High wave vector spin waves in ultrathin Co-films investigated by spin-polarized electron energy loss spectroscopy

The capabilities of spin polarized electron energy loss spectroscopy are demonstrated with results obtained on ultrathin Co-films. Co is investigated in two crystalline phases, the fcc- and the hcp-phase. In both cases, the spin-wave signals for wave-vector transfers up to the surface Brillouin zone for film thickness of only a few atomic layers are measured. In both systems, a pronounced dispersion of the spin waves is found. The measured data is in good agreement with the calculated dispersions of surface spin waves within a nearest neighbor Heisenberg model. From the fits of the calculated to the measured dispersions, a value for the product of the exchange coupling constant and the magnetic moment (JS) of about 15 meV for both crystalline phases is obtained. Due to the similar values of JS, the spin-wave energies are similar for wave-vector transfers much smaller than the surface Brillouin zone boundary. For high wave-vector transfers, the spin-wave energies differ significantly reflecting the larger surface Brillouin zone for the hcp Co-lattice caused by the different arrangement of atoms at the surface.