Ferromagnetic resonance study of polycrystalline cobalt ultrathin films

We present room-temperature ferromagnetic resonance (FMR) studies of polycrystalline ∥Pt∕10nmCu∕tCo∕10nmCu∕Pt∥ films as a function of Co layer thickness (1⩽t⩽10nm) grown by evaporation and magnetron sputtering. FMR was studied with a high-frequency broadband coplanar waveguide (up to 25 GHz) using a flip-chip method. The resonance field and the linewidth were measured as functions of the ferromagnetic layer thickness. The evaporated films exhibit a lower magnetization density (Ms=1131emu∕cm3) compared to the sputtered films (Ms=1333emu∕cm3), with practically equal perpendicular surface anisotropy (Ks≃−0.5erg∕cm2). For both series of films, a strong increase of the linewidth was observed for Co layer thickness below 3 nm. For films with a ferromagnetic layer thinner than 4 nm, the damping of the sputtered films is larger than that of the evaporated films. The dependence of the linewidth can be understood in terms of the spin-pumping effect, from which the interface spin-mixing conductance is deduced.