Brillouin scattering from thermal magnons in a thin Co film

We report a room-temperature Brillouin scattering study of surface and standing spin-wave modes in a 100-nm film of polycrystalline hcp cobalt. The film was prepared by vapor deposition of Co metal onto a single-crystal sapphire substrate with the crystalline $c$ axis in the substrate plane. X-ray measurements show the film is predominantly aligned with the [001] axis normal to the film plane. Measurements were made with a five-pass plus four-pass tandem vernier Fabry-Perot interferometer with an instrumental full width of 700 MHz. The magnetic field $H_0$, $150\le H_0\le 3500$ Oe, was applied in the plane of the film, perpendicular to the scattering plane. Fitting the theoretical expressions for surface and standing spin-wave dispersion relations to the experimentally determined frequency shift versus applied field gives values for the gyromagnetic ratio and saturation magnetization in satisfactory agreement with literature values obtained from the application of other experimental techniques. However, the exchange stiffness constant $D$ obtained from analysis of the light scattering measurements is significantly smaller than that deduced from both neutron scattering and magnetization measurements.