  The domain wall response under constant external magnetic fields reveals a
complex behavior where sample disorder plays a key role. Furthermore, the
response to alternating magnetic fields has only been explored in limited cases
and analyzed in terms of the constant field solution. Here we unveil phenomena
in the evolution of magnetic domain walls under the application of alternating
magnetic fields within the creep regime, well beyond a small fuctuation limit
of the domain wall position. Magnetic field pulses were applied in ultra-thin
ferromagnetic films with perpendicular anisotropy, and the resulting domain
wall evolution was characterized by polar magneto-optical Kerr effect
microscopy. Whereas the DC characterization is well predicted by the elastic
interface model, striking unexpected features are observed under the
application of alternating square pulses: magneto-optical images show that
after a transient number of cycles, domain walls evolve toward strongly
distorted shapes concomitantly with a modification of domain area. The
morphology of domain walls is characterized with a roughness exponent when
possible and contrasted with alternative observables which result to be more
suitable for the characterization of this transient evolution. The final
stationary convergence as well as the underlying physics is discussed.
