  We describe a novel type of magnetic domain wall which, in contrast to Bloch
or Neel walls, is non-localized and, in a certain temperature range,
non-monotonic. The wall appears as the mean-field solution of the
two-dimensional ferromagnetic Ising model frustrated by the long-ranged dipolar
interaction. We provide experimental evidence of this wall delocalization in
the stripe-domain phase of perpendicularly magnetized ultrathin magnetic films.
In agreement with experimental results, we find that the stripe width decreases
with increasing temperature and approaches a finite value at the
Curie-temperature following a power law. The same kind of wall and a similar
temperature dependence of the stripe width is expected in the mean-field
approximation of the two-dimensional Coulomb frustrated Ising ferromagnet.
