Brillouin study of long-wavelength spin waves in quasimonatomic Co films with uniaxial perpendicular magnetic anisotropy

We have observed spin-wave Brillouin light scattering from ultrathin Co/Au/ Cu(111) films with Co thicknesses tc, down to 1 monolayer (ML) and with a 1 -ML Au interlayer. The detection of a well-defined spin-wave spectrum and t he field dependence of its frequency show directly long-range collective an d ferromagnetic ordering in these films at room temperature. From the field dependence of the spin-wave frequency, we derive uniaxial perpendicular ma gnetic anisotropy constants as a function of t(Co) with various overlayer m aterials, including Cu, Pd, and Au. With a Cu overlayer, we observe that th e first-order perpendicular anisotropy K-u((1)) obeys well a linear relatio n between K-u((1)) t(Co) and t(Co) for t(Co) greater than or equal to 1.5 M L, which indicates a constant contribution of the interface anisotropy of 0 .16 mJ/m(2) in addition to the volume anisotropy of 0.73 MJ/m(3). With an A u or a Pd overlayer, we find that both the interface and volume anisotropie s are significantly larger than those with the Cu overlayer. We quantify ma gnetic inhomogeneities from the field dependence of the spectrum width. Wit h the Au or Pd overlayer, K-u((1)) shows a steep decrease with decreasing t (Co) for t(Co) <3.0 ML, which agrees well with a significant increase in th e structure-related magnetic inhomogeneity. We show directly that long-rang ed ferromagnetic ordering exists, with the perpendicular anisotropy, in our quasimonatomic Co films thinner than 1.5 ML. K-u((1)) for each overlayer t ends to be zero at 1 ML of Co, accompanied by heavy damping of the spin wav e. In addition, we find the second-order perpendicular anisotropy is still maintained with a comparable value to K-u((1)) in such quasimonatomic Co fi lms, indicating significant deformation of the uniaxial anisotropy.