EXTENSION THEORY FOR MAGNON OF A FERROMAGNETIC SUPERLATTICE

We prove that the extension theory for lattice Green's functions can b e used to study Heisenberg ferromagnetic systems. As an example, the m agnon spectrum of a ferromagnetic superlattice is calculated by the ex tension theory for lattice Green's functions cooperated with the trans fer matrix technique. The spectrum of magnetic excitation near the sur face of a ferromagnetic superlattice is also presented.