Magnetic Composites: Magnonic Excitations vs. Three-Dimensional   Structural Periodicity
  This study deals with the spin wave spectrum in magnetic macrostructure (composed of two ferromagnetic materials) showing a 3D periodicity: spherical ferromagnetic grains disposed in the nodes of a simple cubic crystal lattice are embedded in a matrix with different ferromagnetic properties. It is shown that the \textit{magnonic spectrum} of this composite structure exhibits frequency regions \textit{forbidden} for magnon propagation, and the energy gaps are found to be sensitive to the exchange contrast between the constituent materials as well as to the magnetization contrast. The widths of the respective magnonic gaps are studied as functions of parameters characterizing the magnetic structure. 
M. Krawczyk, H. Puszkarski