Ferromagnetic resonance linewidths in ultrathin structures: A theoretical study of spin pumping

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-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.