Phase transition in a 2-dimensional Heisenberg model
  We investigate the two-dimensional classical Heisenberg model with a nonlinear nearest-neighbor interaction   V(s,s')=2K[(1+s.s')/2 ]^p.   The analogous nonlinear interaction for the XY model was introduced by Domany, Schick, and Swendsen, who find that for large p the Kosterlitz-Thouless transition is preempted by a first-order transition. Here we show that, whereas the standard (p=1) Heisenberg model has no phase transition, for large enough p a first-order transition appears. Both phases have only short range order, but with a correlation length that jumps at the transition. 
Henk W. J. Bloete, Wenan Guo and Henk J. Hilhorst