Dissertation

Skyrmionic states in confined helimagnetic nanostructures

Beg, Marijan

2016

Overview

Magnetic skyrmions have the potential to provide solutions for low-power, high-density data storage and processing. One of the major challenges in developing skyrmion-based devices is the skyrmions' magnetic stability in confined helimagnetic nanostructures. Through a systematic study of equilibrium states, using a full three-dimensional micromagnetic model, we demonstrate that skyrmionic states are the lowest energy states in confined helimagnetic nanostructures at zero external magnetic field and in absence of magnetocrystalline anisotropy. We show that bistable skyrmionic states undergo hysteretic behaviour between two energetically equivalent skyrmionic configurations with different core orientation, even in the absence of both magnetocrystalline and demagnetisation-based shape anisotropies, suggesting the existence of novel Dzyaloshinskii-Moriya-based shape anisotropy. We show that the skyrmionic state core reversal is facilitated by the Bloch point occurrence and propagation. In this work, we also study the dynamic properties (resonance frequencies and corresponding eigenmodes) of these skyrmionic states in confined helimagnetic nanostructures. The eigenvalue method allows us to identify all resonance frequencies and corresponding eigenmodes that can exist in the simulated system. However, using a particular experimentally feasible excitation can excite only a limited set of eigenmodes. Because of that, we perform and report ringdown simulations that resemble the experimental setup using both an in-plane and an out-of-plane excitations. In addition, we report the nonlinear dependence of resonance frequencies on the external magnetic bias field and disk sample diameter and report the possible reversal mode of skyrmionic states. Finally, we show that neglecting the demagnetisation energy contribution or ignoring the magnetisation variation in the out-of-film direction in either static or dynamic simulations is not always justified.

• Publisher: ProQuest Dissertations & Theses