  The Pair Approximation method has been formulated for the isotropic
ferromagnetic Heisenberg model with spin $S=1$. The exchange interactions of
arbitrary range have been taken into account. The single-ion anisotropy has
been considered as well as the external magnetic field. Within the method, the
Gibbs free-energy has been derived, from which all thermodynamic properties can
be self-consistently obtained. In order to illustrate the developed formalism,
the numerical calculations have been performed for CrIAs planar magnetic
semiconductor, a hypothetical material whose existence has been recently
predicted by the Density Functional Theory-based calculations. For this model
material, all the relevant thermodynamic magnetic properties have been studied.
The numerical results have been presented in the figures and discussed.
