Exploration of Vortex Stabilization and Its Conversion into Topologically Protected Bimeron Spin Textures in Ferromagnetic Heterostructures

Abstract

This thesis presents an in-depth study of the stability and control of magnetic spin configurations, particularly focusing on vortices and bimerons within Permalloy (Py) material, offering crucial insights for the advancement of future spintronic devices. We examined the vortex magnetization stability in circular Py nanodots using micromagnetic simulation techniques, exploring the effect of magnetocrystalline anisotropy on hysteresis loops, spin configurations, and energy profiles. Our results indicate that vortex stability in a 64×20 nm2 Py nanodot persists till the critical anisotropy (CK) of 170 kJ/m3, after which it transitions into a single-domain state. Energy analysis emphasizes the interplay between exchange and demagnetization energies in this process, and we also calculated the energy barrier for the transformation from single-domain to a vortex state. Further developing this insight, we studied asymmetric Py annular discs, which are considered promising structures for advanced spintronic applications because their geometry facilitates an extra degree of freedom for manipulating the magnetic states. Utilizing micromagnetic simulations, we explored the impact of ring thickness, the shape of the void, and variations in in-plane and out-of-plane anisotropy on magnetization reversal mechanisms and spin configurations. By varying the annular disc thickness (t), we observed fascinating spin configurations at zero magnetic field. Specifically, the discs with t lt 12 nm exhibited a 360° domain wall, while thicker discs with t gt 36 nm exhibited the presence of a bi-vortex wall and enhanced the stability range for the global-vortex state during magnetization reversal. Within specific an in-plane anisotropy ranges, we observed multiple vortex cores emerged with differing polarities and rotation senses, owing to the demagnetization energy contributed to their formation newline