Ferromagnetic Stability and Elementary Excitations in aS=12Heisenberg Ferromagnet with Dipolar Interaction by the Green's-Function Method

The temperature dependence of the elementary excitation energy in a dipolar Hamiltonian has been studied by the Green's-function equation-of-motion method. The result differs from that given by Charap and reduces to it for $B=F=0\mathrm{}$. The magnetization calculated at zero order of an iterative procedure coincides with the Holstein-Primakoff result. The criterion of stability of the ferromagnetic state has been found to be temperature dependent.