Theory of spin excitations in Fe(110) multilayers

We present theoretical studies of the nature of the spin excitations in ultrathin free standing Fe(110) films, and such films adsorbed on a semi-infinite W(110) substrate. The calculations are carried out within the framework of an itinerant electron theory, with a realistic underlying electronic structure. The energy bands of the film and substrate are described by a nine band empirical tight binding picture, to include the relevant d bands and overlapping sp complex. Ferromagnetism in the Fe film is driven by on site Coulomb interactions between the $3d$ electrons, treated in mean field theory while a description of the spin wave excitations is generated through use of the random phase approximation. Results are reported and discussed for Fe films three, four, and five layers in thickness. A principal conclusion which emerges from these studies is that the “frozen magnon” or adiabatic description of spin wave excitations proves inadequate in a qualitative sense, for systems such as those studied here. The spin waves are embedded in the Stoner continuum, with the consequence they are heavily damped by decay to Stoner excitations, save for the lowest lying mode at long wavelengths. A consequence is that throughout much of the surface Brillouin zone, the spectrum of spin excitations at each wave vector consists of a broad feature which shows dispersion, with no evidence of the sequence of standing spin wave modes predicted by a theory based on the adiabatic approximation. For the five layer film, even near the center of the surface Brillouin zone, while we find a weakly damped low lying acoustic spin wave, the first standing wave is quite broad, and the second standing wave structure is so broad it cannot be viewed as a well defined excitation. At large wave vectors, we find a single broad feature in the spectral density which shows dispersion, very similar to a spin wave, rather than a sequence of standing modes. The results are very similar in character to recent SPEELS data on the Co/Cu system. The physical reasons for the breakdown of the adiabatic method are discussed.