Domain walls are the regions between two domains in which the magnetic moments gradually change direction.
A zero length domain wall (i.e. the magnetic moments are oriented 180 degree with respect to each other at the domain interface)
costs energy due to the exchange interaction that tends to align the magnetic moments.
It is therefore energetically much more favourable to cant the magnetic moments gradually over a region at the interface: the domain wall.
There exist two kinds of domain walls as is shown in the images below that differ in the direction of canting.
The more common one is the Bloch wall, but in thinner films a Neel wall is often favoured.
FIG. X1: Simple sketch of magnetic domains. The arrows show the direction of the local magnetization. The exact magnetic
structure is more complicated and includes domain walls and closure domains.
Closure domains are regions at the interfaces of the film in which the moments lie in the plane of the surface
that reduces the energetically unfavourable stray field setup by the sample.