Simulations on Ferromagnetic Nanostructures



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Understanding domain wall (DW) motion in ferromagnetic nanostructures is crucial for realizing proposed magnetic data storage and logic devices. Simulations offer time-wise and cost-wise efficiency when testing theories of magnetism, which structure the nucleation, movement, and annihilation of DWs. During my work with the Aidala Group as summarized by this thesis, we have explored two topics: One constituted the effects of curvature on DW motion in elliptical nanorings, and the other did the nucleation of 360 deg DWs in a wire by using a local circular field. In this thesis, we investigate the effects of curvature on DW pinning and motion by studying elliptical rings via micromagnetic simulations. [1] Elliptical rings with constant width have varying curvature: The minor axis has the lowest curvature and the major the greatest. We can nucleate DWs anywhere along the ellipse by applying the appropriate uniform magnetic field. However, DWs do not pin at all locations along the ellipse once the field is removed. We study the pining and depinning of DWs by applying a slowly increasing elliptical magnetic field to determine the magnitude of the field at which DWs begin to move. By varying the major to minor axis ratio, we examine the effects of curvature on DW pinning: A larger field is requisite for moving DWs in regions of higher curvature (near the major axis) than that for lower curvature (near the minor axis). Overall, increasing the major to minor axis ratio of elliptical nanorings requires an increasing field strength to depin the DWs along the major axis. Furthermore, our simulations demonstrate the feasibility of nucleating a 360 deg DW at a specific location along a ferromagnetic wire by applying a local circular field that is centered in close proximity to the wire. We simulate the field strength as if from a current carrying wire, which we can experimentally realize by passing current through the tip of an AFM. [2] The successful nucleation of a 360 deg DW depends on the dimensions of the ferromagnet, on the strength of the circular field, and on the distance of the center of the field from the wire. Once a 360 deg is nucleated, its position shifts with time. We use notches to pin the 360 deg DWs. We explore the optimal size and spacing of notches that allow the greatest packing density with control over the nucleation and annihilation of individual DWs. 1: M. J. Donahue and D. G. Porter, OOMMF User’s Guide, Version 1.0 (Interagency Report NISTIR 6376 (National Institute of Standards and Technology), MD, USA, 1999). 2: T. Yang, N. R. Pradhan, A. Goldman, A. S. Licht, Y. Li, M. Kemei, M. T. Tuominen, and K. E. Aidala, Appl. Phys. Lett. 98 (2011).



domain wall, simulation, ferromagnet, OOMMF, nucleation, exchange energy, magnetostatic energy, zeeman field, stray field, 360 deg domain wall, 180 deg domain wall, topological edge defect and charge