Abstract
This thesis investigates the principles and properties of magnetic skyrmions, focusing on their morphologies, sizes, and characteristic energies to identify methods for manipulating their shapes and behaviors. By exploring these topics in the wider context of skyrmionic physics, this study provides insights that contribute to the advancement of skyrmion-based spintronic devices.
Magnetic skyrmions are nontrivial topological solitons found in magnetic films and present significant potential for academic research and industrial applications, particularly with regards to information storage devices. However, effectively controlling these enigmatic entities remains a challenge. Despite extensive experimental efforts and the accumulation of a wealth of data and theories from over the past decade, a coherent framework is lacking.
This thesis addresses this need by bridging several gaps in our understanding skyrmions as guided by recent revelations about their comprehensive landscape. As such, we focus on stripe skyrmions and their intriguing metastable behaviors. Key skyrmionic components, such as their condensed morphology and vital energies that govern their creation, transition, and annihilation are examined with the purpose of providing a general framework for controlling them. This thesis employs a general chiral magnetic film model to enable the comprehensive examination of skyrmions across various parameter spaces. Specifically, by modeling the spin profiles of skyrmions within skyrmionic crystals (SkXs), we gain a new understanding of a skyrmion’s morphological sensitivity to density, which differs from that of an isolated skyrmion. Using the nudged elastic band method, we determine the nucleation energy and energy barrier of skyrmions and analyze the transition paths of isolated skyrmions, further identifying the transition state as an infinitesimal skyrmion by linking the nucleation energy solely to its exchange stiffness. We demonstrate that the skyrmion energy barrier is uniquely determined by its exchange stiffness parameter, κ, which reaches its maximum when the skyrmion formation energy is zero. These energies remain independent of skyrmionic size under a constant κ and exchange stiffness. Building upon these insights, we provide a general method for precise skyrmionic control via the manipulation of SkX stripes with a skyrmion number of one within a predesigned structure. We also provide strategies for mutual transformations between helical states and SkXs. For chiral magnetic films, this study illustrates how patterned external forces can generate various striped phases and other exotic patterns. We demonstrate that by dividing one stripe into multiple segments and coalescing several skyrmions into a single entity using various external fields, an effective means to interconvert helical states into SkXs is obtained.
In summary, this thesis attempts to operationally understand magnetic skyrmions by studying their morphologies, topologies, and other essential characteristics. As a result, strategies for the precise and systematic control of these enigmatic entities are provided, which bridges several gaps in the understanding of skyrmionic behavior while offering strategies for their manipulation, which is expected to enrich the theoretical foundations of applied physics while enabling groundbreaking academic research and industrial material science applications.