  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.
