Theory of spin waves in ultrathin ferromagnetic films: The case of Co on Cu(100)

We present theoretical studies of the nature of spin waves in ultrathin ferromagnetic Co films deposited on the Cu(100) surface. Motivation for the calculations are the recent experimental SPEELS data reported by Vollmer et al. Our calculations employ a realistic electronic structure for both the Co film and the semi-infinite Cu substrate, modeled through use of an empirical tight binding model with ferromagnetism in the Co film driven by intra-atomic Coulomb interactions within the Co $3d$ shell. Mean field theory describes the ground state, and we use the Random Phase Approximation to generate spin wave spectra. We obtain an excellent quantitative account of the value of the exchange stiffness reported in previous Brillouin lights scattering studies of epitaxially deposited Co films on Cu(100). As in the SPEELS experiment, we find a single very broad feature dominates the spin wave spectral density throughout much of the surface Brillouin zone, in contrast to the sequence of sharp standing spin wave resonances which result from generation of spin wave spectra via adiabatic approaches. Landau damping absent from such treatments influence the calculated spectra qualitatively, as in our earlier theoretical studies of related systems. It is the case, however, that the spin waves reported in the SPEELS experiments are very considerably softer than those generated by our theory.