  We study magnetic domain walls in double exchange materials. The domain wall
width is proportional to the square root of the stiffness. In the double
exchange model the stiffness has two terms: the kinetic energy and the Hartree
term. The kinetic energy term comes from the decrease of the tunneling
amplitude in the domain wall region. The Hartree term appears only in double
exchange materials and it comes from the connection between band-width and
magnetization. We also calculate the low-field magnetoresistance associated
with the existence of magnetic domains. We find a magnetoresistance of 1-2%.
The magnetoresistance c an be considerably larger in magnetically constrained
nanocontacts.
