  Utilizing an atomistic computational model which handles both translational
and spin degrees of freedom, combined molecular and spin dynamics simulations
have been performed to investigate the effect of vacancy defects on spin wave
excitations in ferromagnetic iron. Fourier transforms of space and
time-displaced correlation functions yield the dynamic structure factor,
providing characteristic frequencies and lifetimes of the spin wave modes.
Comparison of the system with a 5% vacancy concentration with pure lattice data
shows a decrease in frequency as well as a decrease in lifetime for all
transverse spin wave excitations observed. Additionally, a rugged spin wave
line shape for low-q spin waves indicates the presence of multiple localized
excitations near defect sites resulting in reduced excitation lifetimes due to
increased magnon-magnon scattering. We observe further evidence of increased
magnon-magnon scattering as the peaks in the longitudinal spin wave spectrum
become less distinct.
