Computational methods and studies in nanomagnetics

Abstract

Computational study of magnetic materials has been crucial for the development of new technologies in areas such as data storage. One challenge with current computational methods is that the dipolar field calculation dominates the computation time. In this work we show how the fast multipole method can be applied to this problem, and other long range force and potential calculations, through symbolic generation of operator functions in a generic fashion. We study the equilibrium states found in triangular and square samples of the helimagnetic material FeGe, in which skyrmions have been observed, by varying the size and applied field. We show that the equilibrium states of such systems is modified in comparison to previously studied disk systems of this material, with larger sample sizes required for skyrmions to form the ground state. We show the final states obtained from relaxation of a uniform magnetisation in order to provide data for experimental comparison. We then study the energy barriers between ferromagnetic and skyrmion states in Cobalt monolayers when triangular, square and Bezier edge defects are introduced, and show how this varies by size and by whether the dipolar field is included in the calculation. Finally, we study the equilibrium behaviour of Bloch points in FeGe disks and nanotracks made up of two layers in which the Dzyaloshinskii-Moriya interaction has opposing chirality. We then study the dynamic behaviour under an in-plane magnetic field, showing that the Bloch point reaches a velocity linearly proportional to the applied field.

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ORCID for Hans Fangohr: orcid.org/0000-0001-5494-7193

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