Reversal Dynamics of Magnetic Vortex Core in Soft Magnetic Nanoelements

Abstract

This thesis addresses dynamics of ultrafast magnetic vortex core switching (≤ 1 ns) in soft magnetic nanoelements by micromagetic simulations. To find more efficient way to reverse the core magnetization, we used the radial and azimuthal spin-wave modes in thin magnetic nanodisks. We also examined the core switching in three-dimensional magnetic half-spheres. First, we observed the radial spin-wave mode driven vortex core reversals using oscillating magnetic fields applied perpendicularly to the disk plane. It was found that when the field frequencies are tuned to the eigenfrequencies of radial spin-wave modes of the vortex structure, the threshold field amplitudes required for vortex-core switching are an order of magnitude smaller than those of static perpendicular fields within 1 ns. The reversal mechanism and associated underlying physics are completely different from well-known vortex-antivortex-pair-mediated core reversals. Next, we also examined the azimuthal spin-wave modes driven vortex-core switching in soft magnetic nano-disks. We addressed the similarities and differences between the asymmetric core reversals in terms of the temporal evolutions of the correlated core-motion speed, locally concentrated perpendicular gyrofield, and magnetization dip near the original vortex core. The core-motion speed and the associated perpendicular gyrofield must reach their threshold values to meet the ultimate core-reversal criterion mz,dip ~ -p , where p is the polarization of the given vortex state. Also, we determined the perpendicularly bias field strength and direction dependence of the core-switching time and threshold exciting field strength required for the core reversals, which parameters are essential in the application aspect. Finally, we studied the magnetization-reversal dynamics of soft magnetic half-spheres with vertically non-uniform vortex cores, and found a heretofore-unknown mechanism of vortex-core reversals in cylindrical dots. In the half-sphere geometry, another vortex core, the magnetization of which is opposite to the original core, is formed at the edge of the half-sphere after expulsion of the original core, thereby reversing the core. Core switching occurs either with or without chirality switching. The specific curved three-dimensional geometry of half-spheres can affect the core-reversal mechanisms. The present studies offer deeper insights into the core switching dynamics as well as imparts further momentum to the realization of information storages based on the vortex structure and its dynamic properties.