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Magnetic domain walls (DWs) in ferromagnetic nanowires are candidates for both Boolean [1–3] and non-Boolean computing architectures such as neuromorphic/stochastic computing [4]. In this context, interconnected nanorings are particularly interesting as they show emergent behaviour which make them interesting for reservoir computing [5-6]. Since these approaches use magnetic DW motion for logic operations, it is crucial to understand the dynamics of DWs in these structures for device realisation. We used micromagnetic simulations (using MuMax3 [7]) to simulate DWs passing through junctions of 2 μm diameter, 5 nm-thick Permalloy bi-ring structures with different ring widths and degrees of ring overlap. An example structure is shown in the inset of Fig. 1. The system was initialized with DWs in the upper and lower halves of the left and right ring. A rotating field was then applied to move the DWs towards the junction following previous experimental approaches [8]. The field amplitude was varied to record the depinning field HDP when the DWs separated from the junction. The variation of HDP for different overlap and wire widths is shown in Fig. 1. A minimum in HDP is observed for most widths indicating that competing mechanisms are involved in the depinning mechanism. In order to better understand these competing mechanisms, we look at the magnetic energy density [7] E=-½M.Beff (1) for a wire width of 100 nm with 12 (Fig. 2a) and 100 nm (Fig. 2b) overlap . In Fig. 2 (a), the DWs form a ‘diamond’ state which forms an energy minima requiring a high HDP to depin the DW. For the largest overlap in Fig. 2 (b), the DWs get ‘pinned’ before and after the junction and a high HDP is required to overcome this pinned energy threshold. Thus the intermediate overlap states (of 25 &amp; 50 nm) require lower HDP due to competition between these states. We expect that the understanding of these dynamics is crucial for the development of ring based computing applications.References: [1] S. S. P. Parkin, M. Hayashi, L. Thomas, Science 320, 190 (2008). [2] D. A. Allwood et. al., Science 309, 1688 (2005). [3] K. A. Omari et. al., Adv. Func. Mater. 29, 1807282 (2019). [4] J. Grollier, et. al., Nat. Electron. 1-11 (2020). [5] R. Dawidek et al., MMM conference (2017). [6] R. Dawidek et al., submitted [7] A. Vansteenkiste et. al., AIP Adv. 4 (10), 107133 (2014). [8] M. Negoita et. al., J. Appl. Phys. 114.1, 013904 (2013).

MMM 2020

Domain Wall Dynamics in Junctions of Bi-Ring Structures

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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Magnetic domain walls (DWs) in ferromagnetic nanowires are candidates for both Boolean [1–3] and non-Boolean computing architectures such as neuromorphic/stochastic computing [4]. In this context, interconnected nanorings are particularly interesting as they show emergent behaviour which make them interesting for reservoir computing [5-6]. Since these approaches use magnetic DW motion for logic operations, it is crucial to understand the dynamics of DWs in these structures for device realisation. We used micromagnetic simulations (using MuMax3 [7]) to simulate DWs passing through junctions of 2 μm diameter, 5 nm-thick Permalloy bi-ring structures with different ring widths and degrees of ring overlap. An example structure is shown in the inset of Fig. 1. The system was initialized with DWs in the upper and lower halves of the left and right ring. A rotating field was then applied to move the DWs towards the junction following previous experimental approaches [8]. The field amplitude was varied to record the depinning field HDP when the DWs separated from the junction. The variation of HDP for different overlap and wire widths is shown in Fig. 1. A minimum in HDP is observed for most widths indicating that competing mechanisms are involved in the depinning mechanism. In order to better understand these competing mechanisms, we look at the magnetic energy density [7] E=-½M.Beff (1) for a wire width of 100 nm with 12 (Fig. 2a) and 100 nm (Fig. 2b) overlap . In Fig. 2 (a), the DWs form a ‘diamond’ state which forms an energy minima requiring a high HDP to depin the DW. For the largest overlap in Fig. 2 (b), the DWs get ‘pinned’ before and after the junction and a high HDP is required to overcome this pinned energy threshold. Thus the intermediate overlap states (of 25 & 50 nm) require lower HDP due to competition between these states. We expect that the understanding of these dynamics is crucial for the development of ring based computing applications.References: [1] S. S. P. Parkin, M. Hayashi, L. Thomas, Science 320, 190 (2008). [2] D. A. Allwood et. al., Science 309, 1688 (2005). [3] K. A. Omari et. al., Adv. Func. Mater. 29, 1807282 (2019). [4] J. Grollier, et. al., Nat. Electron. 1-11 (2020). [5] R. Dawidek et al., MMM conference (2017). [6] R. Dawidek et al., submitted [7] A. Vansteenkiste et. al., AIP Adv. 4 (10), 107133 (2014). [8] M. Negoita et. al., J. Appl. Phys. 114.1, 013904 (2013).

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Domain Wall Dynamics in Junctions of Bi-Ring Structures

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Manipulation of coupling and magnon transport in magnetic metal-insulator hybrid structures

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