The ability to manipulate the topological spin textures, such as magnetic skyrmions, is one of the crucial requirements in advance spintronics research. Recently, attempts have been made to understand the mechanism of Domain wall (DW) pair to skyrmion conversion in a specially designed nanotrack which may be useful in memory and logic operation [1,2]. Here, using micromagnetic simulation, we investigate the role of geometry in controlling the DW to skyrmion conversion in a three-channel narrow nanotrack adjoined to a wide nanotrack. Dimension of the nanotrack is as follows; length - 800 nm, thickness - 1 nm, width of narrow nanochannel - 20 nm, width of wider nanochannel - 200 nm, with spacing between the three narrow channels (lower -1, middle - 2, and upper - 3) varied symmetrically and asymmetrically from 10 - 30 nm in step of 10 nm. The simulations are performed using a python interfaced OOMMF known as Ubermag, where a Zhang-Li torque evolver in the Landau-Lifshitz-Gilbert equation is incorporated to understand the magnetization dynamics under the influence of spin current [3]. In figure 1, by keeping the separation between narrow channels 1 and 2 as well as 2 and 3 fixed as 10 nm, Gilbert damping α = 0.3 and non-adiabatic torque parameters β = 0.3, conversion of DWs to two skyrmions are shown [4]. Interestingly, we note that while the steady state velocity for both the skyrmions are same, the time scale involved in DW to skyrmion for individual skyrmion varies resulting in difference in the x-position. By increasing the separation between channel 2 and 3 to 30 nm and keeping all other parameters fixed, formation of a skyrmion and a meron is observed in figure 2. We envision that such difference in the observation of topological spin texture by changing the separation between the narrow nanochannels will be useful for implementing futuristic skyrmion-based logic gate devices. 

**References** 
[1] A. Fert, V. Cross, J. Sampaio Nature Nanotech., Vol. 8, p. 152, (2013). 
[2] Y. Zhou, M. Ezawa, Nat. Commun., Vol. 5, p. 4652, (2014). 
[3] M. Beg, M. Lang, H. Fangohr., IEEE Trans. Magn. Vol. 58, p. 1-5 (2021) 
[4] P. Hari Prasanth, Syamlal S K, B. Priyanka, J. Sinha, Manuscript under preparation. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/d1d0c5f3d1bfa99da4e93dfe7637c1cd.png) 
Fig 1: Snapshot of DW to skyrmion conversion from the three symmetrically separated narrow nano-channels for α = β = 0.3 and separation - 10 nm. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/f37168fd86d14542429e1d5a3a107039.png) 

Fig 2: From asymmetrically separated narrow nano-channels (separation between 1 and 2 - 10 nm and between 2 and 3 - 30 nm) and for α = β = 0.3 conversion of DWs into a skyrmion and a meron.

Controlled skyrmion creation from a Domain wall pair for logic gate applications

Analysis of magnetization reversal in ferromagnetic nanodots of different shapes with dimension in the range of 100 nm in the presence of Dzyaloshinskii-Moriya Interaction (DMI) is extremely important for technological application [1]. Though a few studies have explored the effect of DMI on the magnetization reversal mechanism in nanostructures, to disentangle the influence of shape and DMI requires further systematic studies [2-4]. Here, we investigate the effect of DMI (D) and in-plane bias field (H) on magnetization reversal in perpendicularly magnetized isolated square, circular, and triangular nanodot using the micromagnetic simulation Mumax3 [5]. The side length/diameter of the nanodot is chosen as 128 nm and the thickness as 1 nm and the material parameters of Co is selected. Our results indicate that the coercive field monotonically decreases as the strength of D increases and loop becomes asymmetric in the presence of both D and H in all three shapes. Interestingly, for D = 0 and H ≠  0, nucleation of the reverse domain occurs at the central region of the nanodot (Figs. 1(a-c)), and in case of D ≠  0 and H ≠  0 chiral nucleation occurs at the far edge (Figs. 1(d-f)). Due to lateral asymmetry of the triangular nanodot, the total energy value is smaller for the triangular nanodots (for -100 mT < H < +100 mT, and -1 mJ/m2 < D < + 1 mJ/m2) thereby making it preferred for energy efficient magnetization reversal as compared to other two shapes. However, at sufficiently higher value of the magnitude of H (> 200 mT) and D (> 1 mJ/m2), due to the laterally symmetric sides of the square the energy barrier for the magnetization reversal reduces drastically (cf. Fig.1(f)). These findings indicate that the effect of D and H in lowering the energy barrier for the magnetization reversal is intricately related to the symmetry associated with the shape of the nanodots. 

**References** [1] F. Hellman, A. Hoffmann, Y. Tserkovnyak et al., Rev. Mod. Phys., Vol.89, p.025006 (2017) 
[2] S. Pizzini, J. Vogel, S. Rohart et al., Phys. Rev. Lett., Vol.113, p.047203 (2014) 
[3] D.-S. Han, N.-H. Kim, J.-S. Kim et al., Nano Lett., Vol.16, p.4438 (2016) 
[4] Syamlal S K, Hari Prasanth Perumal, and Jaivardhan Sinha., Mater. Lett., Vol.303, p.130492 (2021) 
[5] A. Vansteenkiste, J. Leliaert, M. Dvornik et al., AIP Adv., Vol.4, p.107133 (2014) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/55a7db279d3bf5b28b8f2b50d2d2ee8a.png) 
Figure 1: Snapshot of the spin configuration for square, circular and triangular nanodot during down to up (D-U) switching at H = -100 mT and D = 0 (a-c), H = -100 mT and D = 3 mJ/m2
(d-f). (g) Variation of total energy as a function of in-plane bias field for square, circular, and triangular nanodots at different value of D is plotted during D-U switching.

Effect of Dzyaloshinskii Moriya Interactions on Magnetization Reversal in Perpendicularly Magnetized Nanodots of Different Shape

In-depth investigation of structural and magnetic property correlation in technologically important heavy metal/ferromagnet/oxide heterostructures are crucial for the development of spintronics based devices [1]. In recent times, in Ta/CoFeB/TaOx heterostructures, zero-field magnetization switching, reasonably large Dzyaloshinskii-Moriya interaction, and stabilization of skyrmions have been reported [2-4]. Here, we study the change in magnetic and structural properties of sputter deposited (1 nm) Ta/(1.5 nm) CoFeB/(0.5 nm) TaOx heterostructures induced by post-deposition annealing at 300 °C. Both as-deposited and annealed film stacks possess in-plane magnetic anisotropy and we observe nearly two times increase in the saturation magnetization (Ms) of annealed film stack (~ 800 emu/cc) in comparison to the as-deposited one (~ 400 emu/cc). The surface topography study suggests reduced surface roughness for the annealed film stack as compared to the as-deposited stack. Using, x-ray photoelectron spectroscopy we analyze the B 1s, Ta 4f and Fe 2p spectra for both as-deposited and annealed heterostructures. Interestingly, the analysis of Ta spectra revealed that B has intermixed with Ta layers in the annealed stack, thus forming new chemical states such as TaB and TaOB (cf. Fig.1). Boron 1s peak analysis suggests that the annealing causes B diffusion out of the CoFeB layer and subsequent exposure to the atmosphere results in the formation of BOx. Analysis of Fe 2p spectra in the annealed stack shows an additional signature of metallic Fe0 2p1/2 peak, which likely plays a critical role in enhancing the Ms (cf. Fig.2). These XPS results are crucial in thorough understanding of the mechanism that governs Ms and ultimately, spin-orbit coupling induced effects in ferromagnetic thin film heterostructures.

![](https://assets.underline.io/uploads/markdown_image/1/image/a5c66219d056c0e02fc86687536ed4ca.jpg)

Fig. 1. XPS spectra of Ta 4f core levels of as-deposited and annealed films. The binding energy shift in the annealed spectra are due to formation of new chemical state such as TaB.


![](https://assets.underline.io/uploads/markdown_image/1/image/92c1c60f482d4ee564ad597e655c00b7.jpg)

Fig. 2. XPS spectra of Fe 2p core levels of as-deposited and annealed films. An additional peak Fe0 2p1/2 is observed in the annealed film stack.

Study of As-deposited and Annealed Ta/CoFeB/TaOx Heterostructures Using X-Ray Photoelectron Spectroscopy

In order to develop energy efficient race track memory devices, it is crucial to create and controllably move chiral spin textures, namely, skyrmions in magnetic nanotrack. In recent time, intense efforts have been dedicated to understand the mechanism of domain wall pair to skyrmion conversion in different junction geometry that may fit into the memory applications [1,2]. Here, we study the mechanism of DW pair to skyrmion conversion in a typical junction geometry which consists of the combination of two narrow nanotracks (length – 800 nm, thickness – 1 nm, width varying from 20 nm to 60 nm in steps of 20 nm) and a wide track (width 200 nm). The separation between narrow channels are selected as 10 nm, 30 nm and 50 nm. The micromagnetic simulations have been performed using Ubermag to investigate the magnetization dynamics and the Zhang-Li torque evolver is implemented in Landau Lifshitz Gibert equation to drive the DWs using spin polarized current [3]. For a fixed Gilbert damping parameter (α = 0.3) and different values of non-adiabatic spin torque parameter (β = 0.2, 0.3 and 0.5) we investigate the DW to skyrmion conversion in these nanotracks. Interestingly, under the influence of spin torque, the DW pair gets converted to skyrmion only if the geometrical constraints allow the favorable instability to set in at the junction between optimally separated narrow channels and wide channel (cf. Fig. 1(a-d)) [4]. For certain width and separation between the narrow channels, the DW pair to skyrmion conversion cannot be observed for sufficiently large time scale (cf. Fig. 2(a-d)). We believe these results will be helpful in designing the skyrmion based racetrack memory where the controlled creation of skyrmion is expected to play crucial role.

![](https://assets.underline.io/uploads/markdown_image/1/image/af3301ae9ac634fc6c9411eabcfd6b01.jpg)

Fig 1: Snapshot of DW pair to skyrmions conversion from two narrow channels for α = β = 0.3, width of the narrow nanochannel = 60 nm, separation between the narrow channel = 30 nm

![](https://assets.underline.io/uploads/markdown_image/1/image/ed5b1617a9c98ac080b569cc57b93e0f.jpg)

Fig 2: DW dynamics from two channels for α = β = 0.3, width of the narrow nanochannel = 40 nm, separation between the narrow channel = 10 nm


Influence of Geometry on Domain-Wall Pair to Skyrmion Conversion in Typical Magnetic Nanochannel

HOD: Skyrmions: Characterization, Dynamics and Control

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HPA: Magnetization Dynamics and Micromagnetics

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**Session chairs:** Ricardo Ferreira, INL - International Iberian Nanotechnology Laboratory and Akinobu Yamaguchi, University of Hyogo


EOD - Thin Films Ι

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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Hari Prasanth Perumal

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Controlled skyrmion creation from a Domain wall pair for logic gate applications

Effect of Dzyaloshinskii Moriya Interactions on Magnetization Reversal in Perpendicularly Magnetized Nanodots of Different Shape

Study of As-deposited and Annealed Ta/CoFeB/TaOx Heterostructures Using X-Ray Photoelectron Spectroscopy

Influence of Geometry on Domain-Wall Pair to Skyrmion Conversion in Typical Magnetic Nanochannel

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