Three-dimensional micromagnetic simulations of multidomain bubble-state excitation spectrum in ferromagnetic cylindrical nanodots

Abstract

The excitation spectrum of multidomain bubble states in cylindrical nanodots with a large perpendicular anisotropy has been investigated by means of three-dimensional micromagnetic simulations for various dot diameters. In a first step, the evolution of the zero-field susceptibility spectra as a function of the dot diameter is studied for bidomain bubble states. Increasing dot diameter leads to shifts of the resonance frequencies toward the low frequencies for the fundamental domain wall and domain modes, whereas the spectral positions of surface domain wall and domain modes are weakly affected. Second, the high-frequency response of a tridomain bubble state stabilized in larger dot diameters is analyzed. Dynamic interaction effects are revealed mainly through the identification of coupled domain-wall modes, the resonance frequencies of which depend on the relative domain-wall chiralities. Lastly, the magnetization dynamics of a metastable bidomain bubble state with a pair of vertical Bloch lines, each of them possessing a Bloch point, is addressed. In such a case, the presence of magnetic singularities leads to a splitting of the fundamental domain-wall mode into two resonance lines assigned to oscillations of domain-wall parts with different phase relations.