Magnetic order in heisenberg models on non-bravais lattice: Popov-fedotov functional method

We study magnetic properties of ordered phases in the Heisenberg model on a nonBravais lattice by means of a Popov - Fedotov trick, which takes into account a rigorous constraint of a single occupancy. We derive the magnetization and the free energy using sadle point approximation in the functional integral formalism. We illustrate the application of the Popov – Fedotov approach to the Heisenberg antiferromagnet on a honeycomb lattice.