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In this last decade, experimental results and their understanding have led to magnetic skyrmions becoming promising candidates for the storage and processing of digital information [1]. However, some key barriers such as reducing their size, enhancing their stability at room temperature, or their low-power manipulation under electrical current have not yet been overcome, meaning that they are rarely integrated in actual devices [2]. To surmount these limitations, it has been proposed to study these magnetic textures in antiferromagnets or synthetic antiferromagnets (SAFs) instead of the conventional ferromagnetic layers that are usually considered [3,4].&lt;br/&gt;&lt;br/&gt;In this more complex system, multiple ferromagnetic layers are coupled antiferromagnetically through a non-magnetic spacer layer &lt;i&gt;via&lt;/i&gt; the RKKY interaction (Fig a) [4,5]. By switching to these systems, two crucial points need to be addressed. First, a drastic reduction of stray fields is expected in SAFs due to the anti-alignment of magnetization, which should reduce the size of the skyrmions [6]. Second, it is not possible to manipulate a conventional skyrmion in the driving force direction in a ferromagnet due to its topology that intrinsically induces a deflection known as the skyrmion Hall effect [7]. In SAF systems, since the magnetisation of the skyrmions in each magnetic layer are opposite, it is expected that the two deflections cancel each other out and the two skyrmions move in the current direction [3]. Even if skyrmions have recently been observed in SAFs [4,8,9], a clear and simple micromagnetic description of both the static and dynamic properties of such skyrmions is so far lacking to further understand this phenomenon.&lt;br/&gt;&lt;br/&gt;In order to improve the description of these properties and highlight the expected benefits of using SAFs rather than conventional ferromagnets, we simulate numerically the behavior of skyrmions in bilayer SAFs for a large range of parameters. We also adapt the ferromagnetic analytical formalism [10] to SAF systems. This leads to results that are in good agreement with the numerical simulations whilst also highlighting the underlying mechanisms. First, we investigate the stability region of SAF skyrmions and how their radii &lt;i&gt;r&lt;/i&gt; evolve with micromagnetic parameters. We show that they are only slightly smaller than for ferromagnetic systems (Fig b), but that they exist for a larger range of parameters. Second, we study the dynamics of these skyrmions under spin currents. We find, as expected, that SAF skyrmions move at a faster velocity &lt;i&gt;v&lt;/i&gt; than in a conventional ferromagnet (Fig c) for a given current density &lt;i&gt;J&lt;/i&gt;, and move in the applied current direction (Fig d) with zero skyrmion Hall angle &lt;i&gt;θ&lt;/i&gt;&lt;i&gt;Sky&lt;/i&gt;. By using the analytical model based on the Thiele equation, we are able to recover these dynamics (Fig c and d) and identify the required condition for the vanishing of the topological deflection. Finally, we unbalanced the SAF by asymmetrizing the skyrmions in each layer in different ways in order to verify the condition for vanishing deflection.&lt;br/&gt;The results obtained highlight the qualitative benefits of SAF skyrmions and propose some methods for more quantitative optimizations.

**References:**

[1]        The 2020 skyrmionics roadmap: C. Back et al., J. Phys. D: Appl. Phys. 53 (2020)&lt;br/&gt;[2]        Magnetic skyrmions: advances in physics and potential applications: A. Fert, et al. Nat. Rev. Mat. 2, 17031 (2017)&lt;br/&gt;[3]        Static and dynamical properties of antiferromagnetic skyrmions in the presence of applied current and temperature: J. Barker, et al. Phys. Rev. Lett. 116, 147203 (2016)&lt;br/&gt;[4]        Room-temperature stabilization of antiferromagnetic skyrmions in synthetic antiferromagnets. W. Legrand et al., Nat. Mat. 19, 34–42 (2020)&lt;br/&gt;[5]        Oscillatory magnetic exchange coupling through thin copper layers: S. S. P. Parkin, et. al . Phys. Rev. Lett. 66, 2152–2155 (1991)&lt;br/&gt;[6]        Theory of isolated magnetic skyrmions: from fundamentals to room temperature applications: F.Büttner, et al,. Sci. Rep. 8 4464 (2018)&lt;br/&gt;[7]        Nucleation, stability and current-induced motion of isolated magnetic skyrmions in nanostructures: J. Sampaio, Nat. Nano. 8, 839-844 (2013)&lt;br/&gt;[8]        Imaging non-collinear antiferromagnetic textures via single spin relaxometry: A. Finco et al., arXiv:2006.13130v1 (2020)&lt;br/&gt;[9]        Exploring different facets of magnetic skyrmion nucleation and dynamics in ultra-thin films: R. Juge, PhD Thesis, Université Grenoble-Alpes (2020)&lt;br/&gt;[10]      The skyrmion-bubble transition in a ferromagnetic thin film: A. Bernand-Mantel et.al. SciPost Phys. 4, 027 (2018)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/060a538fb9d9988c2db214e60cb7057e.jpg)
(a) Sketch of the skyrmion structure and motion under current in ferromagnetic and SAF systems. Radii (b), velocity (c) and deflection (d) for a ferromagnetic skyrmion (in blue) and a SAF skyrmion (in red) with the same micromagnetic parameters versus the current density&lt;i&gt; J&lt;/i&gt;. Points correspond to the micromagnetic numerical simulations and lines to the analytical model.



INTERMAG 2021

 Statics and Dynamics of Skyrmions in Synthetic Antiferromagnets : Benefits and Micromagnetic Understanding

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ACL-IJCNLP 2021

The great event is jointly organized by the Association for Computational Linguistics (ACL) and Asian Federation of Natural Language Processing (AFNLP).

In this last decade, experimental results and their understanding have led to magnetic skyrmions becoming promising candidates for the storage and processing of digital information [1]. However, some key barriers such as reducing their size, enhancing their stability at room temperature, or their low-power manipulation under electrical current have not yet been overcome, meaning that they are rarely integrated in actual devices [2]. To surmount these limitations, it has been proposed to study these magnetic textures in antiferromagnets or synthetic antiferromagnets (SAFs) instead of the conventional ferromagnetic layers that are usually considered [3,4]. In this more complex system, multiple ferromagnetic layers are coupled antiferromagnetically through a non-magnetic spacer layer via the RKKY interaction (Fig a) [4,5]. By switching to these systems, two crucial points need to be addressed. First, a drastic reduction of stray fields is expected in SAFs due to the anti-alignment of magnetization, which should reduce the size of the skyrmions [6]. Second, it is not possible to manipulate a conventional skyrmion in the driving force direction in a ferromagnet due to its topology that intrinsically induces a deflection known as the skyrmion Hall effect [7]. In SAF systems, since the magnetisation of the skyrmions in each magnetic layer are opposite, it is expected that the two deflections cancel each other out and the two skyrmions move in the current direction [3]. Even if skyrmions have recently been observed in SAFs [4,8,9], a clear and simple micromagnetic description of both the static and dynamic properties of such skyrmions is so far lacking to further understand this phenomenon. In order to improve the description of these properties and highlight the expected benefits of using SAFs rather than conventional ferromagnets, we simulate numerically the behavior of skyrmions in bilayer SAFs for a large range of parameters. We also adapt the ferromagnetic analytical formalism [10] to SAF systems. This leads to results that are in good agreement with the numerical simulations whilst also highlighting the underlying mechanisms. First, we investigate the stability region of SAF skyrmions and how their radii r evolve with micromagnetic parameters. We show that they are only slightly smaller than for ferromagnetic systems (Fig b), but that they exist for a larger range of parameters. Second, we study the dynamics of these skyrmions under spin currents. We find, as expected, that SAF skyrmions move at a faster velocity v than in a conventional ferromagnet (Fig c) for a given current density J, and move in the applied current direction (Fig d) with zero skyrmion Hall angle θSky. By using the analytical model based on the Thiele equation, we are able to recover these dynamics (Fig c and d) and identify the required condition for the vanishing of the topological deflection. Finally, we unbalanced the SAF by asymmetrizing the skyrmions in each layer in different ways in order to verify the condition for vanishing deflection. The results obtained highlight the qualitative benefits of SAF skyrmions and propose some methods for more quantitative optimizations.

**References:**

[1] The 2020 skyrmionics roadmap: C. Back et al., J. Phys. D: Appl. Phys. 53 (2020) [2] Magnetic skyrmions: advances in physics and potential applications: A. Fert, et al. Nat. Rev. Mat. 2, 17031 (2017) [3] Static and dynamical properties of antiferromagnetic skyrmions in the presence of applied current and temperature: J. Barker, et al. Phys. Rev. Lett. 116, 147203 (2016) [4] Room-temperature stabilization of antiferromagnetic skyrmions in synthetic antiferromagnets. W. Legrand et al., Nat. Mat. 19, 34–42 (2020) [5] Oscillatory magnetic exchange coupling through thin copper layers: S. S. P. Parkin, et. al . Phys. Rev. Lett. 66, 2152–2155 (1991) [6] Theory of isolated magnetic skyrmions: from fundamentals to room temperature applications: F.Büttner, et al,. Sci. Rep. 8 4464 (2018) [7] Nucleation, stability and current-induced motion of isolated magnetic skyrmions in nanostructures: J. Sampaio, Nat. Nano. 8, 839-844 (2013) [8] Imaging non-collinear antiferromagnetic textures via single spin relaxometry: A. Finco et al., arXiv:2006.13130v1 (2020) [9] Exploring different facets of magnetic skyrmion nucleation and dynamics in ultra-thin films: R. Juge, PhD Thesis, Université Grenoble-Alpes (2020) [10] The skyrmion-bubble transition in a ferromagnetic thin film: A. Bernand-Mantel et.al. SciPost Phys. 4, 027 (2018)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/060a538fb9d9988c2db214e60cb7057e.jpg)
(a) Sketch of the skyrmion structure and motion under current in ferromagnetic and SAF systems. Radii (b), velocity (c) and deflection (d) for a ferromagnetic skyrmion (in blue) and a SAF skyrmion (in red) with the same micromagnetic parameters versus the current density J. Points correspond to the micromagnetic numerical simulations and lines to the analytical model.

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The discovery of a significant interfacial Dzyaloshinskii-Moriya Interaction (iDMI) in multi-layer thin films launched a fervent research effort into its impact on the behavior of chiral magnetic objects including domain walls (DWs).[1] The field driven creep of these Dzyaloshinskii domain walls as observed by Kerr microscopy remains a popular tool for evaluating the strength of the iDMI, but results are notoriously challenging to interpret. In this talk, we focus on experimental and theoretical investigations into the asymmetric field-driven expansion of magnetic domains in thin films with iDMI, which provide a fascinating landscape to understand creep of 1D elastic manifolds with an internal order parameter.[2] We will review early models of creep based on the DW energy, which matched well with certain experimental observations – namely, that an in-plane field (Hx) coupled with a perpendicular driving field (Hz) leads to asymmetric expansion of the domain as well as minima in the plot of x-velocity vs Hx. However, other common observations were not explained by this model including that, at large values of Hx, the high energy wall orientations reached or exceeded the velocity of low energy orientations. To explain this, Pellegren at el considered the more general elastic energy of the DW, given by the stiffness [σ(θ) + σ''(θ)].[3] By introducing a dispersive stiffness, which accounts for the finite lengthscale over which the wall deforms, it was indeed shown that the elastic energy of opposite extrema on the domain converge as Hx → ∞. This more general model explains some of the features that have also been explained by an attempt frequency that depends on wall chirality (via a chiral damping parameter).[4] The possible role of chiral damping will be discussed briefly. In more recent experiments, Brock et al showed that, in some cases – especially for weak DMI, the domain adopts a highly preferred growth direction with a dendritic structure that is neither parallel or anti-parallel to Hx.[5] The results cannot be explained by a chiral Bloch component to the DW, which would violate the symmetry of the system. Instead, we identify a steady-state DW substructure that results from the z-field torque on the internal DW magnetization (see figure 1c). This breaks the energetic mirror symmetry along the x-axis and leads to orientations of the DW with vanishing elastic energy (i.e. rapidly increased creep velocity). Remarkable agreement between an augmented form of the dispersive stiffness model (now accounting for steady-state configurations) and the experimental measurements is found. The material parameters that give rise to this seemingly anomalous behavior will be discussed. Other important factors such as internal DW excitations, field-dependent pinning, and a field-dependent DW width will be considered in this presentation as well.

**References:**

[1] Thiaville, A., Rohart, S., Jué, É., Cros, V. and Fert, A., 2012. Dynamics of Dzyaloshinskii domain walls in ultrathin magnetic films. EPL (Europhysics Letters), 100(5), p.57002. [2] Fournier, J. B. "Generalized gibbs-thomson equation and surface stiffness for materials with an orientational order parameter." Physical review letters 75.5 (1995): 854. [3] Pellegren, J.P., Lau, D. and Sokalski, V., 2017. Dispersive stiffness of Dzyaloshinskii domain walls. Physical Review Letters, 119(2), p.027203. [4] Jué, E., Safeer, C.K., Drouard, M., Lopez, A., Balint, P., Buda-Prejbeanu, L., Boulle, O., Auffret, S., Schuhl, A., Manchon, A. and Miron, I.M., 2016. Chiral damping of magnetic domain walls. Nature materials, 15(3), pp.272-277. [5] Brock, J., Medapalli, R. and Fullerton, E., 2020. Chirality-driven growth of stripe domains in Co/Ni/Pt-based multilayers. Bulletin of the American Physical Society, 65.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/d041ea67aa2fad976e5c8d2fe441cb85.jpg)
Fig 1. Schematic of a Néel bubble subject to external magnetic field. Arrows indicate the direction of internal magnetization along the circumference. Green lines indicate time-reversal mirror planes, m'. a) No external field. Magnetic point symmetry: ∞m'm’. b) Hx only. Magnetic point symmetry: m’. c) Hx and Hz field. Magnetic point symmetry: 1.

Dzyaloshinskii Domain Wall Creep INVITED

Despite numerous recent studies focusing on the dynamics of pinned chiral magnetic textures [1-6], their interactions with weak pinning disorder is far from being understood. Basic issues such as the length-scale and the strength of interaction between domain walls (DWs) and defects in magnetic materials remain open. Our works [7] evidence a strong correlation between controlled variations of DW width and the characteristic length of pinning. The DW-disorder interaction strength is also shown to depend on DW energy and width. These findings should be also relevant for a wide variety of elastic interfaces moving in weak pinning disordered media. We explore the magnetic field-driven motion of domain walls with different chiralities in thin ferromagnetic films made of Pt/Co/Pt, Au/Co/Pt, and Pt/Co/Au. The displacement of DWs was observed by polar Kerr microscopy. An in-plane magnetic field Hx was used to control the magnetization direction in the DW, as well as its width. A pulse out-of-plane field H was used to move DW. The measured velocity corresponds to the ratio between the measured DW displacement and the pulse duration. The velocity curves obtained for Pt/Co/Au with different values of Hx are reported in Fig. 1. Their analysis with the self-consistent description of the universal creep and depinning regimes [8] was used to extract parameters (depinning field Hd and temperature Td) characterizing the interaction between domain wall and weak random pinning disorder of the films. Surprisingly, the values of Hd and Td vary (not-shown) with the in-plane magnetic field Hx, which strongly suggests that the pinning properties of DWs depend on their magnetic texture, in contrast to the usual assumption found in the literature [1-6]. To predict accurately the variation of DW energy and structure with in-plane and DMI fields, we have calculated the orientation of the magnetization M (x) for a DW plane perpendicular to Hx, from numerical micromagnetic calculations (MuMax3), using the micromagnetic parameters of the three samples (not-shown). For the DW width, we both use the geometrical Hubert and dynamic Thiele definitions. Upon varying Hx away from the magnetic field H_DMI associated to the Dzyaloshinskii-Moriya interaction (DMI), the magnetization within the DW progressively switches from Bloch to Néel configuration, and the DW width increases due to the Zeeman contribution, as expected. The DW surface energy decreases as Hx departs fromH_DMI, in qualitative agreement with the predictions of simplified models [3-5]. In order to discuss the DW pinning at microscopic scale as a function of in-plane field Hx, we use standard scaling arguments [8] based on the variation of the energy produced by the deformation of a DW. The DW energy is assumed to be controlled by the gain of Zeeman energy due to magnetization reversal, the elastic energy produced by the increase of DW length, and the pinning energy due to the fluctuations of DW energy produced by the weak disorder. From scaling arguments, we obtain the relations which link the characteristic range and force of the interaction between DW and pinning defects, to the measured depinning field Hd(Hx), temperature Td(Hx), and the predicted DW surface energy. For a comparison (see Fig. 2) with the predicted variations of the DW width, the values of the pinning range were rescaled with a constant factor, which is the only adjustable parameter. As it can be observed, there is a strong correlation between the variations of the pinning range and the DW width. This strong correlation suggests that the variations of domain wall structure, controlled by an in-plane field, modify the characteristic lengthscale of pinning, whatever the DW chirality and the interaction strength between domain wall and pinning defects.

**References:**

[1] S.-G. Je, D.-H. Kim, et al., Phys. Rev. B 88, 214401 (2013). [2] R. Lavrijsen, D. M. F. Hartmann, et al., Phys. Rev. B 91, 104414 (2015). [3] D. Lau, V. Sundar, et al., Phys. Rev. B 94, 060401 (2016). [4] J. P. Pellegren, D. Lau, and V. Sokalski, Phys. Rev. Lett. 119, 027203 (2017). [5] D. Lau, J. P. Pellegren, et al., Phys. Rev. B 98, 184410 (2018). [6] D. M. F. Hartmann, R. A. Duine, et al. , Phys. Rev. B 100, 094417 (2019). [7] P. Géhanne, S. Rohart, A. Thiaville, and V. Jeudy, Phys. Rev. Res. 2, 043134 (2020). [8] V. Jeudy, R. Diaz Pardo, et al., Phys. Rev. B 98, 054406 (2018).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/4cb670d00ac41c65fa6b4dd07aff8bed.jpg)
Domain wall velocity versus out-of-plane magnetic field measured for a Pt/Co/Au film and different values of the in-plane field. The solid and dash lines are predictions for the creep and depinning regimes. The stars correspond to the coordinates of the depinning thresholds Hd(Hx).

![](https://assets.underline.io/uploads/markdown_image/1/image/1ff28aae80963ce5024f1005ff81dcb7.jpg)
Domain wall widths (Hubert: solid line) and (Thiele: dash line), and correlation length of the pinning versus in-plane magnetic field for the three films. For each film, the correlation length was rescaled to highlight its strong correlation with the DW widths.

Interaction of chiral domain walls with pinning disorder in thin ferromagnetic films

I will present an overview of our recent results on modelling field-driven dynamics of domain walls in ferromagnetic thin films and strips with perpendicular magnetic anisotropy, focusing on the effect of internal degrees of freedom on the domain wall dynamics. In the geometries considered, the dynamics associated with the internal degrees of freedom are given by that of the magnetisation at the domain wall mid-point, and is manifested as repeated nucleation, propagation and annihilation of topological defects known as Bloch lines within the moving domain wall. Our aim is to understand the effect of such internal dynamics on the large-scale dynamics of domain walls. First, I will discuss micromagnetic simulations of magnetic field-driven domain wall dynamics in thin ferromagnetic strips with perpendicular magnetic anisotropy which demonstrate that in wide enough strips the Walker breakdown - the sudden drop of domain wall velocity as the field is increased above the Walker field - is a multistep process: It consists of several distinct velocity drops separated by short linear parts of the velocity vs field curve. These features originate from the repeated nucleation, propagation, and annihilation of an increasing number of Bloch lines within the domain wall as the driving field magnitude is increased [1]. Second, I will consider micromagnetic simulations of Barkhausen noise in disordered Pt/Co/Pt thin films due to precessional motion of domain walls. In this regime the domain walls contain Bloch lines which repeatedly nucleate, propagate, and annihilate within the domain wall during the Barkhausen jumps. In addition to bursts of domain wall propagation, the in-plane Bloch line dynamics within the domain wall exhibits crackling noise and constitutes the majority of the overall spin rotation activity [2]. Finally, I will present our very recent work on developing a reduced model of line-like domain walls in thin films with perpendicular magnetic anisotropy including a description of internal degrees of freedom, i.e., Bloch lines and their dynamics. The model, which is computationally much more efficient than full micromagnetic simulations, is applied to study the depinning transition of field-driven domain walls in disordered ferromagnetic thin films [3].

**References:**

[1] J. Hütner, T. Herranen, and L. Laurson, Phys. Rev. B 99, 174427 (2019). [2] T. Herranen and L. Laurson, Phys. Rev. Lett. 122, 117205 (2019). [3] A. Skaugen and L. Laurson, in preparation (2020).

**Images:**

On dynamics of domain walls with internal degrees of freedom

With recent advancements in fabrication techniques, in particular through FEBID technology, three-dimensional (3D) nanomagnetism has emerged as a new field of interest [1]. Over the past decades, extensive studies on patterned two-dimensional thin-film elements have demonstrated that the properties of magnetic nanostructures are significantly affected by their shape, such that certain magnetic features can be generated and modified by changing the geometry. Following this principle, the access to an additional degree of freedom in 3D nanomagnetism can introduce new effects unknown in planar structures and may open up possibilities for new devices with artificially designed properties [2]. In this context, a promising category of novel nanomagnetic nano-architectures with remarkable potential for applications are 3D arrays of interconnected magnetic nanowires. The assembly of magnetic nanowires into an interconnected array can lead to interactions and magnetic structures with features known from artificial spin ice (ASI) lattices. A particular micromagnetic situation occurs at the intersections of the wires, where the magnetization must adapt to a change of both the geometry and the magnetization direction. At the vertices, characteristic magnetic configurations and phenomena of magnetic frustration unfold. Here we show that these frustrated states can strongly impact the overall properties of a bucky-ball type geometry, which can be considered a prototype of a 3D ASI. Using finite-element micromagnetic simulations, we investigate the hysteretic behavior, the static magnetization structure, and the high-frequency properties of the magnetization in artificial buckyball-type nanostructures. The buckyball geometry [3, 4] acts as a model system to study the transition from two-dimensional to three-dimensional ASI structures. It has, on one hand, important similarities with a two-dimensional Kagomé lattice, while, on the other hand, it also incorporates distinct three-dimensional properties resulting from the spatial arrangement of the nanowires and the spherical shape of the network that they form. The geometry consists of 90 nanowires connected at 60 vertices, resulting in a closed network of hexagons and pentagons. Our model system is made of soft-magnetic Co, and its size is typically in the micron range. More specifically, we have investigated various buckyball-type geometries with individual nanowire lengths ranging between 25 nm and 250 nm. At zero field, the static magnetization structure of the magnetic bucky-balls is characterized by their configurations at the vertices. Due to the prevailing impact of shape anisotropy, each nanowire is magnetized along its axis. Based on the orientation of the magnetization in the three nanowires meeting at a vertex, the magnetic structures at the vertices can be classified into four types: two-out / one-in, two-in / one-out, three-out, and three-in. A specific magnetic charge, +1,-1,+3 and -3, can be assigned to each of these configurations, as is done in Kagomé lattices [5]. Neglecting the sign of the charges, one can distinguish between ice-rule obeying single-charge and defect-like triple-charge configurations. By means of hysteresis loop simulations, we find that triple-charged vertices can be created and removed through minor loops: If the field is reduced to zero from saturation, two triple-charge vertex configurations form at diametrically opposite sites. These triple charge defects dissolve after applying a moderate negative field, and they do not reappear if the field is subsequently removed again. Hence, we show that in these 3D ASI geometries, it is possible to inject and to remove magnetic triple-charge defect structures in a controlled way by varying the external magnetic field within a specific range. Going further in the study, we analyze the fundamental high-frequency modes of the magnetization in these geometries and their dependence on the magnetic structure. To extract the modes, the equilibrium states at zero field are excited by a picosecond field pulse, leading to a magnetic ringing over several nanoseconds. From these high-frequency dynamics, the magnetic modes are derived by using spatially-resolved windowed Fourier analysis [6]. We observe a distinct correlation between the distribution of magnetic charges at the vertices and the frequency of the modes. The modes can be distinguished as oscillations that are localized either at the vertices or within the nanowires. In particular, we find that the dynamics of the triple-charge states is very different from the dynamics at the single-charge vertices. The triple-charge states generate a strong, often dominating signal at low frequencies, which is absent in configurations without triple-charge defects. Moreover, we show that the frequency of this mode can be varied in an almost linear fashion by an external magnetic field. The combination of this feature with the particular hysteretic properties of the system opens a promising perspective to develop tailored devices with desired high-frequency properties that can be manipulated by varying the type of magnetic frustrations.

**References:**

[1] A. Fernández-Pacheco et al., Nat Commun 8, 15756 (2017) [2] P. Fischer et al., APL Materials 8, 010701 (2020) [3] L. Keller and M. Huth, Beilstein J. Nanotechnol. 9, 2581 (2018) [4] C. Donnelly et al., Phys. Rev. Lett. 114, 115501 (2015) [5] F. Montaigne et al., Sci. Rep. 4 : 5702 (2014) [6] M. Yan, R. Hertel, C. M. Schneider, Phys. Rev. B 76, 094407 (2007)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/d48aacc7c01215a7aa016921fa518153.jpg)
Fig.1: Finite-element discretization of a 3D buckyball-type nanostructure. Depending on the buckyball size, we use up to 1.9 million elements.

![](https://assets.underline.io/uploads/markdown_image/1/image/7c6422b1b28117f73a2f02635cba7c87.jpg)
Fig. 2: a) Different zero-field configurations can be accessed through hysteresis loops and minor loops, in particular the triple-charge state and the single-charge state. b) Simulated Fourier spectrum of the magnetic modes of the triple-charge and single-charge state.

Micromagnetics of Frustrated States and High-frequency Modes in Artificial Buckyball Nanostructures

The manipulation of magnetic textures in ferromagnets (FM) has been the focus of intense research in recent years. More recently, this focus has been displaced towards the use of antiferromagnets (AFM), and ferrimagnets (FiM), which are more robust against external perturbation and can sustain faster magnetization dynamics. In this context, simplified models have been proved very useful to describe and clarify the phenomenology on FM [1], AFM [2], and FiM [3] materials giving in some cases also good quantitative predictions. Nevertheless, AFM and FiM are described by two sublattices strongly couple through an exchange interaction while FM are described by only one magnetic lattice. In order to recover the FM case from the FiM description, one needs not only to set the second sublattice magnetization to zero but also its corresponding energy densities. It is possible to describe all these cases in a unified framework by a simple renormalization of the energy densities to the squared of the sublattices magnetization modulus. To give an example, considering Ku1 and Ku2 the common uniaxial anisotropy parameters, we substitute them by ku1*(MS1)2 and ku2*(MS2)2 respectively. This procedure is followed for all the energy density terms and included in Petaspin micromagnetic solver [4]. Here, we focus on the statics and dynamics of domain walls (DWs) when excited by spin-orbit torque (SOT). Regarding the static states of AFM, we show that as in FM, AFM also may exhibit periodic patterns due to the interfacial Dzyaloshinskii-Moriya interaction (iDMI) [5]. In this case, the ‘effective’ inhomogeneous exchange accounting for the two intralattice inhomogeneous exchange contributions and the one interlattice inhomogeneous exchange plays the role of the exchange interaction in FM. Correspondingly, the ‘effective’ iDMI to consider is the sum of the iDMI parameter of the two sublattices. Additionally, we show that similarly to FM, iDMI promotes the nucleation of new domains at the edges of and AFM strip even for moderate current densities [6]. Differently from FM, in AFM this effect is more efficient due to the robustness of the Néel DW against SOT excitation dynamics. This effect is one of our main results since it could limit the maximum DW velocity in and AFM racetrack memory because the nucleation of new domains would change the stored information, thus limiting the maximum current density that can be applied. We have also compared full micromagnetic simulations with a collective coordinates (CC) model which also includes the DW width changes due to the SOT and the spatial tilting of the domain wall [7] for different saturation magnetization ratios. The CC model properly predicts the domain wall width at equilibrium. On the other hand, when the magnetic texture is excited by a current through the SOT we can find three scenarios: (i) when the exchange and anisotropy parameters are larger enough, the DW dynamics can be described by the CC model even if we neglect the DW tilting and width variation. When we neglect these effects we talk about the ‘rigid CC model’. (ii) The cases where the exchange is lower but the anisotropy is large enough cannot be properly described by the rigid CC model. In these cases, the effect of the tilting and the DW width reduced the stationary DW velocity and, thus, they should be taken into account to describe the dynamics. This is shown in Fig. 1 where the solid linear line corresponding to the rigid CC model cannot predict the stationary DW velocities from the full micromagnetic simulations. Nonetheless, the dashed line corresponding to the CC model follows the saturation of the velocity due to the DW tilting. (iii) For low anisotropy parameters the excitation of self-oscillation threshold can be found for moderate current densities (red line in Fig. 2). We observe that before this threshold, where the Gilbert damping is almost compensated by the antidamping term induced by the SOT, the DW velocity increases faster than the usual linear trend being able to reach much larger velocities. Neither the rigid CC model nor the CC model can describe this effect and these cases required a full micromagnetic approach. From a fundamental point of view, this super-linear trend is another of our main results. The threshold current for self-oscillations depends on the anisotropy [8] which can be tuned, opening a path to exploit the super-linear DW mobility under moderate current densities.

**References:**

[1] S. Emori, U. Bauer, S.-M. Ahn, et.al., Nat. Mater., Vol. 12, 611 (2013). [2] H. V Gomonay and V. M. Loktev, Phys. Rev. B, Vol. 81, 144427 (2010). [3] L. Sánchez-Tejerina, et. al., IEEE Magn. Lett., Vol. 11, 2505005 (2020). [4] A. Giordano, G. Finocchio, L. Torres, et. al., J. Appl. Phys. , Vol. 111, 07D112 (2012). [5] R. Tomasello, et al., Phys. Rev B, Vol. 102, 224432 (2020) [6] L. Sánchez-Tejerina, et. al., Phys. Rev. B, Vol. 101, 14433 (2020). [7] E. Martinez, S. Emori, N. Perez, et. al., J. Appl. Phys., Vol. 115, 213909 (2014). [8] V. Puliafito, R. Khymyn, M. Carpentieri, et. al., Phys. Rev. B, Vol. 99, 024405 (2019).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/161fbbefb0726562a28ef3d0cc27a4c9.jpg)
Fig. 1 Domain wall velocity near the angular momentum compensation point as a function of the current density for a case with low exchange and large anisotropy. Dots stand for full micromagnetic simulations, solid lines for the rigid domain wall model, and the dashed line for the non-rigid domain wall model.

![](https://assets.underline.io/uploads/markdown_image/1/image/bc6cd85cb0155057350b7edfb0a26663.jpg)
Fig. 2 Domain wall velocity near the angular momentum compensation point as a function of the current density for a case with low exchange and low anisotropy. Dots stand for full micromagnetic simulations, solid lines for the rigid domain wall model, and the dashed line for the non-rigid domain wall model.

Micromagnetic modelling of ferro-, ferri-, and antiferromagnetic materials.

Research into geometrical frustration over the past two decades has revealed fundamentally new exotic behavior [1]. The magnetic interactions of the structure, coupled with the specific geometry causes frustration in the system, referred to as artificial spin ices. Several geometries exist, but for the purpose of this research, the kagome arrangement was used. In general, the geometry of these frustrated systems follows the ice rule, that being two interacting moments point in while two point out. The kagome however follows the modified ice rule of two moments pointing in while one points out or vice versa. These systems appear as 2D analogues to the 3D structure and it is the lattice configuration along with the specific frustration created that causes these exotic behaviors. Presented here are the magnetoresistances obtained for kagome magnetically frustrated systems in a single vertex and lattice format. Both geometries are compared in a magnetically connected format and a magnetically disconnected format in order to observe the exchange and dipolar energies. The structures were simulated with parameters for permalloy, using BORIS [2], and dimensions of 1µm long, 100nm wide and 10nm thick. The aim was to evaluate the magneto resistance, which was done assuming the current density aligns with the long axis and a basic resistor network. By comparing the simulated data for the connected and disconnected single vertex with the measurements taken previously for a lattice, a greater understanding of the effect of the disconnected versus connected structures can be achieved. Measurements of the kagome lattice showed that the disconnected lattice had a change in magnetoresistance that was 2~3 times greater than the connected lattice, experimentally. The simulations of the single vertex’s magnetoresistance differ by approximately an order of magnitude to those of the experiments. While the simulations are not for the full lattice, they shed some light on the experimental results obtained and the single vertex interactions that occur within the lattice. The simulation of the hybrid vertex appears to mirror that of the experimental data obtained for the hybrid lattice while the simulation of the connected vertex appears to differ somewhat from the experimental data obtained for the connected lattice. This highlights the implications of the experimental work in that the connected kagome has more complex interactions throughout the lattice while the magnetically disconnected hybrid is rather a simplified system. The subtle difference in the geometries of the connected and disconnected lattices aids in the explanation of the effects and interplay of both the spin structures and the current density. This work demonstrates the potential for using simulations to expose magnetic interactions in a lattice type structure and other interesting geometries, along with the enhanced magnetoresistance signals that were obtained.

**References:**

References: [1] Nisoli, C., Moessner, R., & Schiffer, P. (2013). Colloquium: Artificial spin ice: Designing and imaging magnetic frustration. Reviews of Modern Physics, 85(4), 1473. [2] Lepadatu, Serban. (2019). Boris Computational Spintronics User Manual.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/bc02a8200bfe4ba73c8d18a80397bf3d.jpg)
Figure 1. SEM images of an electrically connected but magnetically disconnected (hybrid) lattice (left). Also shown is the standard honeycomb lattice for comparison.

![](https://assets.underline.io/uploads/markdown_image/1/image/65475fb3f43153712262934a633adf6d.jpg)
Figure 2: Simulated single vertex and experimental lattice for the magnetically disconnected, electronically connected (hybrid) kagome plotted against the applied magnetic field. Data shown is the positive part of a full sweep of the magnetic field i.e. from a positive applied field through to the negative of the same applied field.

Micromagnetic simulations of artificial spin ice lattices and vertices

The micromagnetic configuration of a ferromagnetic sample results from the balance between different energy terms, in particular the magnetic anisotropy, the exchange energy and the magnetostatic energy. A very large variety of domain structures is observed depending on the intrinsic properties of the ferromagnetic material (anisotropy, exchange, spontaneous magnetization) and on its shape. Of particular interest are domains presenting a cylindrical symmetry that appear in thin films or patterned cylindrical micro/nanostructures. As examples, magnetic bubble domains have been extensively studied in micron-thick rare-earth orthoferrites films where they were considered for magnetic recording applications. A theory linking the static stability of these structures to both the material properties and sample geometry was developed, highlighting the requirement that the uniaxial anisotropy must overcome the stray field energy in order to stabilize the bubble. Stable cylindrical target domains have also been observed in a variety of patterned magnetic thin films, characterized by multiple nested concentric magnetic rings. Such micromagnetic configurations have been reported in systems with out-of-plane magnetization such as high magnetic anisotropy Co and FePt films [1] or perpendicular anisotropy multilayers [2,3], as well as in low anisotropy NiFe and Ni films [4,5] in which the magnetization is mostly in-plane with a weak out-of-plane component. Several numerical simulation studies have been undertaken to investigate these magnetic configurations [1-3,5-7]. Most of these simulations assumed the presence of a strong perpendicular anisotropy. Numerical simulations with low anisotropy systems have received less attention [4,5], and still require to consider a rather large perpendicular uniaxial anisotropy - attributed to built-in magnetostriction or resulting from a columnar grain structure [8] - to explain the formation of bubble or target domains. In this paper, we present experimental and numerical results obtained on patterned micron-size permalloy disks showing magnetic target domains. We conducted a thorough investigation of the evolution of the magnetic structure as a function of the Py thickness. Arrays of microdisks were patterned by optical lithography, e-beam evaporation and lift-off step. Disks diameter is constant at 1.3 µm and the thicknesses was varied from 90 nm to 400 nm. The MFM imaging of the remanent magnetization for disks in the as deposited state are shown in Figure 1. Thinner disks exhibit the expected vortex structure (Fig. 1a, b). For thicker disks, imaging reveals a structure with spiraling or concentric ring domains with alternating, up and down, perpendicular component of the magnetization (Fig. 1c - 1h). Micromagnetic numerical simulation have been carried out where we consider that the disks exhibit a columnar magnetic grain structure with variable inter-grain coupling and, in contrast with previous calculations, no contribution from magnetocrystalline or intrinsic uniaxial anisotropies. In our case, the inter-grain coupling intensity σ allows for the modulation of the demagnetizing field energy, giving rise to various magnetic configurations that will be described in details (Fig. 2). It is found that, for the thinnest disks, despite the columnar structure, the most stable magnetic structure is the vortex structure. Above a critical thickness the lowest energy structure evolves into a target domain structure, with a number of concentric domains that decreases as the thickness increases. It is also observed that, in addition to the target structure, less symmetrical spiral or disordered metastable structures can be observed. These observations closely match the experimental results for disks with similar aspect ratio, indicating that, together with σ, these are the two governing parameters in this case. We will also present details of the different energy contributions and describe the internal structure of the magnetization. This model based on a columnar texture of the film constitutes an alternative to explain the formation of target domain structures without relying on the existence of a perpendicular anisotropy difficult to justify in materials such as permalloy.

**References:**

[1] C. Moutafis, S. Komineas and C. A. F. Vaz, Phys. Rev. B 76, 104426 (2007). [2] F. Garcia, H. Westfahl and J. Schoenmaker, Appl. Phys. Lett. 97, 022501 (2010). [3] T. Liu, V. Puliafito and F. Montaigne, J. Phys. D: Appl. Phys. 49, 245002 (2016). [4] P. Eames and E. Dan Dahlberg, J. Appl. Phys. 91, 7986 (2002). [5] G. D. Skidmore, A. Kunz and C. E. Campbell, Phys. Rev. B 70, 012410 (2004). [6] J. K. Ha, R. Hertel and J. Kirschner, Europhys. Lett. 64, 810 (2003). [7] S. Komineas, C. A. F. Vaz and J. A. C. Bland, Phys. Rev. B 71, 060405 (2005). [8] S. Finizio, S. Wintz and D. Bracher, Phys. Rev. B 98, 104415 (2018).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/45753e3a6b6a4c36a98fc4ede0ad4c75.jpg)
Fig. 1: MFM images of permalloy disks with 1.3 µm diameter, in the as-deposited state, with different thicknesses. (a, b) Vortex structure; (c, e) Four concentric ring domains structure; (d, f) Spiraling domains structure; (g, h) Three concentric ring domains structure.

![](https://assets.underline.io/uploads/markdown_image/1/image/f9c8a31341f86902d48f4d2469dc767b.jpg)
Fig. 2: (a) Calculated magnetic structure in the median plane of the disk (plane at half thickness), as a function of the thickness and σ coupling parameter. (b) Examples of metastable configurations with disordered, spiral or incomplete ring domains, obtained with σ = 0.1 and 180 nm thickness (top); σ = 0.2 and 150 nm (middle); and σ = 0.1 and 150 nm (bottom). The red/blue colors refer to the ±Mz magnetization component.

Concentric target domains in magnetic nanodisks

Recent advances on the stabilization and manipulation of chiral magnetic configurations hosted by systems that consist in alternating atomic layers of ferromagnetic and non-magnetic materials hold promise of innovative spintronic devices. The low dimensionality of the systems promotes spin-orbit driven interfacial effects[1] like the antisymmetric Dzyaloshinskii-Moriya interaction (DMI) and interfacial magnetic anisotropy (iMAE), whose relative strengths may be tuned to achieve stable nanometer sized magnetic objects with fixed chirality called magnetic skyrmions[2], which are proposed as information carriers in future spin-orbitronics technologies. While in most of the cases this is obtained by engineering complex multilayers stacks in which interlayer dipolar fields become relevant[3], we consider here a simple epitaxial trilayer in which a ferromagnet (namely Cobalt, Co), with variable thickness, is embedded between a heavy metal and graphene[4]. The latter enhances the perpendicular magnetic anisotropy of the system, promotes a Rashba-type DMI, and can sustain very long spin diffusion length. The work consists in the development of a layer-resolved micromagnetic model capable to account for the interfacial nature of the interactions[5], which leads to macroscopic parameters that depend on the thickness of the ferromagnetic layer. We demonstrate that our model correctly reproduces the experimental magnetization configurations and the spin reorientation transition (SRT) of a gr/Co(tCo)/Pt sample[4]. Next, we study the stabilization of magnetic skyrmions in patterned magnetic nanodots with a diameter of 256 nm. In particular, we analyze the thickness-dependent stability of the skyrmionic state in contrast with a single domain state. Our results demonstrate that, for samples with Co thicknesses larger than 3.6 nm, intrinsic mixed Bloch-Néel (predominantly Bloch-type) skyrmions are stabilized in 256 nm wide dots. For thicknesses larger than ~5 nm, the results show that the geometrical confinement imposed by the nanodot promotes the stabilization of chiral spin textures similar to skyrmions but with complex and fluctuating domain walls.

**References:**

[1] A. Soumyanarayanan, N. Reyren, A. Fert and C. Panagopoulos, Nature, Vol. 539, p. 509-517 (2016) [2] A. Fert, N. Reyren and V. Cros, Nature Reviews Materials, Vol. 2, p. 170301 (2017) [3] W. Legrand, J.-Y. Chauleau, D. Maccariello et al., Science Advances, Vol. 4(7), p. eaat0415 (2018) [4] F. Ajejas, A. Gudín, R. Guerrero et al., Nano Letters, Vol. 18(9), p. 5364-5372 (2018) [5] P. Olleros-Rodríguez, R. Guerrero, J. Camarero et al., Applied Materials and Interfaces, Vol. 12(22), p. 25419-25427 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/b0a17000d2dcf3624f3186e212665bbe.jpg)
Fig.1 Sketch of modelled magnetic nanodots at selected cobalt thicknesses. The stabilized spin textures are represented by the arrows at the top of the structures. The color of the arrows represents the out-of-plane component of the magnetization. The color of the surface represents the X-component of the magnetization.

Micromagnetic Study of Stable Skyrmions in Dot-Patterned Graphene-Based Magnetic Trilayers

Magnetic skyrmions in ferromagnetic materials are mainly stabilized by the interplay of several magnetic energy contributions, i.e. exchange, dipole, anisotropy, and Zeeman. In particular, the asymmetric Dzyaloshinskii-Moriya exchange interaction (DMI) introduces chiral canting between neighboring spins and favors skyrmion stability [1]. Skyrmions are particle-like chiral spin textures found in magnetic films and patterned nanostructures with out-of-plane anisotropy and are considered to be potential candidates as information carriers in next-generation data storage devices [2]. The challenge of the experimental study of skyrmions is to find an efficient technique for forming stable skyrmions. In particular, the outstanding issues are the reproducible generation, stabilization, and confinement of skyrmions at room temperature [2,3,4]. It is well known that various magnetic textures can be stabilized by geometrical confinement using artificial nanostructures, however, there are several key challenges like reproducible skyrmion formation that still need to be addressed. We present two methods of skyrmions formation in nanodots and antidots arrays. For these first structures, we have shown the formation of skyrmion using the tip of a magnetic force microscope [3], and during the remagnetization process for antidots arrays [4]. Here, we present the results of the investigation of a skyrmion formation in Pt/Co/Au nanodots. We demonstrate that the high magnetic momentum probe induces individual skyrmions during the scan. A specific path of the MFM tip causes a systematic change in the state of the magnetic domains during scanning. The process of annihilation of remaining domains is ruled by magnetostatic interactions where the magnetic domains are simultaneously repelled from the edges of the nanodots and from the tip field. Micromagnetic simulations have shown that specific conditions restricting the movement of magnetic domains must be met in order to form the magnetic skyrmion [3]. In this work, we present a method to form nanometer-sized magnetic skyrmions in an array of magnetic topological defects in the form of an antidot lattice. In particular, the introduction of the holes in a thin magnetic film modifies the spatial distribution of the effective field, which has a profound influence on the formation mechanism of the skyrmions. The position of skyrmion formation was recognized at the saddle point of the lattice as a result of the spatial distribution of the effective field. With micromagnetic simulations, we elucidate the steps of the skyrmion formation process and showed that this process depends on the antidot lattice parameters. This behavior is confirmed with scanning transmission x-ray microscopy measurements [4]. The work was supported by National Science Centre of Poland, Projects Nos. UMO-2017/27/N/ST3/00419 and UMO-2018/30/Q/ST3/00416.

**References:**

[1] Fert et al. "Magnetic skyrmions: advances in physics and potential applications", Nat. Rev. Mater. 2, 17031 (2017). [2] C Back et al. "The 2020 skyrmionics roadmap", J. Phys. D: Appl. Phys. 53 363001 (2020). [3] Iu. V Vetrova et al. "Skyrmion formation in nanodisks using magnetic force microscopy tip", arXiv preprint arXiv:2002.11685. [4] S. Saha et al. "Formation of Néel-type skyrmions in an antidot lattice with perpendicular magnetic anisotropy", Phys. Rev. B 100, 144435.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/f61ffb0cc45868d21b91dd067ad9f3f4.jpg)
Fig. 1: Simulated images of the evolution of the magnetic configuration in nanodots for random initial state and different positions of the tip field. x (nm) Simulations were performed for 400 nm disks with 4 repetition of the stack with DMI=1.0 mJ/m 2 . The red circle represents the z-component of the magnetic field induced by the high-momentum tip. The amplitude of tip field was 200 mT at center of the spot, and spot diameter was equal to 25 nm. The colors represent the orientation of the magnetization.

Néel type skyrmions formation in nanodots and antidot arrays.

Skyrmions in magnetic thin films are solitonic excitations that could open new possibilities for spintronic devices. The recent successes obtained in ferromagnetic films have shown some limitations, with dipolar-coupling induced large skyrmion sizes and topology induced gyrotropic deflection. Antiferromagnetic films and their artificial equivalents that are ferrimagnets and synthetic antiferromagnets could overcome these limitations. In this talk, we explore the skyrmion dynamics in such materials. Separating them into two antiferromagnetically coupled sublattices, we show that the dynamics can be described by two coupled Thiele equations. This framework is used to address the consequences of the small skyrmion size and the angular momentum compensation. It provides a simple approach to describe this problematic. Small skyrmions, that are possible due to the vanishing total magnetization, are shown to be significantly slower (velocity proportional to radius) than larger ones (size independent velocity). This result is a direct consequence of the skyrmion topology, which induces an excess of dissipation per size unit at small sizes. Angular momentum compensation is shown to cancel the gyrotropic deflection, without the need to modify the topology and maximizes the velocity. In a track, where the edge repulsion cancels the gyrotropic deflection, the angular momentum compensation has no effect and skyrmions travel faster that non confined skyrmions. From an applied perspective, the data rate is given by the velocity to radius ratio and correspond to the inverse of the time needed for a skyrmion to jump from one site to another. This shows that small skyrmions in ferrimagnetic films would still be preferable to large ones, providing fast and dense devices.

**References:**



**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/255c7d94b2bc6cbde230f79959b9396a.jpg)
(a) Variation of the skyrmion velocity in a CoGd thin film and (b) variation of the skyrmion velocity to skyrmion radius ratio (proportionnal to an application data rate), when the skyrmion is confined in a track, as a function of the alloy compostion, close to the angular momentum (LS) composition (79.4%), and for different DMI values. The points are the results of simulations, and the lines are the result of the model. Note that the skyrmion size increases with DMI at a given composition, and that the size is minimual at the magnetic moment compensation (77.5%).

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