Magnetic vortex based magnons in one-dimensional periodic arrays of soft ferromagnetic nanodisks

Abstract

Abstract Magnetic Vortex Based Magnons in One-dimensional Periodic Arrays of Soft Ferromagnetic Nanodisks

Dong-Soo Han Department of Materials Science and Engineering Seoul National University

Lattice vibration modes are collective excitations in periodic arrays of atoms or molecules. These modes determine novel transport properties in solid crystals. Analogously, in periodical arrangements of magnetic vortex-state disks, collective vortex motions and vortex gyration mediated signal transport have been predicted. In this thesis, in particular, we focus on magnetic vortex-based magnonic behavior in one-dimensional (1D) periodic arrays of soft ferromagnetic nanodisks, and also provide a foundation for manipulation of the vortex-gyration based signal transfer. We, for the first time, experimentally demonstrate wave modes of collective vortex gyration in 1D periodic arrays of magnetic disks by using time-resolved scanning transmission x-ray microscopy. The observed modes are interpreted based on micromagnetic simulation and numerical calculation of coupled Thiele equations. Dispersion of the modes is found to be strongly affected by vortex polarization, chirality ordering, dimensional parameters of the constituent disk, and interdistance between neighboring disks. The effects of change in the primitive unit cells of 1D vortex arrays on collective vortex-gyration dispersion are also investigated through micromagnetic numerical and analytical calculations. As the primitive basis, we consider alternating constituent materials (NiMnSb vs. Permalloy) and alternating dimensions including constituent disk diameter and thickness. In the simplest case, that of one vortex-state disk of given dimensions and single material in the primitive cell, only a single branch of collective vortex-gyration dispersion appears. By contrast, two constituent disks different alternating materials, thicknesses and diameters yield characteristic two-branch dispersions the band widths and gaps of which differ in each case. Furthermore, we propose and demonstrate an efficient way to control coupled-vortex dynamics by means of an external perpendicular bias field. The results reveal that the dynamics properties, such as the eigenfrequencies and dispersion relations, of coupled-vortex arrays can be manipulated by the strength and direction of bias field. More interestingly, for antiparallel polarization ordering case, a single branch splits into two distinct branches under the non-zero bias field, and thus resulting in bandgap opening. This substantial work offers potential implementation into vortex-gyration based information processing devices with the advantages of endless endurance of switchable vortex states and vortex-gyration propagation, low-power signal input through resonant excitation of vortex gyrations, and low energy dissipation.

Keywords: magnetic vortex, vortex dynamics, coupled dynamics, spin dynamics, magnonic crystal, spin waves Student number: 2008-20696