form of magnetization dynamics – commonly referred to as spin superfluidity [1] or exchange mediated spin transport [2,3] – may enable more efficient transport owing to its topologically-protected easy-plane precession of the magnetization and associated linear decay of spin flow. Our work addresses the open question of how defects impact this easy-plane spin transport at low drive current densities. We performed micromagnetic simulations on easy-plane precessional dynamics in synthetic antiferromagnet nanostrips with and without notch defects. The material parameters were taken from our experimentally measured, low-damping FeV alloys [4]. We view the dynamics as a train of interacting, homochiral domain walls (DWs) rather than as hydrodynamic flow [2]. We observe the transition from (1) isolated, diffusively propagating DWs, which are susceptible to pinning, to (2) a fluid-like flow of a closely packed DW train that is unimpeded by pinning defects. This transition is quantified by computing the superfluid velocity ▽Φ, where Φ is the in-plane azimuthal angle of the magnetization, at a position beyond the notch defect (see Fig. 1). In Fig. 2, ▽Φ differs between the perfect and notched nanostrips as a result of pinning of the DWs at low drive current densities, Jc &lt; 5 x 1011 A/m2. As Jc increases, ▽Φ becomes identical between the two geometries, indicating that the DWs have effectively condensed to form a fluid flowing unaffected by the notch defect. Our findings are promising for future realization of superfluid-like spin transport via easy-plane precession in ferromagnetic metal devices, which are compatible with established fabrication techniques [5].

![](https://assets.underline.io/uploads/markdown_image/1/image/b29ae1464b62923fd11997a38ae0428e.jpg)

Figure 1: Micromagnetic snapshots of the perfect and notched nanostrips at Jc = 4 x 1011 A/m2, labeled with location of notch defect at the center and where ▽Φ is calculated.


![](https://assets.underline.io/uploads/markdown_image/1/image/329eb8448836511a5d16e9a2b0225809.jpg)

Figure 2: Superfluid velocity ▽Φ as a function of charge current density Jc for the perfect (black squares) and notched (red circle) nanostrips computed at a fixed position beyond the notch defects.

Diffusive and Fluid-like Motion of Homochiral Domain Walls in Easy-Plane Magnetic Strips

As three-dimensional nanomagnetism evolves, novel non-trivial magnetic textures emerge as appealing information carriers for recording and spintronics based on curved nanosystems, and particularly on Cylindrical Nanowires (NWs) [1,2]. One of the most fascinating candidates that is likely to reach the high velocities required for fast recording technologies is the Bloch Point (BP) domain wall (DW). Recently, theoretical evidence indicated that BPs in NWs could reach high velocities close to 2 km/s in the magnonic regime [3]. While the observation of the BP DW in cylindrical NWs is no longer recent [2], scarce numerical studies that combine both current and Oersted field have been published in NWs [4], despite first attempts to measure DW velocities are in progress [5]. In this work we evaluate the dynamics of the BP DW under both current directions in a Ni NW with 100 nm in diameter. We investigate two cases: i) pre-nucleated BP Wall, and ii) the BP DW originated from the transformation of a Vortex-Antivortex DW. The effects of both spin-polarized current and Oersted field are considered. We discuss in detail the role of the chirality of the BP in relation to the Oersted field, also reported previously in precursors of BPs [4]. We show that while the pre-nucleated DW with the same chirality as that of the Oersted field propagates always against the current direction, the BP originated either from the transformation of the BP with the opposite chirality or from the vortex-antivortex DW can either stop the propagation or propagate parallel to the current. We attribute this effect to the role played by the inertia of the BP in its dynamics. Finally, we provide values of the velocities achieved by the BP in the NW as a function of applied current in Fig. 1. We conclude that BPs with vanishing momentum propagate opposite to the current with velocities that may be suppressed by the Oersted field. Importantly for spintronic applications, both momentum and inertial mass play a major role in the dynamics of BPs that has not been envisaged up to know.

![](https://assets.underline.io/uploads/markdown_image/1/image/6ff528b21478a289ec02bc86a4e8eae4.jpg)

The average velocity of a head-to-head BPW driven by a spin-polarized current in a Ni nanowire, far from the ends of the nanowire, as a function of the current density.

Oersted-field- and current- induced Bloch Point domain wall dynamics in cylindrical Ni nanowires

Highly-magnetostrictive amorphous microwires show bistable behaviour, i.e. magnetization reverses by depinning/propagation of a single Domain Wall, DW, along the entire wire. This response makes these microwires an ideal system for fundamental studies on DWs dynamics [1,2]. On the other hand, the classical Matteucci effect, ME, consists of the induction of an emf voltage at the ends of the microwire under applied axial magnetic field [3], and requires a net azimuthal component of magnetization, Mφ. That emf is associated with the DW propagation and its amplitude is proportional to the DW speed, vDW, and the time derivative of Mφ, tailored by the torsion magnetoelastic anisotropy. emf has been measured in a bistable FeSiB microwire (12mm-diam, 10cm-long) under torsion (maximum of ξmax=57 πrad/m) and axial driving magnetic field, Hdr (square wave). The setup is designed to generate unidirectional motion of the DW that unpins from the same end irrespective of the Hdr direction [4], and its speed is quantified by the voltage induced in tiny pickup coils along the microwire. The clockwise/counterclockwise, CW/CCW, torsion dependences of emf and vDW are experimentally determined for parallel/antiparallel Hdr. Asymmetric behaviour is observed for both, the sense of x and the Hdr direction (Fig.1). The DW dynamics is analysed in terms of Landau-Lifschitz-Gilbert eq. considering a bell-like DW (Fig.2). The observed asymmetries are numerically interpreted to derive from the magnetochiral effect related to the DW chirality. The switching of the Hdr direction changes the Head-to-Head, H-H, or Tail-to-Tail, T-T, DW character and chirality (i.e., Mφ circulation). As a consequence of this magnetochiral effect, we proof that H-H and T-T DWs propagate at different velocities, vDW, as experimentally observed. Here we provide a first evidence of DWs magnetochiral effects in microwires that offers an opportunity for investigation of topological properties of spin-textures in microwires that has not been envisaged up to now.

![](https://assets.underline.io/uploads/markdown_image/1/image/3aa87066cea42d5386ea12ea8fba69a6.jpg)

CW, ξ+, and CCW, ξ-, torsion dependence of DW speed under Hdr = 232 A/m

![](https://assets.underline.io/uploads/markdown_image/1/image/83f8fc99b2ba0f6840b6466bd0d04e32.jpg)

Scheme of a longitudinal slice with a propagating bell-shaped DW. Magnetization in the domains makes a small angle with the microwires axis

Magnetochiral effect of single domain wall propagation in the Matteucci effect of magnetostrictive microwire

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

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