Dynamics of the soliton instability in the easy-plane ferromagnetic chain

We present results on the nonlinear dynamics of a realistic classical easy-plane ferromagnetic chain in an external magnetic field, concerning in particular the dependence of soliton solutions on the strength of the single-ion anisotropy. After giving two exact static solutions, slowly moving permanent profile solutions are investigated. The velocity dependent contribution to the energy as well as the amplitude of the out-of-plane distortion of these solutions is shown to diverge when the Magyari-Thomas-Kumar instability of the static Sine-Gordon soliton is approached. The dynamic origin of this instability is shown to be an unstable behaviour of the soliton towards spontaneous motion instead of a soft mode.