Method of effective potential for quantum Heisenberg ferromagnet theory

Self - consistent temperature dependence of the average magnetization in quantum Heisenberg ferromagnet is obtained as a first approximation of perturbation theory on an inverse radius by application of the functional method for quantum ferromagnet. The experimental data are compared with the theoretical results for {\bf EuO} ferromagnet. A quantitative agreement is observed between theoretical and experimental results outside interval near Curie temperature. The theory is of critical behavior in the vicinity of Curie temperature $T_c$. Theoretical $T_c$ value itself is agreed with the experimental one with an accuracy of several percents.