Spin waves in ultrathin hexagonal cobalt films on W(110), Cu(111), and Au(111) surfaces

Spin wave spectra of ultrathin epitaxial cobalt films deposited on W(110), Cu(111), and Au(111) surfaces are studied in the wave-vector regime between 0.10.16em0ex^-1 and 0.70.16em0ex^-1 using inelastic electron scattering with 6 meV energy resolution. Up to three different spin wave modes are resolved for wave vectors q_∥<0.350.16em0ex^-1. The modes are identified as the acoustic mode and standing modes with one and two nodes inside the film. The relative weight of the modes in a particular spectrum may depend critically on the electron impact energy. For larger wave vectors beyond q_∥>0.350.16em0ex^-1 and layers thicker than five atom layers the separate modes merge into a single, broad loss feature. Since the shape and position of the loss feature depend on the electron impact energy, a separation into different modes is nevertheless possible for not too large wave vectors. The spin wave dispersion curves of films grown on W(110) agree with those observed on Cu(111) if one takes into account that on copper the cobalt grows in islands so that the mean height of the islands is higher than the nominal coverage. On films grown on Au(111) the low wave vector spin waves are buried in the high elastic diffuse scattering caused by the considerable disorder in the films. The broader appearance of the spectra at higher wave vectors compared to films grown on W(110) and Cu(111) is quantitatively accounted for by disorder-induced kinematic broadening. Because of the granular growth on copper and gold primarily the spin wave spectrum of cobalt films on W(110) is amenable to quantitative theoretical analysis. Such an analysis is not available at present. We show however, that the dispersion curves are incompatible with the Heisenberg model as long as only a single, layer-independent exchange coupling constant is invoked.