Introduction of the factor of renormalization of the spin–wave energy to the Bogolyubov–Tyablikov theory

The decoupling parameter has been chosen in the form $\langle S \rangle R_s(T)$ for the compensation of error in the Tyablikov decoupling. The function $R_s$ defines the renormalization of the spin wave spectrum. The high-temperature limit value of this function has been estimated from the Curie temperature, and the temperature dependence has been obtained on the base of the Callen–Swendsen method. The suggested theory has been proved to be sufficiently simple for the calculation, accurate enough, and free from some difficulties of the previous methods.