  his study is an investigation of spin wave spectrum in macrostructures
composed of two ferromagnetic materials and showing a 3D periodicity: spherical
ferromagnetic grains disposed in the nodes of a 3D crystal lattice are embedded
in a matrix with different ferromagnetic properties. Frequency ranges forbidden
to spin wave propagation are found in the calculated magnonic spectra. Both the
position and the width of the gaps are found to depend on the magnetic
(exchange and magnetization) contrasts in the composite material, as well as on
its structural parameters (filling fraction, crystal lattice type). Having
applied our theory to interpretation of the existence of a spin wave gap in
doped manganites, recently revealed in neutron scattering experiments by S.
Hennion {\em et al.}, we obtained a good (though approximate) quantitative
agreement with the experimental results. A working hypothesis is proposed on
this basis, supposing that certain manganites can be treated as natural 3D
magnonic crystals.
