Wick theorem and linked-diagram expansion for the Heisenberg model ferromagnet

A finite-temperature perturbation theory for the Heisenberg model of ferromagnetism is presented. A modified Wick theorem proved for S = ½ and finite temperature allows the terms in the expansion for the temperature-dependent Green function to be expressed by linked diagrams of the Feynman type. These differ from the normal boson and fermion diagrams in the appearance of time(temperature)-dependent broken lines between propagators, a modification caused by the spin statistics.