  Spinful superfluids of ultracold atoms are ideal for investigating the
intrinsic properties of spin current and texture because they are realized in
an isolated, nondissipative system free from impurities, dislocations, and
thermal fluctuations. This study theoretically reveals the impact of spin
current on a magnetic domain wall in spinful superfluids. An exact wall
solution is obtained in the ferromagnetic phase of a spin-1 Bose--Einstein
condensate with easy-axis anisotropy at zero temperature. The
bosonic-quasiparticle mechanics analytically show that the spin current along
the wall becomes unstable if the velocity exceeds the critical spin-current
velocities, leading to complicated situations because of the competition
between transverse magnons and ripplons. Our direct numerical simulation
reveals that this system has a mechanism to generate an eccentric fractional
skyrmion, which has a fractional topological charge, but its texture is not
similar to that of a meron. This mechanism is in contrast to the generation of
conventional skyrmions in easy-axis magnets. The theoretical findings can be
examined in the same situation as in a recent experiment on ultracold atoms. In
terms of the universality of spontaneous symmetry breaking, unexplored similar
phenomena are expected in different physical systems with the same broken
symmetry.
