Micromagnetic study of size-dependent picosecond dynamics in single nanomagnets

We present the micromagnetic simulation study of the dynamics of single cylindrical nickel nanomagnets of varying size (aspect ratio), as the shapes of the elements change from disc-like to pillar-like structure. The magnetic ground states change from multi-domain to vortex to out-of-plane magnetized single domain structure and the time-resolved dynamics reveal three distinct modes originating from the centre and end modes of the out-of-plane magnetization (mode 1 and mode 3) and precession of the in-plane magnetization (mode 2). The mode frequencies show strong size dependence including a sharp jump at diameter D = 250 nm (aspect ratio = 0.6), followed by a sharp increase in the frequency with reduced diameter in mode 1. Macrospin simulations reproduce the size variation of frequency of mode 1 only for D <= 250 nm, where the ground states take quasi-single domain configurations. Further analyses after the application of monochromatic harmonic fields to the samples show that size dependence of frequency stems from the variation of the dynamical modes with the continuously varying static magnetic configurations with size and the sharp jump in frequency at D = 250 nm is due to the transition from vortex gyration to precession of the out-of-plane magnetization. Further simulations of local excitation of the sample show the formation of perpendicular standing spin waves and spatial oscillation of the vortex core in nanomagnets with varying aspect ratios.