Micro-magnetic Study On Skyrmionic Spin Configurations In Geometrically Confined Systems

Magnetic skyrmion is a kind of topological soliton with particle properties.It has the advantages of small size,low driving current density and so on.It is very hopeful to become the basic unit of ultra-high-density information storage in the future.In this paper,the physical properties of the skyrmionic spin structures(including skyrmions,skyrmioniums,target skyrmions)are studied by means of micromagnetic simulation.In the first and second chapters,starting from the origin and development of magnetics and the research content and current situation of spintronics,the origin,production mechanism,materials and application prospects of magnetic are introduced,and the research objectives and contents of this paper are introduced.Then it introduces the development and basic theory of micromagnetic calculation,and discusses the micromagnetic theory including Brown steady-state equation,LLG equation,spin transfer torque effect in detail.Finally,the simulation software Mumax3 used in this work is introduced.Chapter 3:the topological transformation between skyrmions and skyrmioniums.We theoretically investigate the micromagnetic dynamics and spin configurations in a CoFeB/Pt nanodisk with varying effective anisotropy constant and radius.It is found that skyrmion and skyrmionium could be stabilized well in a certain region of the phase diagram of topological structures,and their formations physically originated from the competition between exchange interactions and anisotropy contributions.Furthermore,we achieve the transition between the skyrmion and the skyrmionium through fine tuning the magnetic anisotropy.Our results reveal the specific conditions needed to form the equilibrium states of skyrmions and skyrmionium and have significance for the development of skyrmion-based devices.Chapter 4:polarity transition of the target skyrmions.We investigated the current-induced polarity reversal for a ground-state target skyrmion in a spin-valve nanopillar structure,demonstrating two reversal modes distinguished by current density.In view of the considerable writing energy consumption(?E)caused by Joule heating effect,we proposed a multivariate optmization method considering the coupling of the current density,the applied area and the effective excitation time.This optimization can significantly improve the system stability and reduce ?E to about 30%without sacrificing reversal efficiency.Our results explain the physical origin of polarity reversal of target skyrmion and overcome a major difficulty for the practical application of skyrmionic spin structures as high-density elements in memory devices.