Stripe-like topological excitations in a two-dimensional Heisenberg ferromagnet

For a two-dimensional Heisenberg ferromagnet, a class of steady-state nonlinear excitations above the ground state is considered. The excitations have the form of stripes and exhibit quasiparticle properties. The effect of an external magnetic field on the basic characteristics of these nonlinear topological excitations is investigated. The magnetic field is found to destroy the instanton-type solutions (kinks) and to generate new particles with the properties of vortex-antivortex pairs: each particle has a zero topological charge and an energy close to the double skyrmion energy $8\pi JS^{2}$. The dispersion of the quasiparticles and the dependences of their energy and momentum on the number of magnons localized by one excitation are discussed.