Computer simulation studies of complex magnetic materials

Abstract

With the development of both computing power and software engineering, computer simulation of the micromagnetic model or atomistic spin model, has become an important tool for studying a wide range of different complex phenomena in magnetic materials. Meanwhile, the rapid improvement of advanced measurement techniques has allowed the probing of ultrafast magnetization dynamics, as well as the magnetic phenomena involving charge current, heat and light. The simulation of magnetism is now moving towards a multiphysics method. Therefore, fast, user-friendly, and extensible codes with accurate algorithms are helpful in understanding the physics and designing novel magnetic devices on the nanoscale.

In the preparation of this thesis we have developed Fidimag, which is a Python/C simulation tool supporting both micromagnetic and atomistic spin models. The software has also been extended to support the Landau-Lifshitz-Baryakhtar (LLBar) equation. Using Fidimag, we have performed simulations to study the domain-wall motion and spin-wave decay with the LLBar equation. We also explain the exchange damping in the LLBar equation as the phenomenological nonlocal damping by linking it to spin pumping, therefore, LLBar equation can be considered as a phenomenological equation of the nonlocal damping.

We studied magnon-induced domain-wall motion in the presence of Dzyaloshinskii-Moriya interaction (DMI) numerically and theoretically. We find that the presence of DMI and easy-plane anisotropy can drive the domain wall very effectively and that the domain-wall velocity depends on the sign of DMI constant. While the negative velocity is considered as a result of angular momentum conservation, we attribute this fast domain-wall motion to linear momentum transfer between magnons and the domain wall. By numerically solving the Landau-Lifshitz-Gilbert equation with a classical spin model on a two-dimensional system, we show that both magnetic skyrmions and skyrmion lattices can be moved with microwave magnetic fields. The mechanism is enabled by breaking the axial symmetry of the skyrmion with a static in-plane external field.

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Identifiers

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

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