  The present paper gives a new method of attack on the Nambu-Goldstone
dynamics in spontaneously broken theories. Since the target space of the
Nambu-Goldstone fields is a group coset space, their effective quantum dynamics
can be naturally phrased in terms of generalized coherent-state functional
integrals. As an explicit example of this line of reasoning we construct a
low-energy effective Lagrangian for the Heisenberg ferromagnet in broken phase.
The leading field configuration in the WKB approximation leads to the
Landau-Lifshitz equation for quantum ferromagnet. The corresponding linearized
equations allow to identify the Nambu-Goldstone boson with ferromagnetic
magnon.
