  Exact propagating topological solitons are found in the easy-plane phase of
ferromagnetic spin-1 Bose-Einstein condensates, manifesting themselves as kinks
in the transverse magnetization. Propagation is only possible when the
symmetry-breaking longitudinal magnetic field is applied. Such solitons have
two types: a low energy branch with positive inertial mass and a higher branch
solution with negative inertial mass. Both types become identical at the
maximum speed, a new speed bound that is different from speed limits set by the
elementary excitations. The physical mass, which accounts for the number
density dip, is negative for both types. In a finite one-dimensional system
subject to a linear potential, the soliton undergoes oscillations caused by
transitions between the two types occurring at the maximum speed.
