Resonantly exited precession motion of three-dimensional vortex core in magnetic nanospheres

We found resonantly excited precession motions of a three-dimensional vortex core in soft magnetic nanospheres and controllable precession frequency with the sphere diameter 2 R , as studied by micromagnetic numerical and analytical calculations. The precession angular frequency for an applied static field H DC is given as ω MV = γ eff H DC , where γ eff = γ 〈 m Γ 〉 is the effective gyromagnetic ratio in collective vortex dynamics, with the gyromagnetic ratio γ and the average magnetization component 〈 m Γ 〉 of the ground-state vortex in the core direction. Fitting to the micromagnetic simulation data for 〈 m Γ 〉 yields a simple explicit form of 〈 m Γ 〉 ≈ (73.6 ± 3.4)( l ex /2 R ) 2.20±0.14 , where l ex is the exchange length of a given material. This dynamic behavior might serve as a foundation for potential bio-applications of size-specific resonant excitation of magnetic vortex-state nanoparticles, for example, magnetic particle resonance imaging.