  Exact solutions of magnetic field-driven propagating topological solitons are
found in the easy-plane phase of ferromagnetic spin-1 Bose-Einstein
condensates, manifesting themselves as dark-soliton-like magnetic domain walls.
Such solitons have two types: a low energy solution with positive inertial mass
and a higher energy 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, and
transitions between the two types occurring at the maximum speed.
