THERMAL MAGNONS IN BCC COBALT-ITINERANCY AND EXCHANGE STIFFNESS

The stable hcp and the metastable bcc phases of cobalt have almost identical nearest-neighbor distances, but substantially different coordination numbers z (z(bcc) = 8, z(hcp) = 12). While z is central to magnetic behavior in insulating systems, these distinctive structural features of cobalt offer an opportunity to address the role of coordination in strong itinerant ferromagnets. We report a room-temperature Brillouin scattering study of surface and bulk spin waves in molecular-beam-epitaxially grown thin films of bcc cobalt. Fitting expressions for the surface and bulk magnon dispersion to the measured frequency shifts versus applied magnetic field provides values for the gyromagnetic ratio, saturation magnetization M, and the exchange stiffness constant D for bcc cobalt. Our results reveal that M is about 10% lower than that of the hcp phase, while D scales with z, a characteristic of an insulating ferromagnet.