Magnetic skyrmion stabilization, nucleation, and dynamics on magnetic multilayer thin films

• Main Author: Ang, Calvin Ching Ian
• Other Authors: Lew Wen Siang
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Science::Physics::Electricity and magnetism
• Online Access: https://hdl.handle.net/10356/158214
• Institution: Nanyang Technological University

Magnetic skyrmions are particle-like nanoscale magnetic structures currently surging in interest in the spintronics field. Their small size combined with topological stability and efficient manipulation by various electromagnetic excitations make them promising next-generation high-density information carriers. Beyond data storage and memory applications, the diverse magnetic skyrmion interactions and excitations can be further exploited for conventional logic computing and unconventional computing like neuromorphic, probabilistic, and Brownian computing. Skyrmion research is advancing on many fronts to bring these technological ideas into reality, including stabilisation in materials and conditions, reliable nucleation techniques, and precise control of magnetic skyrmions. In this thesis, the spin texture evolution between the labyrinth, stripe, skyrmion, and ferromagnetic states in magnetic multilayer [Pt/Co/Fe/Ir]2 under the first-order reversal curve (FORC) magnetic field sweeps was investigated. Temperature modulation was performed to tune skyrmion phases and acquire their corresponding FORC signatures. Using magneto-optical Kerr microscopy, an analysis technique based on the sweeping field was developed and applied in the derivation of the skyrmion phase. A technique for skyrmion nucleation and deletion by current density modulation independent of current polarity in magnetic multilayer [Pt/Co/Fe/Ir]2 was demonstrated. A high current density induces the nucleation of skyrmions via spin-orbit torque acting on defects. In contrast, the low current density causes a volatile skyrmion-stripe transformation at pinning sites that annihilates other skyrmions. A voltage-induced magnetic skyrmion motion based on voltage-controlled magnetic anisotropy gradients was investigated towards a more energy-efficient skyrmion propagation technique. The dynamics of synthetic antiferromagnetic skyrmions on a magnetic anisotropy gradient was investigated numerically, and an analytical model was developed to describe their motion accurately.