  We present theoretical studies of the spin pumping contribution to the
ferromagnetic resonance linewidth for various ultrathin film ferromagnetic
structures. We consider the isolated film on a substrate, with Fe on Au(100)
and Fe on W(110) as examples. We explore as well the linewidth from this
mechanism for the optical and acoustical collective modes of FM/Cu$_{\rm
N}$/FM/Cu(100) structures. The calculations employ a realistic electronic
structure, with self consistent ground states generated from the empirical
tight binding method, with nine bands for each material in the structure. The
spin excitations are generated through use of the random phase approximation
applied to the system, including the semi infinite substrate on which the
structure is grown. We calculate the frequency response of the system directly
by examining the spectral density associated with collective modes whose wave
vector parallel to the surface is zero. Linewidths with origin in leakage of
spin angular momentum from the adsorbed structure to the semi infinite
substrate may be extracted from these results. We discuss a number of issues,
including the relationship between the interfilm coupling calculated
adiabatically for trilayers, and that extracted from the (dynamical) spin wave
spectrum. We obtain excellent agreement with experimental data, within the
framework of calculations with no adjustable parameters.
