Spin excitation spectrum of biskyrmion in a confined nanodot

A biskyrmion, which consist of two stable Bloch skyrmions with opposite magnetic helicities has been observed in several centrosymmetric ferromagnets such as layered perovskite manganites and MnNiGa. Here, the spin excitation modes of biskyrmion in a confined nanodot were investigated by using micromagnetic simulations. An oscillation mode at the lower frequency and two mixed modes of breathing and rotation at higher frequencies are observed. In addition, the frequencies for mixed modes exhibit remarkable dependence on the change of magnetization profile. However, the in-plane orientation of biskyrmion does not shift the excitation frequencies corresponding to all eigenmodes. Our result indicates novel excitation modes which enrich the theory of eigenmodes for spin textures and offers a direct mean of detecting and characterizing biskyrmion in experiment.