The Darboux-Backlund transformation for the static 2-dimensional   continuum Heisenberg chain
  We construct the Darboux-Backlund transformation for the sigma model describing static configurations of the 2-dimensional classical continuum Heisenberg chain. The transformation is characterized by a non-trivial normalization matrix depending on the background solution. In order to obtain the transformation we use a new, more general, spectral problem. 
Jan L. Cieslinski, Joanna Czarnecka