Current and electric-field driven skyrmion dynamics for magnetic memory applications

• Main Author: Fook, Hiu Tung
• Other Authors: Lew Wen Siang
• Format: Final Year Project
• Language: English
• Published: 2016
• Subjects: DRNTU::Science::Physics::Electricity and magnetism
• Online Access: http://hdl.handle.net/10356/68324
• Institution: Nanyang Technological University

Magnetic skyrmions are topologically stable spin textures that are found in materials with Dzyaloshinskii-Moriya interaction. There has been a growing interest in skyrmions as information carriers in memory devices due to their small size, resistance to pinning by defects and low current required for transport. However, there are obstacles that hinder the realization of skyrmion-based memory devices such as the skyrmion Hall effect. In this thesis, the dynamics and control of current and electric-field driven skyrmions are analyzed by performing micromagnetic simulations. Two methods of mitigating the skyrmion Hall effect are presented to increase skyrmion annihilation threshold velocity. By magnetic anisotropy patterning or adding ferromagnetic strips at the nanowire edges, skyrmions can be guided to traverse only along the nanowire axis even in sharp 180° bends. This allows for more flexible designs in skyrmion-based memory. Skyrmions can also be compressed by narrowing the potential width introduced by these methods, resulting in a significantly higher information density in skyrmion-based memory devices. The methods of confining skyrmions in the nanowire by introducing potential barriers at the edges can be used to also pin skyrmions in the direction transverse to the nanowire axis. A skyrmion diode that utilizes the skyrmion Hall effect induced operation asymmetry when the pinning site is located at only one nanowire edge is also proposed. For symmetric operation, patterned ferromagnetic layers are added to both nanowire edges. Through study of skyrmion response to external magnetic and electric fields, a gateable skyrmion transport is demonstrated by modulating the pinning strength experienced by a skyrmion via external fields. This phenomenon can be utilized for a skyrmion transistor and skyrmion racetrack memory which are proposed in this thesis. Electric-field gradient induced skyrmion motion can be used to assist electric current in driving skyrmions in conducting multilayers. High skyrmion velocities can thus be achieved while maintaining low applied current density. Arrays of electric gates can form potential wells and drive and/or trap skyrmion without the use of physical pinning sites. Our analysis proposes highly functional designs and solutions to the challenges of skyrmion-based memory devices, demonstrating the immense potential and feasibility of using skyrmions as information carriers in memory devices.