Abstract: A thertmodynamic perturbation theory of the Heisenberg ferromagneis is developed, which identifies a perturbation part in the motion equation of the spin operator. After appropriate "renormalization" the perturbation expansion of the Green's function has the same form as that for fermion systems. Thus the Feynman diagrammatic technique can be extended to spin systems. This theory has the distinguishing feature of separating the kinetic interactions from the dynamic interactions.
Then this theory is applied to analyzing various decoupling approximations in Green's function method. And a new decoupling procedure is proposed, which can give the low temperature expansion of magnetization correct up to the T term.