Semitopological solitons in planar ferromagnets

We establish the existence of a finite-energy solitary wave in a two-dimens ional planar ferromagnet which moves rigidly at any constant velocity v tha t is smaller than the magnon velocity c. The shape of the calculated solito n depends crucially on the relative magnitude of v and c. For v << c, the s oliton describes a widely separated vortex-antivortex pair undergoing Kelvi n motion at a relative distance d similar to c/v. There exists a crossover velocity v(0) at which the vortex-antivortex character is lost (d similar t o 0) and the energy-momentum dispersion develops a cusp. For v(0) < v < c, the soliton becomes a lump with no apparent topological features and solves the modified KP equation in the limit v --> c. We also describe briefly a similar calculation of a vortex ring in a three-dimensional planar ferromag net. These results together with the analytically known one-dimensional pi kink provide an interesting set of semitopological solitons whose physical significance is yet to be explored.