  We investigate the dynamical properties of a system of interacting magnetic
dipoles disposed in sites of an sc lattice and forming a cubic-shaped sample of
size determined by the cube edge length (N-1)a (a being the lattice constant, N
representing the number of dipolar planes). The dipolar field resulting from
the dipole-dipole interactions is calculated numerically in points of the axis
connecting opposite cube face centers (central axis) by collecting individual
contributions to this field coming from each of the N atomic planes
perpendicular to the central axis. The applied magnetic field is assumed to be
oriented along the central axis, magnetizing uniformly the whole sample, all
the dipoles being aligned parallelly in the direction of the applied field. The
frequency spectrum of magnetostatic waves propagating in the direction of the
applied field is found numerically by solving the Landau-Lifshitz equation of
motion including the local (nonhomogeneous) dipolar field component; the mode
amplitude spatial distributions (mode profiles) are depicted as well. It is
found that only the two energetically highest modes have bulk-extended
character. All the remaining modes are of localized nature; more precisely, the
modes forming the lower part of the spectrum are localized in the subsurface
region, while the upper-spectrum modes are localized around the sample center.
We show that the mode localization regions narrow down as the cube size, N,
increases (we investigated the range of N=21 to N=101), and in sufficiently
large cubes one obtains practically only center-localized and surface-localized
magnetostatic modes.
