Magnetic skyrmion research using virtual simulation environments

Abstract

Magnetic skyrmions are particle-like structures in the magnetic moment field of a helimagnetic material. Since skyrmions are of the order of nanometres in size, they are a promising replacement for the larger magnetic domain structures proposed for racetrack memory devices. Skyrmions also motivate the design of small logic gate devices. However, skyrmions are only stable in limited regions of the parameter space defined by the strength of an external magnetic field and the properties of the material.

To model the behaviour of skyrmions and the magnetic moment field in a real sample, this thesis contains multiple numerical investigations using the micromagnetic model. Finite-element and finite-difference approaches are used to solve different problems. Due to the complexity of micromagnetic modelling, a software platform is implemented to perform simulations within virtual machines and containers. This novel mechanism has been deployed for public use to improve the reproducibility and accessibility of micromagnetic simulations, and is used for all numerical simulations in this thesis.

This thesis demonstrates the potential for materials with an easy-plane magnetocrystalline anisotropy to support skyrmions. This investigation augments existing analytical approaches by considering demagnetising and thickness effects to more accurately model a real helimagnetic material. Skyrmions are found to exist in easyplane anisotropy systems, expanding the space of possible materials for skyrmionbased devices. The size of the skyrmion is also found to increase with the strength of the easy-plane anisotropy, which impacts negatively on the storage density of these devices.

In addition to investigating the effect of easy-plane anisotropy, the micromagnetic approach is adapted to model arbitrary polycrystalline grain structures. This adaption is used to understand how a polycrystalline grain structure affects the magnetic moment field of a helimagnet. Individual skyrmions are found to be repulsed by grain boundaries, though skyrmions in neighbouring grains attract each other. This finding suggests that an individual skyrmion could be used as the data structure in logic gate devices, where the polycrystalline structure could determine the logic.

These two investigations are combined to model a real bulk helimagnetic material with an arbitrary grain structure and easy-plane anisotropy, and to explain a large topological Hall resistivity observation, which is an indicator of skyrmion presence. The thickness of the material gives rise to two additional skyrmionic structures. This means sufficiently thick materials with both easy-plane anisotropy and an irregular grain structure can support skyrmion states.

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Identifiers

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

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