  Long-wavelength spin waves in disordered magnetic systems have been
investigated. In the framework of the Heisenberg model with magnetic dipole and
exchange interactions between spins it is found that an additional longitudinal
spin wave mode appears. This mode is characterized by variations of the value
of the magnetic moment density. In order to analyse influence of the magnetic
disorder on spin wave dispersion relations, the special case of volume and
surface spin waves in the Damon-Eshbach (DE) geometry in films with magnetic
disorder is considered. It is revealed strong influence of the magnetic
disorder on surface spin waves, which consist of two branches -- the DE mode
and the longitudinal spin wave mode. Decrease of the ordering parameters leads
to a decrease of the initial frequency and the curve slope of the DE mode
dispersion curve and to an increase of the dispersion curve slope of the
longitudinal mode. If the spin noncollinearity is high, then the DE mode
dispersion assumes a curve with the backward character. It is found that the
dispersion relation of the longitudinal mode is weak temperature dependent. The
developed model can explain the observed double-peak structure of FMR spectra
in magnetic nanocomposites.
