We study Heisenberg spins on an infinite plane. In the continuum limit the Hamiltonian of the system is given by the nonlinear model. Following an approach developed by Mikeska and Affleck, we find that the angular momentum associated with the order parameter presents a classical spin part, associated with the gauge freedom of a trihedra. We show that this gauge field may induce a non-trivial topological term, the Hopf term (or Chern-Simons term), as initially suggested by Dzyaloshinski, Polyakov and Wiegmann.