Vortices in the cone phase of a classical quasi-two-dimensional ferromagnet

Vortex phenomenology is developed for the Berezinskii–Kosterlitz–Thouless topological spin transition in a classical weakly anisotropic Heisenberg easy-plane ferromagnet in the so-called cone phase in an external magnetic field directed along its hard axis. A field-induced decrease in the transition temperature is predicted. The structure of static vortices in the applied field is calculated. It is shown that the field produces different effects on oppositely polarized vortices and significantly affects the form of the components of the dynamic structural factor of the magnet.