  In this work, we propose a method for calculating the free energy of
anisotropic quantum spin systems. We use the Hubbard-Stratonovich
transformation to express the partition function of a generic bilinear
super-exchange Hamiltonian in terms of a functional integral over classical
time-dependent fields. In the general case the result is presented as an
infinite series. The series may be summed up in the case of Ising-type models.
For any ordered state we derive a compact expression for the contribution of
Gaussian spin fluctuations to the free energy.
