  Antivortex birth, growth and death due to the propagation of a transverse
domain wall (DW) in magnetic nanostrips are observed and analyzed. Antivortex
formation is an intrinsic process of a strawberry-like transverse DW originated
from magnetostatic interaction. Under an external magnetic field, DW in a wider
width region tends to move faster than that of a narrower part. This speed
mismatch tilts and elongates DW centre line. An antivortex is periodically born
near the tail of the DW centre line. The antivortex either moves along the
centre line and dies on the other side of the strip, or grows to its maximum
size, detaches itself from the DW, and vanishes eventually. The former route
reverses the polarity of DW while the later keeps the DW polarity unchanged.
The evolution of the DW structures is analyzed using winding numbers assigned
to each topological defects. The phase diagram in the field-width plane is
obtained and discussed.
