  We consider two-band double exchange model and calculate the critical
temperature in ferromagnetic regime (Curie temperature). The localized spins
are represented in terms of the Schwinger-bosons, and two spin-singlet Fermion
operators are introduced. In terms of the new Fermi fields the on-site Hund's
interactions are in a diagonal form and one accounts for them exactly.
Integrating out the spin-singlet fermions we derive an effective Heisenberg
type model for a vector which describes the local orientations of the total
magnetization. The transversal fluctuations of the vector are the true magnons
in the theory, which is a base for Curie temperature calculation. The critical
temperature is calculated employing the Schwinger-bosons mean-field theory.
While approximate, this technic of calculation captures the essentials of the
magnon fluctuations in the theory, and for 2D systems one obtains zero Curie
temperature, in accordance with Mermin-Wagner theorem.
